5 Insights that challenge what you think about carbs and exercise

Rethinking Carbs and Athletic Performance | Dr. Andrew Koutnik & Mike Haney

In a recent episode of A Whole New Level, Levels editorial director Mike Haney sits down with Dr. Andrew Koutnik, a researcher specializing in metabolic physiology, nutrition, and human performance. Koutnik, who himself has type 1 diabetes, brings a uniquely personal lens to studying how the body uses fuel — and his research has directly challenged some of the most entrenched assumptions in sports nutrition. His recent landmark review, spanning over a hundred years of evidence and more than 160 sports performance trials, calls into question whether athletes truly need the high carbohydrate loads that current guidelines recommend.

The conversation covers how the body selects and switches between fuel sources, what it means to be metabolically flexible, why the traditional crossover point theory may be wrong, what Koutnik's randomized controlled trials found about carbohydrate intake and performance, and what all of this means for everyday athletes trying to figure out how to eat.

"No athlete's health should be sacrificed at the altar of performance. Ever." — Dr. Andrew Koutnik

Living with type 1 diabetes: manual control of your own metabolism

Mike Haney: So, Dr. Andrew Koutnik, thanks so much for joining us today.

Andrew Koutnik: It's an honor. I appreciate you having me.

Mike Haney: So we've been talking a lot in recent episodes about fuel sources in the body, and we've been talking about them in all kinds of contexts. So we talked about obesity, we've talked recently about evolutionary adaptation and how we've come to use carbs and fat the way we do. But today we're going to talk about this primarily in a kind of athletic context, and particularly some of the research you've done, including a very recent paper that's kind of overturning some of the traditional thought about how fuel sources work with athletics.

Before we get into all that, I thought maybe just on a personal note, I wanted to start here. I know you have type 1 diabetes, and I've heard you describe having type 1 as having fully manual control of your metabolism. And I've never really heard someone describe type 1 before like that — it's almost like an empowering way to frame it. So I'm wondering if you'd mind just talking about what that experience is and how you think about that concept of having manual control of your metabolism.

Andrew Koutnik: Yeah, so when I think about type 1 diabetes — a lot of people who aren't familiar with this disease, it's a disease where you no longer produce insulin. Insulin's the most powerful hormone in all of metabolism, and it really acts as the kind of king over many tissues and mechanistic processes within metabolism.

It tells the liver whether it's going to store or release glucose. It tells the fat whether it's going to store or release fatty acids. And it does this in a number of other key tissues as well. Now, when you lose the ability to produce it, now you're in total control over administering it. You have to be the manual manipulator of your own metabolism. You have to understand the food you're consuming, the exercise you're engaging in, and a bunch of other key variables — sleep, medications, time of day — all these different things affect your metabolism, your insulin sensitivity, and how much glucose is going to come into your body, or how much maybe your body actually has stored that will release into the bloodstream. And you're in control of having to understand these changes in insulin sensitivity at each moment of each day, and then ultimately administering a precise, calculated amount of insulin at each one of those moments.

Now, the risk is that if you get the dose right, then perfect, everything goes well. But if you don't and your blood sugar goes low, it can be quite risky. You can cause lethargic fatigue, or it could be fatal in the worst consequences. Now, if the blood sugar goes too high, then you may feel irritable, reduced cognition, and maybe some other symptoms of fatigue, but it really doesn't become fatal unless it persists for days and days and days at extremely high levels.

So the risk is very high, Mike. It does allow you, by force often, not necessarily by your own choice, to have to become an absolute expert on your own metabolism. What changes in insulin sensitivity? What's going to change your glucose levels? And now you're in total control over managing all of it — because not only do you have access to insulin that you have to administer to essentially survive, but now you're also in control through advanced monitoring, through awareness of continuous glucose monitoring, which, for most people with type 1 diabetes, they've probably had for over 10 years. And also complete control over how much insulin is being administered and how much at any particular moment. So you really have incredible depth of awareness over two of the most important molecules in all of metabolism at all times of every day.

Mike Haney: It is interesting that you mention insulin sensitivity, because I have to say, when I think about this, I think about the concept of it almost as a kind of mathematical equation. Your glucose goes to X, therefore you're going to need Y amount of insulin to sort of bring it down. But I guess it's true, right, that your insulin sensitivity — your body's ability to use insulin to deal with that glucose — changes throughout the day, changes based on different circumstances. So it's not just a math equation. You have to be aware of where is my body right now, what's it going to do with the insulin I give it, and then also kind of tweak the dose.

Andrew Koutnik: Accurate. So imagine most people in the audience could appreciate it because they've probably been told, hey, exercise improves your insulin sensitivity. What they may not appreciate is that the type, duration, and time of that exercise, including even the time of day, can all impact the insulin sensitivity response.

There are some times where I go and exercise — let's say I go do resistance exercise for 30 to 60 minutes — it will subtly improve my insulin sensitivity, maybe upwards of 25 to 50% throughout the rest of the day. But let's say I do a very intense form of resistance exercise or Brazilian jiu-jitsu, and that's incorporated with a very high heart rate for a sustained period of time, and I do that for at least 45 to 60 minutes, sometimes longer than that — the impact on my insulin sensitivity will be dramatically higher. And I will have to know that, Mike, because if I go consume food after my meal, the amount of insulin I would need to have given for that meal pre-exercise is now going to be about half post-exercise. And if you administer insulin at double the dose you actually need, you could have dramatic drops in blood glucose levels, reduce the amount of available metabolites for brain energy. And then in this case people can get into some serious trouble, and some patients end up in the hospital by not being able to accurately calculate just how much their insulin sensitivity does change to various aspects of their life.

How the body uses fuel: glucose, fat, ketones, and lactate

Mike Haney: Right. Well, I think that's a good kind of lead-in into this athletic context about fuel sources. And as you say, it gives you a kind of unique lens on this. I think before we get into some of your research, we should probably just level set with some kind of physiological baseline and some definitions. So I think people are probably familiar with the idea — they've heard that your body has two fuel sources, carbs and fat, and maybe if they follow a certain diet, they've also heard of ketones as a fuel source. So let's just start there. What do we mean when we say fuel sources? And is that an accurate description of what the fuel sources are in our body?

Andrew Koutnik: It is accurate to say that there are two forms of predominant fuel in the body, Mike — in the forms of glucose, which we would categorize as carbohydrates, although they come from other things too, like protein, and also can come a very small percentage, around 10%, from fat. Then you also have fat as a fuel source, which isn't inherently per se fat — it's actually the free fatty acids that are broken down from the triglyceride of a fat that ultimately provide energy in the form of fat.

But those aren't the only things that provide energy. As you mentioned, there's also ketone bodies. Those molecules can ultimately go to various tissues and provide energy, most predominantly the brain. But there's also things like lactate. Many people think of lactate as a byproduct of high-intensity forms of exercise that causes the muscles to burn — lactic acid — but actually lactate itself, independent of the acid component, is actually also a fuel for various tissues, and of particular note, the brain. So there's quite a number of different brain energy metabolites that are present and circulating within the body that all have relevance to overall performance.

Mike Haney: And without having to explain the full Krebs cycle — but when we talk about cells using these substrates as fuel, what's actually happening? How are our cells using, say, glucose versus fat versus lactate to produce — I assume everything's going to ATP, which we've heard is like the energy currency of the body — but they're getting there in kind of different ways.

Andrew Koutnik: Sure. So the body may take, let's say, glucose as a fuel, and if it wants to ultimately metabolize it, it's going to take it through one of these pathways. One of the ones you just mentioned is glycolysis into a Krebs cycle, which may then take it to something called the electron transport chain. Long story short, it can go through various pathways to ultimately create something called ATP, which is the currency of energy in the body. But fat will go through fatty acid oxidation and ultimately be broken down to intermediates as well that ultimately facilitate ATP production through the same kind of end-product ATP production pathway.

Now, that's how our body is mechanistically actually utilizing these fuels, but actually a number of other variables determine what fuel source you will predominantly utilize. One of those is the nutrition you eat. So if you eat a very high carbohydrate diet, your body's going to predominantly burn carbohydrates as fuel. However, if someone were to do a very low carbohydrate diet, or do exercise in a fasted state, their body will shift over to more of a fat-based predominant fuel, and they will rely more on fatty acids — predominantly your own fat storage, the fat you have stored on your body — as the energy.

