“The key is to be efficient at using both fats and carbohydrates for energy. Your ability to flip-flop and efficiently use both of these forms of energy is called metabolic flexibility.”
These questions don’t have simple answers. While we accept that we will become physically tired during training, how do we know if we are getting tired more than necessary because we aren’t fueling correctly? Could this lack of energy be fixed by fueling during our workout? Furthermore, how can metabolic tracking tools like continuous glucose monitors (CGM) provide feedback on energy utilization to optimize performance?
This article will dive into:
- How our muscles use fuel to perform
- Types of fuel
- When we may (or may not) need to supplement our body’s natural energy stores
- How ketogenic diets and intermittent fasting influence fueling strategies
- How a CGM can provide feedback to ensure you are fueling appropriately for your goals
What do you mean by “fuel”?
Calories are your body’s energy currency. Each macronutrient (carbohydrate, protein, fat) contains a different portion of energy. Each gram of carbohydrate contains 4 calories, each gram of protein contains 4 calories, and each gram of fat contains 9 calories.
Your first thought might be that fat is your best bet for athletic performance because it provides the greatest amount of energy for each gram. Unfortunately, it’s not that simple. Fat molecules take longer to dissociate and become available for fuel; carbohydrates, on the other hand, provide less energy but are available for use faster. Both types of fuel can be advantageous for performance when utilized appropriately. The key is to use both fats and carbohydrates efficiently for energy. Your ability to flip-flop and efficiently use both energy forms is termed metabolic flexibility.
What types of fuel are used in exercise?
Fuel during exercise comes from predominantly three sources:
- Fat already in your body (stored fat)
- Glycogen (a stored form of glucose) stored in your skeletal muscle cells and liver
- Carbohydrates that you ingest before or during exercise
Stored fat is primarily used for low-intensity exercise (definitions vary, but generally refer to exercise under approximately 60% VO2max), such as walking or a long easy bike ride). Minimal to no glycogen is used during very low-intensity exercise (less than 25% VO2max). As you increase in intensity, the proportion of glycogen used for fuel gradually increases, but fat is still the predominant fuel source for exercise under about 80% VO2max.
Your stored fat gives you the largest bang for your buck in energy. You can exercise for hours at this pace since your body has a massive store of calories stored as fat, making fat an excellent fuel source for long, sustained efforts. The caveat is that fat is primarily usable under intensities of approximately 60% to 65% VO2max. Thus, while you can travel a long distance, you won’t be able to travel it at max speed.
Think of fuel source number 2, stored glycogen, as your reserve tank. Your skeletal muscles store glycogen, which can be broken down and used to contract these muscles when needed. Your liver’s glycogen “reserve tank” of stored glucose can be used to maintain adequate levels of glucose in the blood. While glycogen is a great source of quick fuel for sprints or a few short, high-intensity efforts, these reserve tanks can only hold a certain amount of fuel. On average, the liver stores 80 g (0-160 g) of glycogen-based energy, and skeletal muscles store an average of 500 g (300-700 g). This amount of fuel will be significantly depleted after only about 90 min of high-intensity exercise. What happens if your high-intensity exercise or competition is longer than 90 minutes or if you start your athletic event with depleted glycogen stores?
In that scenario, you have two main options if you continue to exercise:
- Deplete your muscle and liver reserves, and your muscle cells won’t be able to produce energy (ATP) rapidly enough to maintain exercise intensity, so you “bonk.” This option is neither pleasant nor conducive to a strong performance.
- Avoid the unpleasantries and increase your performance by ingesting carbohydrates or trying a carbohydrate mouth rinse.
That brings us to fuel source number 3: carbohydrates you ingest during exercise. Carbohydrates are the primary energy source needed to facilitate sustained muscle contractions during exercise. So, if you are an athlete in a sport with repeated sprints, interval efforts, or sustained high-intensity exercise lasting longer than 45 minutes, then ingesting small amounts of carbohydrates during exercise will help to spare liver glycogen stores and delay the onset of fatigue. The duration and intensity of exercise largely dictate the amount of carbohydrates needed.
