Are long-term fasts or water fasts good for you?
Depending on your current health, living without solid food for 24 hours or more may help you burn more fat and trigger helpful adaptations, but it may also trigger muscle loss.
Going for stretches without food can have real benefits. We know that fasting for at least 12 hours helps improve the body's metabolic flexibility, or the ability to effectively switch between using glucose and fat for fuel. That's one reason intermittent fasting (IF)---e.g., fasting for 16 hours and limiting your eating to an 8-hour window---has risen in popularity.
But if short-term fasting generally has positive effects, that begs the question: Are longer fasts repeated less frequently (sometimes called periodic fasting) beneficial and safe for metabolic health? Here's what to know about water-only fasts and similar low-calorie liquid fasting approaches that last 24 hours or longer.
Water-only fasts and other common types of longer fasts: The basics
You'll see a lot of different prolonged fasting approaches mentioned on the internet---partly because there's no set definition or parameters for the best way to do one. Often, people get their inspiration for how to take on a longer fast from dietitians, doctors, or influencers who share what works best for them---or what they think might work best based on the human and animal research to date. And to be clear, research on longer fasts in humans is quite limited, so much of what we present here is based on expert opinion.
While water-only fasts are certainly the most common and straightforward approach, they're not the only type of longer fast being promoted. Here are a few variations of the trend:
- Water-only fasts featuring regular or mineral water with or without added electrolytes
- Fasts that allow for water plus other zero-calorie or very-low-calorie beverages, such as teas and coffee, without added milk or sweeteners
- Bone-broth fasts that allow for liberal consumption of water and bone broth, which is often promoted for its gut-healing potential
- Fasts that allow for a minimal amount of calories via liquids such as vegetable soups and juices. (An example of this is Buchinger fasting, which includes no more than 300 calories and 35 grams of carbs from these foods per day. This is a medically supervised protocol lasting 4 or more days that's been the subject of several studies on prolonged fasting.)
As for duration, these fasts may last anywhere from 24 hours to a week or more (sometimes up to 20 days in medically supervised settings). Some proponents recommend repeating them monthly, quarterly, or less frequently. Again, there's no official definition. Generally, though, the most commonly promoted water fasting timeframes are between one and three days. The rationale? There's no consensus, but it may take roughly this long to trigger some beneficial adaptations associated with fasting, such as ketosis and autophagy (a cellular cleanup and repair process associated with longevity). For example:
- It takes 12 to 36 hours of fasting for someone to deplete their glycogen (stored glucose) and switch to burning predominantly fat. As the body mobilizes its fat stores, some free fatty acids are fully oxidized to produce energy, while others are partially oxidized and become ketones, which serve as an alternative fuel source for tissues like the brain, heart, and muscles. Exactly how long it takes to produce ketones depends on your starting glycogen stores and how active you are while fasting. Per Sarah Szal Gottfried, MD, men tend to produce ketones sooner than women do in a fast.
- Additionally, research on mice has shown that it takes about 24 hours of fasting to trigger autophagy and about 48 hours for autophagy to peak. There are no conclusive studies to say exactly how long this takes in humans, but some nutrition experts, including registered dietitian Whitney English, RD, believe it probably takes longer---which is why she chose three days for her water-only fast.
According to longevity expert Peter Attia, MD, fasting should be done strategically (i.e., not too long, not too frequently), particularly in lean people, while fasts lasting longer than about three days may occasionally be appropriate in specific scenarios (more on that later). But regardless of the reason for the fast, intermittent fasting researcher Krista Varady, PhD, says that any fast longer than five days should not be undertaken without medical supervision. Additionally, fasting is not advised if you have a history of eating disorders; and anyone with a medical condition should get the go-ahead from their doctor before embarking on a fast.
Potential health benefits of longer fasts
As mentioned, the rationale for longer fasting periods is that they more effectively flip the "metabolic switch" from burning predominantly glucose to burning stored fat. This subsequently activates processes and metabolic pathways that aren't necessarily triggered with shorter IF approaches, such as time-restricted eating. While it's not always clear how quickly each of these benefits kicks in or how long they might last beyond the fasting period (that remains to be studied for different durations of water fasting), they're worth highlighting.
"There is no denying that some good things happen when we're not eating," writes Dr. Attia in his book Outlive. Here are a few things that might occur when fasting more than a day:
- Initially, blood glucose and insulin levels should drop due to a lack of incoming calories.
- Within 12 to 36 hours, the body will start burning stored fat and producing ketones for energy.
- In turn, elevated ketones have been shown to help curb appetite (which may aid weight loss), improve insulin sensitivity, and reduce inflammation.
- Lack of incoming calories and elevated ketones also activate the AMPK signaling pathway and downregulate the mTOR signaling pathway. Together, this promotes autophagy, a clean-up process in which cells recycle and replace their damaged parts with functional components, thereby improving their function.