Now, the body also, beyond nutrition — which is the most powerful way in which your body chooses what is the energy source — even as we're just sitting here at rest, our bodies are choosing how to ensure that my brain is functioning, how the muscles in my cheeks are moving and creating wavelengths of sound so that people can hear the words we're utilizing. But when we get up and move and exercise, the body starts to shift its priority in what fuel source it's going to utilize.

And we kind of view this as a spectrum. At rest, it was inherently believed that people are predominantly utilizing more fat. Then as the intensity of your activity increases, you'll start to shift more and more towards glucose-based fuel sources. And ultimately it was estimated that around 70% of someone's maximal aerobic capacity — imagine you're giving an all-out effort, you can think of that as 100% of maximal aerobic capacity — if someone does 70% of that, the amount of fuel from fat and carbs was estimated to be somewhat equivalent. Prior to that, at lower intensities, fat would predominate and carbs would be less predominant. But as you get beyond that 70%, carbohydrates would become the overwhelming form of fuel in the body and fat much less important — because of the lack of oxygen availability at higher intensities, glucose can be utilized as a fuel in these lower oxygen environments.

And think of — okay, me and you, Mike, we're going to go all out sprint. We're going to sit up from these chairs that we're sitting in, we're going to start walking. We start needing more oxygen for that extra energetic demand, because oxygen actually takes these fuels and turns them into ATP. Then we increase our intensity, we start running, and then, Mike, you place a bet on me and say, "Oh Andrew, I can run faster than you." I say, that's not true, Mike. And so we put it to the test. We start sprinting, we have an all-out max effort, we're going to sprint as hard as we possibly can for a sustained period of time. As we start moving from sitting to walking to running to running much faster to ultimately sprinting at max capacity, all these start shifting more and more towards glucose-based predominant fuel sources.

However, we know that the type of diet you consume can dramatically shift the point at which your body relies on any one of these fuel sources. We also know that individuals with some form of metabolic dysfunction actually don't burn a lot of fat at rest — they actually burn a lot of sugar at rest. In fact, this is a common metabolic phenotype of someone with metabolic disorders in the context of things like pre-diabetes or diabetes. And so what your body utilizes at any one moment is not only contingent on nutrition and exercise, particularly exercise intensity, but also your metabolic health status.

Mike Haney: I want to come back to maybe unpacking a little bit more of that shift that's happening throughout different intensities — and duration I think also plays a role here when we talk about exercise. But back to that point about switching fuel sources. So we talk a lot about metabolic flexibility, and that ideally you want your body to be very metabolically flexible and you want it to be able to switch between fuel sources easily. And I confess my understanding of that is about at that level — it's pretty hand-wavy. So when we talk about the body switching fuel sources, is "a switch" the right metaphor? Is it a kind of on and off, or at any given time are there certain tissues in my body always using fat and others are using glucose? What's actually happening in the body when we talk about this shift between fuel sources?

Andrew Koutnik: Yeah, so we've actually analyzed this in a number of different settings — athletes, non-athletes, rest all the way up to max effort sprint exercise. And what we see is that there's always a percentage of various fuels being utilized at any one particular moment. So even at rest, as we sit here, assuming you don't have any type of metabolic dysfunction, your body is going to predominantly utilize fat at this kind of seated rested state. However, you are still burning some level of glucose during that. So at all moments, you are still utilizing a percentage of all the fuel sources.

However, as you start shifting towards higher intensity exercise, you shift your diet towards carb predominant or fat predominant diet, or you become healthier, more fit, and lean — or you're less healthy, have more adipose tissue and you're not as active — these are all going to shift where the fuel is coming from. But no matter what, even in unhealthy individuals, it's not 100% glucose being utilized at rest or during activity. There's some amount of fat being utilized, just like in healthy, active individuals as they go — even at rest, they're not 100% using fat. There's always some percentage of carbohydrates. So it's always a spectrum that shifts as intensity, nutrition, or metabolic health status shifts.

Mike Haney: And are there tissue-specific dependencies there? Are some tissues more likely to be using one versus the other in terms of fuel sources?

Andrew Koutnik: Great question. Red blood cells are a primary example — often cited as one of the few sources that does 100% rely on glucose. There are other tissues that have similar dependency on certain substrates. However, the vast majority of tissues and cells are going to have the ability to utilize multiple forms of fuel, particularly if they do have the ability to process glucose and have mitochondria and the ability to actually take these various substrates and turn them into ATP through the electron transport chain.

Mike Haney: I think people may have heard before — I feel like I've read that the brain can only use glucose. But the brain also uses ketones, which are sort of a product of fat metabolism. Is that right? How is the brain taking fuel?

Andrew Koutnik: So the brain used to be believed to be 100% dependent on glucose. It was actually the 1960s and '70s that a prominent physician out of Harvard Medical School and Howard Hughes — a director called George Cahill — discovered that when individuals fasted, they become extremely reliant on fat. And as a result, they dramatically reduced the amount of dependency in the brain on glucose. But at that time, George did not know what was making up the other fuel sources in the brain, because they didn't know of another fuel source that could have reliably fueled the brain at that time.

So he actually stuck an actual catheter in the artery and vein that both perfused or provided nutrients to the brain, and also brought the byproducts of those tissues out of the brain. And what he found is that when individuals fasted and became predominantly reliant on fat as a fuel and reduced the amount of glucose available, the body produced a huge abundance of these molecules as a byproduct of fat metabolism called ketone bodies. And those ketone bodies made up around 60 to 70% of the brain's energy metabolism.

Now that was where it was first discovered, but now we know over time through more advanced analysis that just the presence of additional ketones or other molecules — just like lactate — are used preferentially by various tissues, including the brain, when they're present. Meaning that higher levels of ketones and higher levels of lactate in the blood means that the brain will utilize and break down more of those when they're available.

That's not the case for glucose. Glucose is only utilized in proportion to the amount of energetic demand of the tissue, of the brain. This is a very different thing. So even if you have hyperglycemia or very high levels of glucose in the blood, the body's not going to burn more glucose in the brain. But if you have very high levels of ketones and very high levels of lactate, the body's going to overwhelmingly utilize those instead of glucose as a fuel source.

This was a mechanism the body had developed, Mike, so that it would utilize those metabolites in the context of, let's say, famine. Okay, so let's say a thousand years ago we were traveling from someplace to another on foot. We didn't have fuel for a week and we're becoming completely reliant on our own fat stores as the predominant fuel for the body. We're also breaking down maybe a little bit of muscle as well. And the glucose is being liberated from other tissues to try to make up just the amount of glucose that is needed for things like red blood cells to still function, because they rely on glucose entirely.

Now as the body starts to produce and break down more fat and make more ketone bodies, the body knows that when ketone bodies are present, it should utilize those first. And a number of advanced analyses have shown that when you actually provide ketones and trace them with MRI analysis — so you scan where the ketones are going in the blood and simultaneously where glucose is going — you can see that as ketones become higher and higher in volume in the body, meaning more concentration, higher levels, the brain will break them down more and more. But when glucose is present at higher levels, it doesn't actually change anything about brain energy metabolism.

And this is a way of just understanding that the body has built mechanisms to preference fuels to protect blood glucose levels. Low ketone levels — there are no consequences that we know of for that. There might be some benefits to elevated levels, but there's no consequences to low ketone levels. There are fatal, life-threatening consequences to low glucose. There's also pathological consequences over time with high glucose levels. The body works to keep glucose levels in this very tight range, Mike. We have about five grams on average of glucose floating around in our blood — think one teaspoon of glucose floating around in our blood. And our body's working so hard to keep that in the tightest range possible. And so as a result, when we go through these bouts of famine, or we go through extreme exercise where the body starts to produce more lactate, the body's going to utilize that lactate — or in the case of caloric restriction, ketones first — so that it can protect and make sure that the brain has sufficient amounts of glucose, because something called hypoglycemia can ultimately produce fatal consequences if left unmanaged.

Metabolic flexibility, insulin, and what breaks the switch

Mike Haney: Right. And when we talk about the idea of switching fuel sources, or making your body efficient at doing that, what does it mean for a body to be able to more efficiently, or more healthily, or more easily switch between these fuel sources? What's driving the mechanism of choice there, and what does efficiency look like?