How to fuel properly
To determine how to fuel your exercise properly, we need to specify the duration and intensity of your workout. Please remember that we are talking about fueling during competitions for optimal performance. (In other blog posts, we dive into the intricacies of fueling for training, health/metabolic goals, and building muscle.) This post is about what you need during your athletic event.
While traditional fueling guidelines provide a range of carbohydrate intake per hour for optimal performance, which is a good starting point, newer tools exist that may help track carbohydrate needs even more. For instance, continuous glucose monitors (CGM) can report real-time glucose levels in the body during a workout. This technology provides a unique opportunity to personalize and dial in your fueling strategy to ensure it is most effective and least damaging. These quantitative tools can give a snapshot of how the body utilizes different energy sources.
Duration Less Than 45 Min, Low Intensity (Less Than 60% VO2max)
E.g., easy bike ride/run/swim, walking the dog, yoga
What do you want to see from your CGM to optimize performance during competition? If you perform low-intensity exercise, your blood sugar should not rise. In fact, you may see a brief dip in blood glucose at the onset of exercise while your body uses the readily available glucose already in circulation (it’s a quick and easy fuel source) in combination with stored fat. Since the intensity of exercise is not high enough to cue your liver to mobilize stored glycogen, after you burn through the small amounts of glucose in circulation, your blood sugar should be steady as you primarily use fat for energy.
You can use the feedback provided by your CGM to check that you are exercising at your desired intensity. For example, if your goal is to utilize fat as a fuel source, or you have an easy day planned and want feedback to ensure you are not overexerting yourself, then you should shoot for your blood sugar levels to have a slight dip at the onset of exercise then be steady the rest of the workout. A sudden increase in blood glucose could indicate your body is mobilizing stored glycogen to use as fuel, which is a sign that the intensity of the workout has increased beyond the capabilities of fat oxidation or that you ate something with carbohydrates right before your workout. This should be avoided if the goal is to perform a low-intensity exercise and primarily utilize fat oxidation.
Fueling insights: No extra fuel is needed, nor will additional fuel enhance performance. These exercises’ low intensity and short duration indicate that glucose already in circulation and stored fat will be preferentially utilized as fuel.
Duration Less Than 45 Min, Power Sports
E.g., weightlifting, powerlifting, field events
While many sports can be simplified into “high” or “low” intensity, power sports are in a league of their own. To succeed, maximal muscle contractions are required. In competition, these maximal contractions must be repeated to advance through the rounds and compete for the win.
What do you want to see from your CGM to optimize performance during competition? Muscles need adequate amounts of glucose to produce a maximal contraction. The muscles get this glucose from stored glycogen in the liver and the muscle cells. Often, people may see a rise in glucose during power sports as liver glycogen is released into the bloodstream as glucose to supply the muscles.
Fueling insights: Stored liver and muscle glycogen will get you through the early parts of a power sports event. Later in the event, eating a moderate amount of healthy glucose sources can help fuel maximal muscle contractions when glycogen may be running low. A 2022 review found that consuming carbs before or during an athletic event can help you get in extra exercise volume if the event is longer than 45 minutes.
Another 2022 paper from the International Society of Sports Nutrition recommends increasing carb intake in the days and hours leading up to a competition.
A little can go a long way, and big glucose spikes should be avoided, as they may lead to large insulin spikes that can impair fat oxidation and gluconeogenesis (the process by which the body can make new glucose from other building blocks like lactate, glycerol, certain amino acids). Also, exercise increases insulin sensitivity.
Duration Less Than 60 Min, High Intensity (More Than 80% VO2max)
E.g., interval bike ride/run/swim, tennis match, track events, cross-country skiing, beach volleyball
What do you want to see from your CGM to optimize performance during competition? At high intensities, you will primarily use glucose for energy in the form of circulating glucose, glycogen, and glucose ingested during the workout. On competition day, you want to make sure that you are not starting with extremely low blood sugar; if the athletic event begins at a high intensity, then you will be depleting your glycogen stores quickly, leaving you to rely on in-race feeding for any subsequent high-intensity efforts (aka the sprint finish). On the flip side, you also don’t want to start with extremely high glucose levels from a big carb load before the workout, as this will cause insulin to rise and may impair the ability to utilize fat as a supplemental source of fuel (at high intensities, approximately ⅓ of energy will still come from fat, and you don’t want to impair that). Checking your CGM can give you a sense of how much circulating glucose you have when you begin and is a clue as to whether fueling might be necessary during the workout.