Of course, some of these cellular changes are hard to measure directly in humans, so we have to look at clinical outcomes instead. The problem: Research on water fasting and other prolonged fasts in humans is quite limited. With that in mind, one 2023 review, led by Dr. Varady, analyzed eight small studies on water-only fasting and Buchinger fasting protocols lasting five to 20 days to summarize their metabolic effects. The researchers found that these fasts:
- Significantly increased circulating ketones
- Led to moderate weight loss
- Lowered blood pressure
- Reduced markers of fatty liver in people at risk for non-alcoholic fatty liver disease (NAFLD)
- Improved markers of glycemic control (fasting glucose, fasting insulin, insulin resistance, and HbA1C)
However, some of these markers---fasting glucose, fasting insulin, insulin resistance, and HbA1C---improved in people without diabetes but not in those with diabetes. This result was somewhat surprising, and researchers noted that future studies should ideally track glucose using continuous glucose monitoring (rather than testing at a single point per day) to provide a more accurate picture of glycemic control. The fasts also had a mixed effect on blood lipids, with some but not all studies showing reductions in LDL and triglycerides.
Interestingly, three to four months after subjects completed the fasts, they sustained none of these metabolic benefits, even if they maintained the weight loss.
At the same time, the reviewed studies didn't examine whether any of these metabolic benefits might be maintained or enhanced if someone engaged in several longer fasts per year, as opposed to one single fast. Nor did the studies look at more reasonable fasting durations of less than five days.
Additionally, the review combined water-only fasts and Buchinger fasts, so it's unclear how an alternative approach, like a prolonged bone broth fast (which contains protein), might influence outcomes. Some experts, like Ali Miller, RD, believe that a three- to five-day bone broth fast is an effective way to support the gut lining and quell inflammation (thanks, in part, to bone broth's high concentration of the amino acid glycine) while also supporting autophagy.
Another important finding of the review: Participants who lost weight lost about two-thirds of their weight in lean mass and one-third in fat mass---the opposite of what you want to happen. It's not surprising, though, according to Dr. Varady, because "your body needs a constant intake of protein. If it doesn't have that, then it draws from muscles." You want to hold lean muscle mass, as it's highly metabolically active, improves insulin sensitivity, reduces diabetes risk, and is even a predictor of longevity in older adults. If weight loss or weight management is your primary goal, Dr. Varady encourages a form of intermittent fasting over water fasting, as there's "a lot more data" suggesting IF can help.
The negative impact on muscle mass is a crucial reason Dr. Attia recently changed his opinion on the utility of frequent longer-fasting protocols. On an episode of the Metabolic Optimization Podcast with Thomas DeLauer, Dr. Attia shared that while he used to regularly complete three-day water fasts once a month and up to a 10-day water fast four times a year, he no longer thinks the benefits always outweigh the risk. "I was always accumulating a bit of a debt of lost muscle mass, and over about three years, I probably lost about 10 pounds of lean mass. Today, I just don't feel the tradeoff is worthwhile, at least at that level," he said.
The bottom line: Depending on your baseline body composition, some form of prolonged fasting may or may not be worthwhile. "If you're talking about someone who has 35 percent body fat [and] insulin resistance, I think fasting's a very effective tool, and it's okay that they're going to lose a little lean mass because they're probably going to lose more fat mass along the way," Dr. Attia shared on his own podcast. (Just be sure to loop in a healthcare provider if you decide to try this.)
For leaner people, though, your time may be better spent on muscle-building strategies. If you still want to experiment with longer fasts while limiting muscle loss, the occasional 24-hour fast is also "totally fine," according to Dr. Attia, or you can try a slightly longer fast "very intermittently---maybe once a quarter you do a fast for three days," he adds. Of course, it's hard to say what kind of results this approach might yield (we don't have any studies yet), but it does help ensure that you're periodically flipping that "metabolic switch" and possibly even enhancing longevity-supporting processes like autophagy from time to time.
Risks of longer fasts
According to the 2023 research review, mild adverse events, including insomnia, fatigue, dizziness, dry mouth, and headaches, were frequently observed during fasts lasting five to 20 days, though these symptoms may be less intense for fasting regimens lasting less than five days. After about eight days of water fasting, levels of sodium and chloride also dipped below the acceptable range, suggesting that supplementation with these electrolytes may not be necessary if you're doing a more reasonable one- to three-day water fasting regimen.
Remember, prolonged fasting is also a form of physiological stress. Multi-day fasting has been shown in several studies to elevate levels of the stress hormone cortisol. While some stress is good and triggers beneficial adaptations in the body, it could be a problem if you already have a high baseline stress level.