Andrew Koutnik: Sure. That's a great question, because I think most of the literature evolving this and how we come to appreciate that metabolic flexibility is an attribute of health is because we know that the absence of that occurs in disease. So in the example of things like obesity and ultimately diabetes, we see — as you start to build more and more fat tissue on the body, before you even see rising levels of glucose within the blood — the body becomes more dependent on glucose as a fuel source. The body becomes more resistant to other substrates like fat. And as a result, as it builds up more and more adipose tissue over time, the body starts accumulating damage through inflammation and oxidative stress to various tissues. And then ultimately that can whittle down to the level of the mitochondria as well, where the body starts to become almost reliant on glucose as a fuel source.

Now we see the exact opposite happen for individuals that are able to become lean and start exercising more — that they start utilizing more fat as a fuel source. And that's most apparent actually at rest or at lower levels of activity, because individuals with metabolic disease, even as they start to go from rest to walk to run, are still burning predominantly glucose throughout most of those physical activity intensities. Whereas someone with intact metabolic flexibility can switch between various fuel sources — let's say fat and carbohydrates — and is always utilizing some percentage of both. That is a representation that someone has the ability to leverage the fuel source that the body is most efficient at utilizing in that moment.

So for example, would it make sense at rest for someone to burn the limited fuel source we have in the form of glucose? No, it doesn't. It would make much more sense for us to leverage the fuel source that we have in what is, for all feasible purposes, reasonably unlimited form — in the fat that we have stored on our body. But when we lose that flexibility, that ability to tap into fat as a fuel source, we become very dependent on glucose and very rigid in what fuel sources we're using.

And we've seen that this could even occur in athletes who are healthy but consume extremely high levels of carbohydrates — we see that they also develop a similar phenotype where they also get locked into this dependency on glucose and can't tap into their own fat stores for energy during, let's say, a race.

Mike Haney: It's somewhat ironic and unintuitive that as you gain more fat tissue, as you get into obesity, that that actually makes your body rely more on carbs. Why wouldn't your body say, hey, look, we've got a lot of fat tissue here, let's burn it? Is the body just back to that sort of starvation response, that the body is programmed to just keep that fat tissue as much as it can?

Andrew Koutnik: There's a lot of reasons for that, Mike. But one of the best explanations that I think most people will appreciate is that one of the key drivers of it is that as you develop more adipose tissue — and less physical activity as well — the body starts increasing the levels of insulin in the body. And as insulin levels increase, you're not physically active, you have excess adipose tissue, you also start to release subtle levels of inflammation at above normal baseline levels. And that also starts to cause, amongst other factors, insulin resistance.

And so we know that individuals who develop obesity, even when they first develop it, can see reductions in insulin sensitivity around 33 to 36% across muscle tissue and in fact across the whole body. As they progress even further into obesity, before they even see rises in blood glucose levels in the form of pre-diabetes or diabetes, the levels of insulin can go from twofold higher to sixfold higher than normal. That insulin sensitivity will start to drop by 80 to 90% across all tissues in the body and particularly the muscle. And as a result, you become infinitely more insulin resistant, and then the levels of insulin within the body actually start to rise at even higher levels.

Insulin is the most powerful hormone in the body to block fat breakdown. So when it is present at higher levels within the blood, it binds the fat with high potency — so even small levels of insulin in the body can actually dramatically reduce the amount of fat your body can liberate from its fat storage to then be utilized for various tissues for energy. So as insulin levels start to rise in the body, so does the opposite happen with fat — the ability to break down and liberate fat starts to drop dramatically.

So the best way actually to understand this is through elevated insulin levels, because it's very hard to be glucose predominant at very low levels of insulin. In fact, low levels of insulin are one of the key ways in which the body knows, as a signal, that it needs to shift towards higher levels of fat breakdown as a fuel.

Mike Haney: Okay, that makes sense. Last question on this sort of metabolic flexibility — what is the mechanism that is telling our body to switch? Is this a hormonal signal coming out of some part of the brain that's telling the cells what to do?

Andrew Koutnik: It still comes back down to insulin, because insulin initiates the signals that ultimately stop certain enzymatic pathways. So when insulin's higher, your body will stop enzymatic processes or enzyme production for lipolysis, which is the process that breaks down fat tissue. It'll also go to the liver and lower key enzymes for fatty acid oxidation and also ketogenesis. So the presence of insulin at these tissues binds the tissues and actually stops the production and presence of key enzymes that allow for the process of fat breakdown, that allow for the process of ketone production, and also allow for the process of glucose to be broken down and released into the blood to provide energy as well. So it's actually the insulin that's signaling enzymatic changes within these tissues — where those enzymes were once facilitating processes and now insulin is actually working to inhibit them.

Mike Haney: Right. So that makes sense. It sort of closes the circle on why metabolic flexibility and insulin sensitivity are so related — that if your body is efficient and good at using insulin, it's not running into this insulin-resistant state where it's having to pump out more and more. That allows your body to be more flexible. The less good at using insulin your body gets, the more muddied this metabolic flexibility gets and the worse your body gets at sorting switching fuel sources. Is that right?

Andrew Koutnik: That's a great way of generally describing what's going on in the body. And it's also important to appreciate for our audience — because often you can use extremes as examples, but most of the major issues we see in America with chronic metabolic health conditions — in fact, we know that up to 9 out of 10 Americans, or 88% in one study and 93% in another study, are showing clear biological signs of metabolic dysfunction. Meaning things like elevated waistline, elevated blood glucose levels, elevated triglycerides. And insulin can directly, causally change some of these values. So we know that the overabundance of insulin in the body can be one of the key drivers, and in some cases causally inducing some of these metabolic changes that we see happening in the overwhelming majority of individuals in the developed world, leading to a whole host of metabolic and chronic diseases.

It's not the only thing involved, but it's one of the most important drivers because it is the master regulator of metabolism. It's also the master regulator of whether you're going to break down or store fat tissue, whether you're going to facilitate muscle growth or break it down. So it's a critical molecule and hormone across various tissues that's telling the body what to do or what not to do. And when it becomes higher at pathological levels over extended periods of time, is where we see clear signs of metabolic health decline leading to, in many cases, disease.

The traditional sports nutrition story — and why it may be wrong

Mike Haney: Right. Well, let's shift now, with that kind of baseline in mind, into the athletic context. So what's the traditional story about how the body is using these combinations of fuels when people are exercising? And I imagine we'll have to talk about differences in intensity and maybe differences in duration.

Andrew Koutnik: Sure. So I can actually tell that from a historical perspective, Mike, that I think more people will appreciate. Back in the 1960s there was a famous physician called Jonas Bergström. And Jonas Bergström described the ability to actually stick a syringe into the muscle and pull up muscle tissue. What Bergström and his colleagues discovered was that glucose was stored in the muscle as something called glycogen. And what they also found is that with higher levels of glycogen you would be able to perform exercise for longer.

Now we also found, in the 1970s and 1980s, that as individuals were utilizing techniques in these laboratory settings where they actually were able to capture the amount of oxygen you breathe in and the amount of carbon dioxide you breathe out — so every time you breathe in, you're breathing in oxygen, and every time you breathe out, carbon dioxide goes out — the ratio of oxygen consumed to carbon dioxide released is an indicator of the amount of carbohydrates or fat that your body is burning on the whole.

And back in the 1970s, some famous researchers discovered that as individuals are exercising, the higher levels of glucose that they're burning at various levels of exercise intensity, the longer they're able to perform these types of exercise. And those two key pivotal moments led to a key understanding that happened in the 1980s and 1990s — this concept called the crossover point — where as athletes specifically, when they're at low intensities of exercise and they start to increase their levels of intensity, they will shift from utilizing fat as a fuel to more of carbohydrate as a fuel source. Then as you get high enough in intensity, you become almost 100% reliant on carbohydrates as a fuel.

And this really led to what we now appreciate as the sports nutrition guidelines, which is — to your question, how should someone fuel for exercise, what is important for fueling exercise — well, these studies that I talk about in the 1960s, '70s, '80s, and '90s all led to sports nutrition guidelines, which have persisted with the same kind of thinking since that time point, that you as an athlete should consume somewhere between 5 to 12 grams of carbohydrates for every kilogram of body weight every day. So if you're an average body weight female and you're an athlete performing various levels of exercise intensity, that's going to be anywhere from around 350 to over 900 grams of carbohydrates a day. Whereas if you're an average body weight male doing the same thing, that's going to be anywhere between 450 and over 1,000 grams of carbohydrates per day.