At the beginning of high-intensity exercise, your body secretes specific hormones called catecholamines, which cause glucose release from the liver to increase up to eight-fold. However, muscles’ use of glucose only increases up to four-fold. Thus, it is not unusual to see a rise in blood glucose at the beginning of a high-intensity workout. This rise indicates that you are mobilizing your stored glycogen (meaning the workout is too intense for your body to rely primarily on fat and needs to use glucose as fuel). This increase in blood glucose during intense exercise can increase further immediately at exhaustion and persists for up to one hour post-workout.
If you are competing in a self-paced sport (e.g., cycling time trial or triathlon) and want to check whether you are working at a high intensity, seeing a rise in blood glucose at the beginning of your workout is a clue that you may be mobilizing glucose from your liver to fuel your muscles.
Fueling insights: Some recent studies have shown that mouth rinsing for 5-10 seconds (and not swallowing) a carbohydrate solution (composed of ~6% carbohydrate) can improve performance in short (30 min to 75 min) intense exercise. This is an intriguing finding since less than 60 minutes of exercise should not deplete our glycogen stores fully; therefore, additional carbohydrates should not be needed to fuel muscle contractions. However, the reason behind mouth rinsing’s efficacy may be elucidated by brain imaging studies that have identified areas of the brain activated when carbohydrate is rinsed in the mouth, indicating the central nervous system may be driving the association between mouth rinsing and improved performance. The brain signals triggered by mouth rinsing can produce brain signals that modify motor output. It’s as if the brain senses more energy may become available, and it rises to the challenge. You’ve probably already seen mouth rinsing in action. English Premier League athletes have been rinsing and spitting on the sidelines for years, as well as pro football players and runners.
If you started with low glycogen stores because you were fasting or in a ketogenic diet, you might want to supplement with some ingested carbohydrates towards the end of a workout to optimize performance, but you don’t want to consume so much that you see a significant spike in glucose levels, which would indicate a subsequent insulin surge and possible impairment of fat oxidation or gluconeogenesis for energy.
Duration 45-180+ Min, Low Intensity (Less Than 60% VO2max)
E.g., tai chi, casual hiking, golf, baseball
What do you want to see from your CGM to optimize performance during competition? Same as “Duration less than 45 min, Low Intensity (less than 60% VO2max).”
Fueling insights: Extra fuel is not required, nor will additional fuel enhance performance. Even lean people have enough fat to fuel exercise, even endurance. These exercises’ low intensity and short duration indicate that stored fat will be preferentially utilized as fuel. If you have one or two short sprints throughout your workout, you don’t need to panic and reach for fuel. Your stored glycogen can adequately fuel a couple of quick bursts.
However, if your muscle glycogen was low before beginning exercise because of a low carbohydrate diet or fasting, you might see a drop in blood sugar after those quick bursts. If the rest of your workout is low-intensity, then there is no need to fuel. Your fat can carry you through. However, if you still have a sprint or two left and your workout begins to encroach upon the middle ground between low and high intensity, you can take in small amounts of carbohydrates as tolerated to help you finish the last few sprints strong.
Duration 60-150 Min, High Intensity (More Than 80% VO2max)
E.g., triathlon, open water swimming, marathon
What do you want to see from your CGM to optimize performance during competition? Same idea as “Duration less than 45 min, High Intensity (more than 80% VO2max),” but you will likely need to consume fuel before or during the event if you want to optimize performance in more extended duration exercise and avoid “bonking,” the term often used by athletes to refer to running out of energy and hitting the wall. Remember, the average person who has neither fasted nor followed a low carbohydrate diet has about 90 minutes of high-intensity exercise fuel stored as glycogen.