According to Dr. Szal Gottfried, menstruating women, in particular, tend to be more prone to stress and cortisol issues, and the added stress of a fast could potentially contribute to hormonal imbalances, blood sugar spikes, and other issues. Research has also found that women experience greater self-reported hunger during a 24-hour fast compared to men, which might make it difficult to adhere to longer fasts. This makes timing a fast for periods of lower stress all the more important (more on that below).
Other things to consider before trying a longer fast
It's always good to speak with your doctor before embarking on a fasting regimen, particularly extended fasts. Generally, people with increased nutritional demands---such as children and adolescents, pregnant or breastfeeding women, and people over age 75 (who tend to need more protein and are at greater risk for muscle wasting)---should avoid fasting. Additionally, fasting is not advised for people who are underweight or have a history of eating disorders.
Consider your overall health status, too. Fasting may not be appropriate---or may require close medical guidance or supervision---for people with certain chronic health conditions such as Type 1 diabetes. In some cases, fasting has also been shown to trigger gout attacks. And if you take any medication, ask your doctor if your dosage will need to change during your fast. For example, people on metformin who fast longer than 24 hours may need to adjust their dose.
Practical tips to make life easier if you decide to try a longer fast
If you (and your doctor) decide a longer fast lasting roughly one to three days may be worth trying, consider these tips:
- Plan your fast for a period of lower stress and more flexibility. High stress and the increased hunger and irritability that may come with a fast are a recipe for disaster. Ideally, plan your fast for when you'll be able to prioritize good sleep because sleep deprivation causes cortisol to spike. If you're a woman, experts recommend planning a fast sometime in the first two weeks of your menstrual cycle. Avoid longer fasts in the second half of your cycle (days 15 to 28), especially the week before your period, when the body is more vulnerable to stress.
- Don't overdo your exercise. Your tolerance for exercise during a fast will likely depend on your baseline fitness level and metabolic flexibility. You may be fine with your regular workout during a 24-hour fast, but experts are a bit mixed on the best way to approach exercise during a longer three-day fast. English suggests skipping your workout or opting for a light walk or gentle, restorative yoga if you're eager to move. On the other hand, Kevin Jubbal, MD, keeps up his workouts during his three-day fasts but allows himself more extended rest periods. Do what feels right for your body, and don't push it.
- Prioritize hydration. Because we typically get about 20 percent of our hydration from water-rich foods, it's easy to underhydrate when fasting. Make a point to drink plenty of water during a fast. Consider yourself well-hydrated if your urine is a pale yellow.
- Consider electrolytes. Electrolytes are vital in maintaining proper hydration, healthy blood pressure, and normal nerve and muscle function. Research suggests electrolyte levels should remain within the safe range during a three-day water fast, but to be safe, English recommends drinking a few bottles of electrolyte-infused water daily. During his fasts, Dr. Jubbal includes sparkling mineral water and the occasional cup of bone broth, both of which contain sodium and small amounts of other electrolytes.
- Don't (necessarily) nix the caffeine. If you're seriously caffeine-dependent, consider keeping coffee or tea in your fasting regimen, minus the milk and sweeteners. While some water fasting purists might disagree, these are probably unlikely to impact your fast (negatively or positively), per Dr. Jubbal, and trying to navigate caffeine withdrawals while dealing with fasting side effects like fatigue and headaches might be too much.
- Eat a metabolically healthy diet before and after your fast. Eating a nutrient-dense, balanced diet with plenty of protein and healthy fats---and that isn't loaded with sugars and other refined carbs---should help a fast feel like less of a shock to your system. Plus, given the potentially negative impact of longer fasts on muscle mass, you want to optimize protein intake whenever you're not fasting (strength training helps, too). Finally, to help prolong the metabolic benefits, break your fast with a meal that doesn't spike blood sugar. You could even use your fast to kick off a period of low--carb or ketogenic dieting to extend the health benefits associated with enhanced ketone production, such as reduced appetite, insulin resistance, and inflammation.
- Consider a CGM and ketone monitoring. Continuous glucose monitors (CGMs) provide real-time glucose data during a fast---so instead of just feeling crummy and hungry while your body gradually changes fuel sources, you can look down and see how your glucose levels have declined and stabilized in a healthier range. To get a sense of when your body has flipped the metabolic switch, you can also test your ketone levels. Urine ketone levels of at least 0.5 mmol/L and breath ketone levels of at least 2 ppm or 5 ACEs typically indicate that you've entered ketosis.
If you decide a longer fast isn't right for you, that's okay, too. You could consider a form of intermittent fasting, like time-restricted eating (TRE), which might be easier to sustain. You might start by fasting for 14 hours and limiting your eating to a 10-hour window every day (say 8 am to 6 pm).
But keep in mind, you don't need to fast at all to be metabolically healthy---what's more important is developing long-term, sustainable habits that keep blood glucose stable, such as prioritizing nutrient-dense whole foods (including plenty of protein and healthy fats), limiting highly processed foods, and prioritizing regular physical activity.
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