Those are the current sports nutrition guidelines from the American College of Sports Medicine, the Academy of Dietetics in America, dietitians in Canada, the International Society of Sports Nutrition, and the Gatorade Sports Science Institute. So they all recommend extremely high levels of carbohydrate based on this thinking, Mike — that you need to utilize carbohydrates to replenish the muscle glycogen and burn more carbohydrates during exercise, as it's believed to be essential or critical to performance.

But what we have found over the last hundred years, and also in a number of key randomized controlled trials, is that that isn't necessarily true.

Mike Haney: Yeah, let's get into that then. Because some of your recent work — both trials that you've done, but also this most recent paper that came out about a week ago from when we're recording here, which was actually a review of other studies, a massive review of other studies — is really looking at and questioning that idea that your reserve of muscle glycogen is the thing that is limiting your performance. So maybe talk about what you've found there and why that may not be the case.

Andrew Koutnik: Sure. And I think the best way to describe that is to illustrate a couple of key studies and then ultimately what we found, having been looking at over a hundred years of evidence in 160 different sports performance studies.

So around five years ago we initiated a study. There had been this belief, as we have talked about, that carbohydrates were essential to sports performance. Why? Because you need to be able to fill muscle glycogen, you need to be able to carb load, and you want to burn more carbohydrates during exercise, particularly as you get to higher intensity exercise. But here's the problem, Mike — in science you always test your own assumptions. And so while this was a major assumption, no one had really controlled many of the key variables of performance and actually asked the question: are carbohydrates essential?

And so what we did is we asked athletes who were competitive in their local or regional running — so they're middle-aged athletes who were doing a lot of running, very fit, high VO2 maxes, the epitome of health by every metric that you could visibly see and by their lifestyle — we ask them, hey, we want you to do two types of diet. We want you to do a high carbohydrate diet, which is based on the sports nutrition guidelines, and we also want you to do this extremely low form of, uh, ketogenic diet — very low carbohydrate — because we want to test this idea of whether a very low carbohydrate diet is limiting for these high-intensity forms of performance.

Because based on that crossover effect we're talking about, as you get above 85% of your VO2 max — actually above 70% carbs become increasingly important — and when you get to about 85% of your VO2 max, or your maximal aerobic capacity, you are almost 100% reliant on carbohydrates for fuel according to this thinking.

So we tested that. What we did that was different than all these other prior studies, though, is that we controlled athletes' activity, we controlled their calories, we controlled their changes in body weight. We monitored that they were actually adhering to the diet through continuous glucose monitoring and through measuring ketone levels. Why was that important, Mike? Well, every major study that had led to these major assumptions of what we have to utilize for fuel had many confounders. Many of them didn't control calories, many of them increased activity load while doing the intervention itself, many of them didn't control the changes in body weight in these athletes. And more importantly than any of that, they didn't allow the athletes to be on the diet for sufficiently long.

There are a number of studies that have shown that short-duration diet trials — where you're only on a diet for less than four weeks in duration — don't allow for the full adaptation across various key tissues to things like a ketogenic diet. We have known that for actually over half a century since the 1960s and '70s. We've known that if you were to force the most extreme form of rapid adaptation in the form of fasting, that key brain energy metabolites don't normalize, Mike, until after three weeks. So any study that's less than four weeks in duration, and particularly one requiring fat adaptation or more fat utilization — we knew that longer durations were important, but all these other studies failed to do this.

So we controlled all these key confounders, because in the real world, Mike, you may talk to your friend and say, hey, what should I eat to perform? And they say, oh, you have to do this specific diet. And maybe for them that diet does work. But the truth is, science in a controlled setting allows us to actually tease that apart. It allows us to remove all the other factors in someone's life, their confounding variables, like their genetics, their environment, how much activity they're doing, whether their body weight is changing, whether they're changing their caloric load. All those things can impact performance, they can impact metabolism. And so what we wanted to do is control all of those variables and allow them to adapt to the diet for at least four weeks in duration.

And when we did that, Mike, and we tested very high carb intake versus very low carbohydrate intake, we found that when athletes were performing six by 800 meter sprints, a one-mile time trial — meaning not just a light jog, a max-effort one-miler, one of the most intense and grueling forms of sustained effort of four to six minutes in duration — it's brutal, absolutely brutal, very high intensity — what we found is that athletes on the ketogenic diet saw no impairment in performance. And they were exercising at over 85% of their VO2 max, when carbohydrates were believed to be essential for performance. Because carbohydrates, when you're at 85% of your VO2 max, based on every exercise science textbook that we know of, would say you would be reliant almost entirely on carbohydrates.

But what we found is that not only were they performing the same, but they were burning the highest levels of fat ever recorded in the scientific literature. In fact, some individuals were north of 1.8 grams of fat burned every minute. A huge amount of fat burning was occurring despite these extremely high intensities and despite the assumption that carbohydrates were required to perform at these higher intensities.

So this really shocked our foundation, because every exercise science textbook that we know of would say this was not possible. But when we actually tested these assumptions, the assumptions were not true.

Then we thought, okay, that's interesting — because that shows that on short duration, high intensity, maybe our assumption of how the body is fueling is inherently flawed, or we need to reevaluate and provide more nuance to it. So then we asked the question on the other extreme end of the spectrum. Obviously you don't need carbohydrates to fuel at higher intensities for sustained periods of time. What if you were to do a prolonged, strenuous form of exercise when you may need extra glycogen stores, when you'd also want to burn higher levels of carbohydrate to be more efficient according to many sports science publications and scientists themselves?

And so we did the opposite. We recruited Ironman competitors to come in and again adhere to the diet for at least four weeks — actually six weeks in this case. And we asked them to go as long as they could at a prolonged strenuous exercise at 70% of their maximal aerobic capacity until they hit the wall. Meaning — I'm sure every athlete has heard of bonking or hitting the wall — we had these athletes go until they hit that point, until they could no longer sustain their energy output or until they got some critical biomarker indicating early onset fatigue.

And so when we ran this trial, we assumed that we would of course see some potential deterioration in performance, although I had some speculation that maybe in the ketogenic diet, if you have abundance of fat, maybe the opposite might occur. But yet again, what we found is that these athletes, when we removed all these confounder variables that happen in the real world and just isolated out: when you shift the fuel from carbohydrates to fat, what happens — well, yet again, the athletes performed identically.

Which meant that even if you're doing a short duration, high intensity form of exercise where carbohydrates were thought to be essential to performance, or a prolonged strenuous form of exercise where we're often told through sports nutrition guidelines to consume anywhere between 60 to 90 grams of carbohydrates per hour — we found that by doing virtually no carbohydrates, athletes were performing just as well as the athletes who were consuming very high levels of carbohydrates.

"Every exercise science textbook that we know of would say this was not possible. But when we actually tested these assumptions, the assumptions were not true." — Dr. Andrew Koutnik

The pre-diabetes finding — and what 100 years of evidence shows

Andrew Koutnik: Now, what really shocked us though, Mike, is that we were seeing — in the original study — that these fit, lean, high VO2 max individuals, when we monitored their glucose levels every five minutes of every day over multiple weeks and also did calibration to ensure that these glucose levels were accurate, we were finding that about 30% of these athletes on the high carbohydrate sports nutrition guidelines were developing glucose levels consistent with pre-diabetes.

Now that was a shock. Because you have someone who comes in who's lean, they're doing all the things they're told to do to be the epitome of health, they're doing high volumes of exercise in alignment with that — but yet they're getting signs of metabolic dysfunction. And that deeply concerned us, because if those individuals are developing pre-diabetes, it's no wonder that over 50% of the US population has pre-diabetes.

The reason we know it was due to nutritional intake, Mike, is because when we put them on the ketogenic or low carbohydrate diet, every single one of the athletes had a drop in blood glucose levels. And every single one of those athletes with glucose levels consistent with pre-diabetes immediately reversed those levels of glucose — they immediately dropped into the normal range, and it immediately went away. And they maintained their performance and also started leveraging higher levels of fat for fuel instead of being highly predominantly reliant on glucose as a fuel source during their exercise.

And so this really shocked us, because this goes against much of the sports nutrition guidelines. It goes against much of the dogma that is placed in these textbooks, that is taught to students — including myself — on how you must fuel to perform. But what we are also seeing, Mike, is that there might be an actual health cost for following some of the guidelines that are argued to be essential to physical activity.