At this intensity, you’ll rely mostly on glycogen stores, which will deplete with a longer, high-intensity workout. As these become depleted, glucose values on a CGM may start to dip, indicating that you want to fuel during your workout.
When fueling before a workout to ensure optimal glycogen stores for a longer, high-intensity workout, it seems that a good option is to ingest carbohydrates 2-3 hours before the workout rather than immediately before the workout (meaning any time within 60 minutes of the workout). This is because the carbohydrates will likely generate a glucose and insulin surge, and that insulin can impair fat oxidation and gluconeogenesis. If this happens 2-3 hours before the workout, the glucose will be stored as glycogen, and the insulin will likely recover to normal levels by the time of initiation of the workout. This means that you’ve successfully filled the glycogen tank and have hormones at a good level to tap into fat oxidation during the event, which is a good scenario. On your CGM, this would look like a slight glucose rise 2-3 hours before the workout that returns to baseline by the time of the athletic event.
When fueling during the workout, we also don’t want to generate an excessive spike, which can cause a significant release of insulin and impair fat oxidation and gluconeogenesis during the event. Getting some carbs in, but not in extreme excess, will likely support the optimal hormonal balance.
Fueling insights: Aim for 30-60 grams of carbohydrate per hour (predominantly in the form of glucose and low in fiber). Liquid carbohydrates should be consumed in a 6–8% carbohydrate-electrolyte every 10 to 15 minutes throughout exercise. We need more research assessing the use of mouth rinsing for exercise lasting longer than 75 minutes, but it is unlikely that it will be as effective as consuming carbohydrates, as in this case, we actually need the fuel. Thus, your best bet is to consume fuel rather than rinse and spit. Research also suggests that carb loading (10-12 g/kg of body weight per day) in the day or two before your event can be beneficial.
Duration More Than 150 Min, High Intensity (More Than 80% VO2max)
E.g., road cycling, marathon
What do you want to see from your CGM to optimize performance during competition? Same as “Duration 75-180 min, High Intensity (more than 80% VO2max)”.
Fueling ideas to achieve optimal CGM feedback: Select your liquid, gel, or food of choice and shoot for consuming 75-90 grams of carbohydrate per hour (in a 0.8:1 ratio of fructose to glucose to utilize multiple transport methods optimally). Glucose uses numerous channels to move across the cell membrane, but the GLUT5 channel is specific to fructose. Thus, by utilizing all of these transmembrane carriers, you can maximize the amount of fuel entering the cell. It is commonly recommended to consume snacks or meals high in carbohydrates (1–4 g/kg/day) for several hours before higher-intensity (≥ 70% VO2max), longer duration (> 90 min) exercise.
What about Different Diets?
Ketogenic and Low Carbohydrate Diets
A ketogenic (keto) diet involves an extremely low intake of carbohydrates intended to encourage the body to burn fat as fuel. There are a few types of ketogenic diets, which differ in their macronutrient ratio (some have a slightly higher carbohydrate allowance than others). Keto diets have been shown to improve insulin sensitivity and increase weight loss. How does this diet impact athletic performance?
Studies investigating the effect of a ketogenic diet on anaerobic athletes found that, in general, it was not an effective way to increase anaerobic performance, such as weightlifting. In fact, ketogenic diets have “the potential to negate the expected increases in lean body mass from anaerobic training.” Strength and power athletes, in particular, would likely not want to utilize a ketogenic diet during competition since maximal muscle contractions rely on glucose. Thus, the depleted glycogen stores and lack of carbohydrates accompanying a ketogenic diet could hinder an athlete’s ability to perform a max lift/jump/etc.
Endurance athletes have also experimented with keto diets. Theoretically, the large availability of fat in the body should transfer into increased whole-body and muscle fat utilization rates if these pathways are more active in a keto-adapted athlete. Ketogenic diets result in greater fat oxidation rates, and some studies have shown that consumption of a ketone ester-containing beverage improved endurance performance (read an excellent guide to exogenous ketones for performance at the HVMN blog). However, while energy utilization is altered, studies largely find little to no benefit of a ketogenic diet for performance compared to healthy people on a carbohydrate-rich diet.