And so instead of assuming that our two studies represent what's happening for all — we did do rigorously randomized controlled trials, crossover analysis, we controlled all the key variables unlike many of the major studies done prior — we don't want to just assume that our results represent what's happening for all. And so we went back over a hundred years, over a five-year period, our team went back and looked at a hundred years of evidence on how modulating your nutrition, particularly carbohydrates in the diet, affected not only your metabolism but your sports performance. And that actually involved over 600 research analyses over 160 different sports nutrition trials.

And the findings of that really, I believe, make us rethink how athletes should fuel — not just for performance, but also potentially for health.

Mike Haney: So help me understand that then physiologically. And I'm glad you brought up the short duration one, because the long duration study was the one I read first, and that sort of made sense to me. I also interviewed a few years ago a guy named Mike McKnight, who's a low-carb ultra endurance athlete — he was doing a fastest known time across Arizona, I think at the time, on a very low carb diet. And I know there's this kind of sub-community of very low carb ultra runner people. So I could kind of understand the physiology of the endurance side of it — that you're just training your body to basically have a different shift point and to be able to use fat for longer before it relies on that, if your energy expenditure is staying in that kind of 70% range.

But on those short duration, really high intensity — when you're getting up to that 90%, 100% of your intensity range — that would seem to just violate the physiological processes that we just talked about, meaning that glucose has to be burned in the environment of low oxygen, and fat is better when you've got more oxygen, because of the way those are burned. So what's actually happening physiologically in those high-intensity trials when people aren't burning as much glucose as we think they should be?

Andrew Koutnik: Sure. So what's shifting is that it's not 100% fat that they're utilizing here, Mike. What's happening is actually what we talked about before, when we talk about metabolic flexibility. The athletes on this ketogenic diet actually at these higher intensities were burning more of a mixed form of fuel — meaning around 55% of it was from fat and around 45% of it was from carbohydrates and/or glucose. So they were actually leveraging both forms of fuel.

And what it was showing us is that all these assumptions around carbohydrates being better for performance and being more efficient per unit of oxygen consumed — that alone obviously does not explain why athletes are able to perform at these higher levels. There are a lot of assumptions around what actually leads to higher performance, Mike, and those assumptions are starting to be unraveled.

And I will acknowledge — we don't actually understand exactly in all ways how athletes are adapting, let's say at the muscle level, how muscle glycogen levels relate to how we see fat being utilized and maybe some enzymatic changes. We actually don't know all the reasons why someone who becomes more predominantly utilizing fat through nutritional change is in fact able to sustain the same level of energy as someone who is overwhelmingly reliant on glucose as a form of fuel. Because the assumptions were: muscle glycogen levels are essential to performance, carbohydrate oxidation levels are critical because they're a more efficient fuel source and can be utilized at higher intensities. Well, they obviously failed to be confirmed when we ran randomized rigorous controlled trials that controlled other variables, so we could isolate out whether that effect was in fact real.

And so we don't actually know all the reasons at this point, Mike, but what we do know is that fat can be utilized as a predominant fuel even at these higher intensities, at least up to the levels we've tested — which is around 86% of someone's VO2 max. Which when you think about it, that is virtually every major predominant endurance event at any major high intensity, at least sufficiently long.

But you ask the question, what about 90 to 100%? Well, we have done studies where we looked at athletes who are on a ketogenic or low carbohydrate diet, compared to a high carb diet, and see where they start to shift over — their new crossover point, when they would become more reliant on glucose. Because no matter what, even if you do shift your fuel on a low carbohydrate diet, we still know that there is a point at which oxygen availability becomes so low that carbohydrates become much more essential to just sustaining energy. Simply because low oxygen environments don't allow for fatty acids to be utilized as fuel — they require oxygen to be burned at the electron transport chain.

So as a result, what happens at higher intensities? Well, there have actually been meta-analyses that have looked at athletes who are consuming various amounts of carbohydrates before resistance exercise, because resistance exercise has periods of low intensity and then very, very high intensity, arguably pushing 90 to 100% depending on how intense it could be. And they find that carbohydrate intake doesn't really seem to benefit resistance exercise athletes unless their training sessions are greater than 45 minutes in duration and also are sufficiently strenuous — like a leg exercise such as squats, deadlifts, or other forms of resistance exercise that are more energetically costly.

And so what we're consistently seeing here is that the prevailing belief that athletes have to fuel with higher carbohydrate intake is clearly not consistent with the weight of all the evidence, once athletes adapt to any diet of their choosing for sufficiently long. And this is really important, Mike, because it illustrates that athletes have a choice in what they fuel with, whereas before — in fact, still prevailing today, if you look at sports nutrition guidelines — you would come to the belief that there's only one way to perform, and that one way requires carbohydrates at very high levels. But what we're seeing is that's not the case, that athletes in fact have a choice and should be informed about that choice.

Mike Haney: So when you ran these trials — we've been talking about sort of a low carb diet versus a high carb diet, or a keto diet versus a higher carb diet — there's sort of what you're eating in your normal day-to-day life as you're training, leading up to a race maybe, and then there's what you're actually consuming during the race. So what were these folks, when they were actually doing either the longer term effort or the shorter term effort, taking in as fuel during the effort?

Andrew Koutnik: Great question. So we actually did this when we looked at the Ironman competitors. Not only do we compare low carb versus high carb, but we also wanted to ask a different question. We also wanted to ask, well, what happens if we trickle in just enough glucose during the exercise to offset drops in blood glucose in the circulation?

And you may ask, Mike, why is that important? Well, we know that when we're doing the analysis and comparing high carbohydrate versus low carbohydrate, that reliably produces two distinct metabolic environments. High carbohydrate means you're going to burn lots of carbohydrates, so you're going to oxidize more carbohydrates. You're also going to have higher muscle glycogen levels. Whereas on a ketogenic, low carbohydrate diet, at least within the first three months on the diet, you're going to have lower carbohydrate oxidation or reliance, you're going to increase your fat burning, and you're going to have at least some marginal reductions in muscle glycogen.

So that was the key test — more of a physiologic test of how important is muscle glycogen, how important is carbohydrate oxidation in these environments. And what we're seeing is that when you create a model where you actually test whether low carbohydrate oxidation is better than high carbohydrate oxidation, or low muscle glycogen is better than high muscle glycogen, you do not reliably see a difference in performance. Which made us question all these assumptions back to the 1960s and '70s that muscle glycogen or carbohydrate oxidation were essential to performance.

But there had been studies since the 1920s, Mike. In the Boston Marathon, athletes were running through, and two physicians were observing these athletes. Levins was one of the physician's names — I am actually failing to think of the other. In 1925 and 1926, they published their work in the Journal of the American Medical Association. And when these athletes during the Boston Marathon in 1925 would run through, some wouldn't even finish, some would become pale, some would become ataxic — their pattern of movement, their body — they would become shaky, they couldn't coherently say words, they would say slurred words. And what they were finding when they analyzed the blood of these athletes, which had been known for over a century at this point, is that athletes were seeing low blood glucose levels — a drop in one of the most important metabolites in the blood that fuels the brain and the muscles for energy.

Now, we were also finding in our analysis when we tested Ironman competitors — we said, okay, we have these two models, the high carb diet and the low carb diet, and we're seeing that the amount of sugar or carbs someone burns versus the amount of fat that they burn isn't the reason they're going to perform better or worse. And the amount of muscle glycogen they have isn't going to predict whether they're better or worse, at least in these analyses. But we're going to give them just enough glucose on either of these diets to improve blood glucose levels — which was 10 grams per hour. Now keep in mind, Mike, sports nutrition guidelines recommend that the average weekend warrior consume 30 to 60 grams per hour. But serious athletes doing higher intensities for extended periods of time are told to do 60 to 90 grams per hour. So we were doing six to nine times lower than what Ironman competitors are typically recommended to consume.

What we found is that just giving them 10 grams per hour — the equivalent of one tablespoon of sugar every hour, that's it — we found that they improved their performance 22% and completely eliminated hypoglycemia. So the reduction in glucose in the blood.

What we were observing in more general terms is that the athlete's ability to sustain their brain energy — by maintaining key metabolites that sustain energy for the brain in the form of glucose, by just giving enough glucose to maintain that — they were able to dramatically improve performance. Not just on high carb athletes, Mike, but also on ketogenic low carb athletes. That's so important to appreciate, because it illustrates a key physiologic understanding that anytime someone's exercising for prolonged periods of time, it isn't always reliable to assume that higher muscle glycogen means higher performance or that higher carbohydrate oxidation means higher performance.