A ketogenic diet may impair performance on high-intensity activities due to a lack of quickly available fuel (aka carbohydrates or glycogen). Thus, while it theoretically makes sense for keto diets to improve performance, there are probably better choices on competition day, when readily available glucose can help at high intensities.
“It’s important to remember that everyone’s physiology is different.”
A balance may lie in the concept of “carbohydrate cycling,” where athletes train in a low-carbohydrate state or maintain low carbohydrate intake during rest days, thereby generating adaptations in their fat-burning pathways. However, the body is then supplied carbohydrates on days when maximal performance is necessary.
Indeed, while ketogenic diets are likely not an effective performance advantage, they may have a place outside competition for health benefits and general adaptations in energy utilization. If you want to give the keto diet a try, you can use your CGM to help. While on a ketogenic diet, your glucose readings will likely be pretty low-normal, with minimal swings, indicating that neither glycogen nor carbohydrates are used as a primary substrate for fuel. The lower insulin levels generated from not consuming significant carbohydrates allow for more efficient fat oxidation. Over time, this can increase metabolic flexibility, allowing the body to shift between different fuel sources efficiently based on what is available.
Intermittent Fasting
The concept of fasting is not new, but the idea has gained traction in the media over the past decade. People engage in intermittent fasting for various reasons: religious beliefs, weight loss, decreasing blood pressure, or reducing the risk of metabolic diseases. There are a variety of different protocols utilized to obtain the desired results, such as the 16/8 (16 h fast followed by 8 h of food intake), 5:2 (eating normally for five days and severely restricting calories for two days), or the Muslim fast during Ramadan (1 month with no food or liquid consumed during daylight hours).
When seeking to utilize this strategy for improved athletic performance, we can break it down by exercise type. Training fasted will likely not benefit strength or power athletes since carbohydrates fuel maximal muscle contractions. Thus, if you attempt this type of exercise after a prolonged fast, then it is likely that your blood sugar will remain low and steady or have a minor increase from the release of a small amount of stored liver glycogen but that your performance will be diminished since you don’t have the fuel to perform maximal muscle contractions. Indeed, a 2024 review found that it did not affect strength and power or anaerobic physical capabilities.
Fasting has not been shown to be an effective performance-enhancing technique for sprinters (a high-intensity exercise that relies on a higher percentage of glucose), even when studies controlled for the potential confounding factors of sleep and training load. Intermittent fasting also failed to benefit athletes competing in repeated sprints.
Theoretically, endurance athletes may benefit from alternate-day feeding or intermittent fasting because it allows the body to become better adapted to using fat as an energy source. A study by Marosi et al. found that one month of alternate-day feeding produced mice that ran further and longer than mice that were allowed to eat as desired. However, a 2021 review study found that there is not enough evidence to suggest that intermittent fasting benefits endurance athletes and may have negative effects on them.
While fasting, blood glucose on CGM will likely trend to a low and steady level since you have minimized your ingested glucose and glycogen stores, and fat is being used for fuel. As mentioned previously, in times of low glucose availability, the body can also make new glucose from other sources like lactate, glycerol, and certain amino acids in a process called gluconeogenesis.
Summary
It’s important to remember that while these research-backed guidelines help provide a barometer of how much and what type of fuel is optimal for performance, everyone’s physiology is different. So, while the recommended amount of carbohydrates may work perfectly for one individual, it may not be enough if that person gains muscle mass, works at a higher intensity, has a different diet, competes in a warmer climate, or is at a different phase of their menstrual cycle.
This is where continuous glucose monitors (CGMs) may emerge as helpful adjuncts for athletes. Given so many variables in athletics, CGMs can clarify how the body utilizes different energy sources during athletic performance. They can also take some guesswork out of fueling during exercise, helping to know quantitatively when you might be getting close to bonking. On the flip side, they can ensure that you don’t overshoot your fueling and generate a hormonal cascade that may be counterproductive.