But we have known for over a hundred years that sustaining brain energy is critical to performance. And the amount of glucose required to do that is actually dramatically lower than is recommended in sports nutrition guidelines. What we've seen in another study back almost 30 or 40 years ago has also shown that around 10 grams per hour during exercise is sufficient to maintain or improve performance and eliminate hypoglycemia.

And why that's so important is because when we were looking at over a hundred years of evidence, over 160 different sports nutrition trials that actually administered carbohydrates, what we found is that when carbohydrates improved performance across those 160 different studies, 88% of all those studies showed that not necessarily the group that received carbohydrates saw a change — it was actually the placebo group or the control group who saw drops in blood glucose levels that compromised their brain energy. That was a key finding in this review that we recently published that illustrated that the predominant key driver of sports performance is the sustainment of brain energy metabolism. In the lack of its ability to be sustained, the body will start to shut down key functions that are stripping away and burning glucose, because it's trying to protect its own glucose pool.

So in our analysis, we administered just 10 grams per hour, and we saw a 22% improvement in performance. Now, someone may ask the question, well, what about if you gave 30, 60, 90, 120? Or in some cases there are anecdotal reports that athletes are consuming upwards of 200 grams of carbohydrates per hour on Tour de France events or rides. And we've actually analyzed this. We've looked at all the studies that did dose response analysis, and what we've observed is that the vast majority of these studies show that some level of carbohydrates — whether they did 30 grams or 50 grams — improved performance if the exercise duration was sufficiently long to induce drops in blood glucose levels. But there was no inherent consistent dose response.

Now for your audience, what does dose response mean? If we think that carbohydrates are inherently improving performance, it would be believed that higher must be better. So as you increase them step by step — let's say from zero to 10, 10 to 30, 30 to 60, 60 to 90, 90 to 120 — you would see a consistent improvement in performance across studies. Well, guess what, Mike? We don't. In fact, there are some studies that actually show that once athletes start crossing beyond a certain threshold — let's say, there's an article called Smith et al. that we showed in the publication and the table — where once you get beyond 80 grams per day, athletes start to see a drop in their performance with higher carbohydrate intake.

Why is that? Well, there might be a number of reasons for it, but we also know that when athletes start pushing extremely high levels of carbohydrate intake, there are also intolerance issues. There are side effects to these levels. Keep in mind, 90 to 120 grams per hour is equivalent to eating an entire loaf of bread every hour on a bike. That's what you're doing. That's what these athletes are being told they have to do to perform well. And they're also doing it with arguably the most rapid induced form of glucose in the body.

If we think about this from a health perspective, we would think, okay, you would never want to do that. That's the opposite of what we consider healthy forms of carbohydrates — in fact, they're the polar opposite. They're rapidly absorbable forms of glucose that spike insulin and other things, but the athletes are consuming that at extremely high levels throughout hours and hours of rides. That's what's being recommended to these athletes.

But when we actually control all the variables, what do we see? Because one thing that's always brought up is there are many elite athletes and coaches who ask the question, well, that can't be true, Andrew. We see consistently in our athletes that when they went from, let's say, 60 grams per day to 90 to 120 grams, they got better. And I would say, okay, that's amazing, I'm glad they got better. Was it the new super shoes that are on the market? Was it the new training program you implemented? Was it the new monitoring assessments you have with these new wearables? Was it your new lifestyle? Was it the new environment? Was it your new recovery program? How do we know it was any one of those variables, or how do we know it was the nutrition?

Well, in order to understand if it is in fact the nutrition, Mike, we run controlled laboratory science experiments that remove all those other confounding variables to isolate out the impact of how much carbohydrates contribute to performance in a dose-dependent manner. And we reliably do not see a consistent signal that higher carbohydrate produces higher performance in these athletes.

There is a certain level that does produce performance — that is very clear. Some, over prolonged periods of strenuous exercise, is helpful. However, the dose is in question, because there are many analyses that show no benefits going from 30 grams up to 80 grams. And there are some analyses that show that over 80 grams actually deteriorates performance. But yet most elite athletes are pushed to go upwards of 90 to 120 grams per hour because, look, that's what the guy over here is doing and he's winning.

But yet we've known of Olympic gold medalists, we've known of world record holders, endurance athletes — and of many different athletes in these sports, marathon runners who are elite, who are doing various diets, ketogenic diets, high carb diets, some consuming no carbohydrates during their sports performance, some consuming astronomical amounts of carbohydrates during performance.

What we're seeing is that when we actually do the science and evaluate the science — which we did, and look back to over a hundred years of evidence and over 600 citations and research studies — what we're seeing is that, number one, carbohydrates may be important for some athletes, but there are a lot of athletes for whom it may not be needed at all. In fact, it's very clear that on average, people can do whatever diet they so choose and perform well. And it's also very clear that the amount of carbohydrates currently being recommended is highly in question at these very high levels, because we're not consistently seeing a dose response — we're not consistently seeing studies that illustrate a clear and consistent dose response in these athletes.

So what does that tell me, Mike? It tells me that when you're an athlete, it's very clear in our analysis and otherwise that there's inter-individual variability — meaning that some individuals who are on the ketogenic diets in our study perform better, some athletes perform worse. And so what does that tell us? That tells us that for some athletes, they're going to perform better on these ketogenic diets, and some athletes are going to perform better on a high carb diet. But the problem is that the sports nutrition guidelines right now make it clear that athletes should only do one form of diet. In fact, it completely eliminates their ability to even try, because they will never know, because they're being told that if they do this, they're going to impair their performance.

And we all know athletes — they don't want to see any impairment in performance. And we also see that athletes are pushing astronomical levels of carbohydrates, which again are not consistently shown to improve performance. But you can test this on yourself. You can test to see whether a change from zero to 10 grams makes a difference in your performance. If it doesn't, you're probably not going to see much of a difference at 30 grams or 60 grams or 90 grams. But let's say you do see an improvement at 10 grams — okay, well then try 30 grams. Does it make it better? Then try 60 grams. Does it make it better? Until you find when it's no longer producing any beneficial return.

I've had a number of conversations with elite level athletes about this exact thing and how they've done this over time. And what you find is that there's a lot of variability in any one particular athlete in what they do to fuel from their diet, but also in how they fuel during exercise, with a large focus particularly around carbohydrates. And there's tremendous variability, which illustrates to me that athletes have to figure out what works for them, but also appreciate there isn't a one-size-fits-all approach — which is what is currently being recommended by not just the sports nutrition guidelines, but also the sports nutrition industry that markets these extremely high levels of carbohydrate intake.

"Some individuals who are on the ketogenic diets in our study perform better. Some athletes perform worse. That tells us that for some athletes, they're going to perform better on these ketogenic diets, and some athletes are going to perform better on a high carb diet." — Dr. Andrew Koutnik

What the body is actually doing with all that glucose during exercise

Mike Haney: So it — the finding there makes more sense to me for the fat-adapted folks, right? That in keeping with the story we've been telling, if you've trained your body to just be better at using fat for fuel, that giving yourself a whole bunch of extra glucose during your activity is not necessarily helpful because your body's burning fat and it's perfectly happy doing that.

It seems like a couple of the lessons that have come out of your work: one, we are less reliant on muscle glycogen stores than we think we are in terms of duration, in terms of hitting that wall — that when you hit the wall, it's not because, as the traditional story might go, you have depleted your muscle glycogen stores. I think I've read that in any given study, there's still at least 50% of muscle glycogen stores remaining when somebody hits the wall. The other idea is that blood glucose levels do matter — so exercise-induced hypoglycemia, if your blood sugar goes low, again, no matter what type of athlete you are, that's going to send a signal from your brain: hey, things are going crazy here, we need to slow down, we need to stop. And that may be what makes you hit the wall.

But I think the other place that people think about carbs as fuel during athletics — and the reason people think, oh, I have to take in a lot of carbs — is this idea that, well, I'm running a marathon, I'm working pretty hard, I'm out here for a few hours doing it, I'm going to take in these carbs and my body's going to immediately burn those. So it's not even about muscle glycogen, it's not about trying to refill that, it's not about my blood sugar levels — it's just this idea that I'm going to eat the carbs, my body needs carbs when I'm working hard, it will burn them immediately, and so the more I take in, obviously to a point, the better.

What's wrong with that sort of story? When we see that taking in these extra carbs — 60 grams, 90 grams — isn't improving performance, is it that the body, people's bodies, again no matter which kind of athlete they are, fat adapted or not, is just not actually taking in that glucose and immediately burning it? What's going on, to understand why that extra glucose isn't helping drive better performance?

Andrew Koutnik: So there's a figure in our review in Endocrine Reviews — it's figure 19 — that answers this exact question, actually, under evidence points 9 and 10. So what happens is that when athletes are increasing their carbohydrate intake, the body is seeing this carbs turned into glucose, right? So the higher the carbohydrates consumed, the more glucose that's being released into the bloodstream. The higher the level of glucose, the more insulin that's being released. We talked about this before, Mike. The higher the insulin, the more it'll potently bind to fat. And what do we say it does? We said it blocks fat breakdown.

Well, what happens when this huge sink of fuel is blocked — you're blocking that fat from being utilized as a fuel source. The body during exercise has to leverage some form of fuel. Well, what does it have available to it? It has muscle glycogen and it has circulating blood glucose. So what we see is that when athletes consume high volumes of carbohydrates during exercise, they paradoxically accelerate muscle glycogen breakdown. They paradoxically block the other fuel sources that the body can release and utilize for fuel, and force a huge glucose and sugar reliance during their exercise, and accelerate muscle glycogen breakdown — because it has to use some form of fuel. And the fuel it can use when insulin levels are high, in the context of high carbohydrate intake, is muscle glycogen.

So what happens if you do the opposite? Let's say you have dramatically lower levels of carbohydrates — the body, and we've shown this, at even matched levels of energetic output, where you match the energetic output of the exercise intensity, when you just reduce the amount of carbohydrates being administered, the body just utilizes its own fat source.

So I'm going to give you a distinct example. When athletes are consuming, let's say, 120 grams per hour — this is what's happening in some elite performance endurance sports environments — what happens at that level is the body spikes insulin high. You see that over 85% of the energy is going to come from muscle glycogen and circulating blood glucose levels. And we also see that less than 15% of the fuel, around 10% of total fuel, comes from fat. For over a three-hour ride, Mike, at 70% of someone's VO2 max, it is essentially almost completely reliant on muscle glycogen being accelerated and broken down in order to sustain that performance.

But let's say you dramatically reduce the carbohydrates — not even to zero, but just to 25 grams — okay? You see over that exercise that the athlete's reliance on muscle glycogen goes to less than 50%, and that over 50% becomes fat as the fuel during that exercise ride. And so the body is just utilizing the fuel it already has stored — which makes total sense. The body is constantly taking in food and preparing to utilize that stored fuel for energy. We would have never survived as a species if the only way we could have functioned physically is if we ate throughout the entire physical activity endeavor. We would have never survived as a species if that was the case.

It's only in today's world, Mike, that we have access to nutrients at near unlimited availability, and we now are starting to push these very high levels of nutrient intake during exercise. But the body wasn't even set up to do that. What it's set up to do is leverage the fuel that you gave it previously for future physical demands.

And so what we see happen in layman's terms is: the body just shifts where the fuel's coming from. It shifts towards high glucose reliance, breaking it down, accelerated muscle glycogen breakdown, when you consume very high levels of glucose in the form of gels, sports drinks, et cetera, during your exercise. When you dramatically reduce any of those forms of carbohydrates, your body's just using the endogenous stored fuel that it always had all along.

Mike Haney: Okay, so you're doing your endurance event and you're taking in these very high levels of glucose — and this sort of gets at a point that I think you make in the paper, which is glucose as more of a signaling molecule, I guess would be the way to describe it. That we think of it as purely a fuel source, but we maybe ignore that the glucose coming in is also then sending signals, right? The brain sees all the glucose coming in, it's spiking the insulin, and that's having these sort of downstream effects that are interfering with the process that we are trying to facilitate — which is, I'm taking more glucose, therefore body burn more glucose, and keep me fueled for a long time without having to get into that glycogen. But the response for the body to taking in those high levels of glucose and then spiking that insulin is to actually burn that glycogen faster than it otherwise would at a kind of lower intake of glucose. So maybe talk about that idea of glucose as a kind of signaling molecule, and how that can lead to the exercise-induced hypoglycemia or the kind of crashing sooner than you would think given those carb intakes.

Andrew Koutnik: Sure. So what you're describing is two examples where if you have very high levels of glucose in the blood, that signals the pancreas to release high levels of insulin, and then insulin has this cascade of effects all over virtually every tissue and the metabolic status of what fuel that tissue is going to burn.

Now, what happens in the opposite? What happens when glucose starts to precipitously drop over time and starts to reach these low levels in the blood? Well, that triggers a signal in the brain. The brain sees that as a stress response because it now knows that it's going to be facing glycopenia — a reduction in glucose available to the brain to sustain brain energy metabolism. That sets off a cascade of stress-related signals. Things like glucagon start to elevate. Glucagon is a stress-related hormone that goes to the liver to then tell the liver: hey, take all that stored glycogen you have, break it down immediately and start releasing it to restore the amount of glucose in the blood to normal.

That's one hormone. We also know it releases adrenaline — epinephrine, norepinephrine — the stress-related hormone we see in fight-or-flight response. So you know, you're going up for a public speech, you feel nervous, you feel jittery — that same molecule is also being released because it also helps facilitate the breakdown of glucose in the liver as well, and also sends a signal from the brain to the liver to actually cause glucose production. So not only are you now causing glucose to be broken down and released into the blood, but there's also emergent evidence that came out over the last three to four months, Mike, that also shows that some of these stress-related signals also cause the brain to signal to produce more glucose — to take more nutrients out of the blood or break down other tissues if needed to, and ultimately facilitate glucose production as well.

So the brain is sensing this signal of low glucose and sickling what we call a counter-regulatory neuroendocrine response. Well established and known for years, how it works — low blood glucose changes the neurological signal that then signals down to the adrenal glands to release adrenaline, also causes a stress-related hormone release in the form of cortisol, induces glucagon elevation. Glucagon is a hormone — neuroendocrinology is the study of hormones — and as a result, that hormone now causes glucose to be elevated.

So that's kind of getting into the nitty-gritty weeds. But in short, the way to describe this is: when glucose goes low, the brain is doing everything it can to protect itself and survive, because in the absence of glucose in the brain, you'll die. So your body is doing everything to maintain it at normal levels.

What athletes actually see on a CGM during exercise

Mike Haney: So I've never done this, although I've thought about it — because I've worn CGMs, obviously working for Levels, and I've run marathons. I've never worn a CGM while running a marathon. And I take in relatively low levels of carbs because I just don't tolerate, or I'm too lazy to find the kind of carbs that I really can tolerate. So I'm probably in that 30 grams of carbs per hour range, if that.

What would I be seeing on my CGM? Because we're talking about these sort of two states, right? Of taking in a whole bunch of glucose, and that's driving a high glucose response, lots of insulin, and the downstream effects. We're also talking about this idea of exercise-induced hypoglycemia, where it's going too low. How is my body getting into those kind of states? What would I be seeing on a CGM if I'm running along, and once an hour I stop and take in 30 grams of glucose — or call it 60, whatever number would matter — how is my body responding? Am I spiking and then crashing the way I might be if I wasn't running? Or is my body keeping those levels more stable because I'm in an athletic process?

Andrew Koutnik: That's a really great question. So if someone wanted to actually look at this themselves, there's actually some published literature on this. I'm also privy to some unique kind of world record times where athletes were utilizing CGMs.

So imagine if you just consumed an initial spike in carbohydrates initially, and you'll see a spike in glucose and it may come down. And as long as the carbohydrates are not too high, you're probably going to see a normal level of glucose, because behind in the background, your body's releasing insulin to ensure it's taken into the muscle. And so what's often missed by just looking at only a CGM is that there's also this powerful effect of insulin, which, as someone with type 1 diabetes, I live with. I not only see glucose, but I also see insulin.

So consuming high levels of glucose — while you might see an initial spike in glucose and then leveling off, the only reason it's leveling off is because insulin's working overtime in the background to sustain that glucose level. That might be hidden for most folks if they don't have access to monitoring like they do in type 1 diabetes.

However, athletes who consume astronomical levels of carbohydrates during exercise — particularly these very high ends of sports nutrition guidelines, or even beyond those, let's say 120 grams per hour — what we see is that their blood glucose levels reliably on CGM stay north of 150 milligrams per deciliter. That's just in the reported literature so far, and there's not a lot on this. But I have been privy to a number of world-class athletes — elite world-class athletes, with some world record times — and when they're consuming these levels of carbohydrates during exercise, some of these athletes will sit above 160 to sometimes over 200 milligrams per deciliter throughout multi-hour rides.

So that's a clear indication that the body is not able to utilize all the exogenous forms of carbohydrates during exercise. On a CGM you would see whether the glucose is being maintained into the normal physiologic range, and you'll also see when it becomes astronomically high and the body is unable to sufficiently oxidize all these exogenous forms of carbohydrates being orally consumed.

And in a theoretical world, you'd want the ability for the body to be in a more balanced state during something like this. In fact, there are some examples — although it's very anecdotal — where athletes ran marathon times and used a CGM, and what they found is that in one race, the athlete was running over 150 milligrams per deciliter and had a worse race time. And then they ran much better race times when they were sitting somewhere between 80 to 120 milligrams per deciliter throughout the entire ride. So a reference point for other individuals: around a six to seven millimolar range. But some athletes as a reference value in millimolars are pushing north of 10 millimolars, sustained over the entire multi-hour run or indoor ride.

And so that's what people can see based on various levels of carbohydrate intake. Now, Mike, on the opposite end of the spectrum, what happens if they go low? Well, they may see a persistent but subtle drop in blood glucose over time. And that persistent and subtle drop in blood glucose over time will be detectable on CGM analysis — although CGMs are subtly delayed from the blood, because what happens in the blood is going to take some time to reach the interstitial fluid, although this will be faster during exercise. And ultimately that will translate to a signal that's then read on your phone or some other application. But ultimately that allows you to act ahead of time, right? So that means if you're starting to see indications that you may drop, you can just preemptively consume sufficient levels of carbohydrates to offset that — which in our studies, we see that 10 grams per hour is more than sufficient to do that.

What this means for average athletes and everyday people

Mike Haney: Well, I think that's a good place to kind of bring this home by translating it into what this means for average people — whether any of these lessons about what you've found about how the body is using these fuel sources matters in a non-athletic context, but also what it means for the non-elite kind of athlete, right? How does this impact the kind of traditional advice that your four-hour, five-hour marathoner might be getting — that you need to go hard on pasta the night before, really load up those carbs, and then as you're doing your marathon, make sure you're consistently taking in 50, 60, 70 and on up grams of carbs?

Andrew Koutnik: Sure. So for the average person, if they're doing resistance exercise or aerobic exercise, even if it's strenuous, typically carbohydrates don't do much to improve performance if it's less than 60 minutes in duration. For resistance exercise, if you're doing a really strenuous leg workout, sure, maybe 45 minutes — but generally speaking, hour or less of exercise, carbohydrates don't reliably improve performance for most individuals.

However, once you get beyond that point and exercise is sufficiently strenuous for sufficiently long, our studies have shown that at least 10 grams per hour is sufficient to offset exercise-induced hypoglycemia. And there is some analysis that shows that maybe around 30 grams or so can improve some athletes' performance. And maybe some individuals can benefit from even more than that, but they would have to trial and error that to figure it out.

Now, let's say you're moving into higher levels of athletics and you're more of a serious athlete. Well, the number one thing to consider is — and actually, to be fair, this also applies to the average person — what's your foundational diet? Is it aligned with your overall health? One error that is often made is that athletes, and even the general population, are hyper-focused on their performance and they don't consider the ramifications of those choices for performance on their health.

So athletes should probably go and get an assessment of their full blood workup, just like anyone else, to ensure they're in adequate health, that they're not seeing any changes in particular things like HbA1C or glucose levels. And ensure that everything is good there. And then try various diets. Find a diet that's sustainable for them. Find a diet that's aligned with their overall health.

For someone who has pre-diabetes or diabetes or obesity, we know that very low carbohydrate ketogenic diets can be extremely powerful therapeutic strategies to not only manage but also reverse some of those conditions. And our studies have shown that they don't impair performance either. So athletes can both have an improvement in health in those circumstances, but also feel confident that the literature supports that they can sustain physical performance.

Now, more of the individuals who are listening to this podcast or watching this online are probably going to fit a different category. That's a set of individuals who aren't necessarily elite level athletes, or even weekend warriors. They're probably actually more so going to be individuals who are at some precipice of early signs of metabolic changes that are probably moving in the wrong direction. Because we know that 9 out of 10 Americans in the United States have markers of elevated waistline, elevated triglycerides, elevated blood glucose levels — all indicating that their health is starting to shift or move in the wrong direction. And for those individuals — again, 9 out of 10 adults in the United States — they probably need to start thinking more so along the lines of what diet is aligned with my health, and stop thinking as much about performance.

Because the overwhelming evidence right now suggests that diet, while important to overall performance, means that athletes, as long as they're sustaining a well-formulated diet, are able to perform nearly identically to athletes who are on high or very low carbohydrate diets. And so athletes have a choice. But they should be focused on nutrition because nutrition has been shown over multiple analyses to explain over 90% of the obesity epidemic. We know that nutrition and diet is also one of the most powerful therapeutic strategies to prevent, manage, or reverse many of the chronic metabolic conditions we see in the United States of America, which affect an overwhelming majority of probably the people who are listening to this today.

And so in that case, people should start focusing on their health first. And I always say this: no athlete's health should be sacrificed at the altar of performance. Ever. No athlete's health should ever be sacrificed at the altar of performance. And the fact that we're seeing incidences of pre-diabetes in athletes who are just told you have to do this one type of diet and load up these astronomical amounts of carbohydrates — some percentage of athletes, somewhere between probably 10 to 30%, are going to run into issues with those carbohydrate intake levels based on the evidence that we currently have today. And that's unacceptable to me. It's unacceptable that athletes who are doing everything right — lean, normal body weight, high fitness levels — are ever going to experience early signs of sub-chronic metabolic disease.

And now we know that the weight of the evidence suggests that they have a choice in what they do with their diet to improve their health, without being concerned that they're also going to sacrifice their performance. And so I think the general takeaway from this for most individuals is: do what works, and make sure you're also prioritizing your own health. But know that you'll never know what works for you or what's optimal for you unless you try. And that requires taking the first step in your journey to figuring out what is best for you — for not only your health, but also your performance.

Mike Haney: Right. I think that's a really good lesson to end it on — that particularly if you are somebody starting out, or kind of a light weekend warrior, and somebody who needs to do something about their metabolic health, don't assume that exercise requires an absolute ton of carbs. And we didn't say it, but it probably goes without saying — the carbs that you do eat, it's not added sugar. You don't need to down a candy bar before you go to the gym. That's not actually going to help you perform, and it's just going to worsen the metabolic health that you have.

So, back to that sort of focus on your overall metabolic health: eat a diet that works for you, whether that's low carb or slightly higher carb but all healthy carbs. And don't feel like you have to ingest tons of carbs either while you're working out or before you go work out, because it's not actually going to improve your performance that much.

Andrew Koutnik: I think that's a great take home, Mike. And I'll also say that no matter what, the best way to find out what works for you is to try it. And ultimately, you'll find out what works and what doesn't work if you do that. But in order to do that, you need to find a trusted resource — understanding how to do these things correctly, because there's a lot of misinformation out there. Obviously you guys do a great job on this platform, Mike, on getting evidence-based information out there. But find individuals who are trusted, credible, who know how to do this and who have done it before. That's also important, because experience matters. There's a lot of people who just sit in a lab and never actually go out in the real world and understand how these things apply.

You know, as an example on myself — I've had type 1 diabetes, I've done various diets from high carb, carb cycling, if it fits your macros, ketogenic diets, all these other things. And I've also done some things like ketogenic diets for over 10 years. So I've seen the trial and error of these things, I understand what works. But finding individuals like that who understand how these things work, but also have lived experience, and maybe have helped other people along their journey — just look for what they have to share, because oftentimes it comes with unique nuggets of truth around how to actually practically apply these things in the real world to make them fit your goals for health and performance.

Mike Haney: Right. I think that's a great place to end. So, Dr. Andrew Koutnik, thanks so much for joining us.

Andrew Koutnik: Mike, appreciate the time.