Does intermittent fasting cause muscle loss?
As intermittent fasting gains popularity as a weight loss strategy, the medical community is evaluating its potential drawbacks. One area of concern is whether intermittent fasting causes loss of lean body mass, specifically muscle tissue.
Why are people worried about the answer to this question? Even though the data for intermittent fasting are still evolving, we already have data showing that those who lose weight by chronically restricting calories lose approximately one-fourth to one-third of the weight as lean tissue.1
We know that healthy weight loss involves losing fat while preserving as much muscle as possible. Therefore, if engaging in intermittent fasting to lose weight causes an equal or greater amount of muscle loss compared to chronic caloric restriction, intermittent fasting would not be as useful of a tool in the weight loss toolkit.
This guide explains the controversy, reviews what happens in the body during short-term fasting, and synthesizes the clinical trial data looking at body composition with intermittent fasting.
Finally, we’ll present our conclusions about whether intermittent fasting is likely to cause muscle loss.
For those who prefer to know our conclusions in advance, to give context to the rest of the article, we’ve got you covered:
- As intermittent fasting is typically practiced, with fasting intervals of 16-24 hours several times per week, it is unlikely to cause significant loss of lean body mass/muscle tissue.
- The data are conflicting, with some data showing loss of muscle mass with intermittent fasting. However, consuming a moderate to high amount of protein appears likely to mitigate or completely prevent this loss.
- In the context of intermittent fasting, adding resistance training to an adequate- or high-protein diet appears to make muscle loss very unlikely.
- There are insufficient data to draw conclusions about the risk of muscle loss as the frequency of intermittent fasting increases. In other words, fasting for less than 24 hours “too often” over extended periods may or may not increase the risk of muscle loss.
What is intermittent fasting?
Intermittent fasting is an umbrella term that refers to deliberately not consuming any calories for short periods. If you want to learn more about different versions of intermittent fasting, check out our guides to time-restricted eating and one meal a day.
In this guide, we focus solely on short-term fasting for 16 to 24-hour windows several times per week, also known as time-restricted eating. However, we will use the more colloquially recognized term “intermittent fasting.”2
We will specifically exclude any discussion of fasting periods beyond 24 hours, since most research about contemporary forms of intermittent fasting examines fasting periods of less than 24 hours.
While there is a body of literature looking at what happens to body composition with chronic starvation (days to weeks of calorie deprivation), we exclude it because — compared to short, intermittent fasts — longer fasts lead to very different changes in the body. Therefore, the research about chronic starvation is almost entirely irrelevant to the current debate.
Why is there controversy about whether intermittent fasting causes muscle loss?
When talking about intermittent fasting as a weight loss tool, there is debate about whether its effect on weight is due solely to cutting calories or whether short fasts flip a metabolic switch, shifting the body from sugar-burning into fat-burning mode.3
If intermittent fasting is just a clever way to cut calories without having to count them, it could be an easier and more sustainable way to lose weight — as opposed to chronic caloric restriction, which is hard to maintain and generally ineffective over long periods of time.4
The downside of this simple explanation is that intermittent fasting would therefore also carry the same drawback as caloric restriction; namely, one-third to one-quarter of weight lost would be lean tissue.
On the other hand, if intermittent fasting causes weight loss by preferentially encouraging the body to mobilize fat stores for energy while preserving lean tissue, that would make it an ideal weight management strategy.
While trying to figure out the “right” explanation for why intermittent fasting helps with weight loss, many have lost sight of a third possibility: both explanations can be correct.5
When we break a fast, we don’t necessarily “make up” for the calories we missed while fasting; this leads to built-in calorie restriction. Also, when fasting — especially in the context of a low-carb diet — the liver’s glycogen stores are usually depleted within the first 24 hours, leading to the mobilization of fatty acids from fat stores for use as energy.6
Bringing the conversation back to whether intermittent fasting causes loss of lean body mass, we need to know more about how the body obtains energy when a person eliminates food for 16 to 24 hours.
Does the body use mostly glycogen from the liver and fatty acids from fat stores, thereby sparing muscle tissue? Or does it break down muscle so amino acids can be used by the liver to make new glucose? Read on to find out.
Understanding short-term fasting
One major problem with the debate about intermittent fasting and muscle loss is that it focuses too much on how the body reacts during long-term fasting. This is because much of the literature in this field includes studies of fasting that lasts for weeks.
But if you’re only fasting for up to 24 hours at a time, then you only care about what’s happening in the 24 hours or so leading up to a fast and then the fasting period itself — not what’s happening over the course of several weeks without food.7
Because current scientific evidence doesn’t give us hour-by-hour details of how our bodies use and store energy in the first 24 hours of a fast, we are left to extrapolate from the existing — and less specific — literature.
That’s quite difficult to do; it’s like referring to a general map of the state of New York to try to find your way around Manhattan. You’ll end up making a lot of “best-guess” navigational decisions, based mostly on where you are in relation to the water.
Given the state of the science, we are left to look closely at the most relevant data, extrapolating where necessary, to describe what’s happening in the body when fasting for 16 to 24 hours.
The first thing we need to discuss is how the body uses energy over the course of 16 to 24 hours of fasting. During that time period, the liver is breaking down a fair amount of its glycogen to glucose, which the rest of the body can then use for energy. As glycogen stores decrease, the body needs to start looking for other energy sources.8
At this point, the body’s options for obtaining energy are:
- Gluconeogenesis: The liver can use amino acids, lactate, or glycerol to make glucose, which can be used directly for energy.
- Fat tissue: Fatty acids can be released from fat tissue and metabolized to ketones, which can be used directly for energy.
- Muscle: If there aren’t enough amino acids readily available for gluconeogenesis, muscle can be broken down into amino acids, and those can be sent to the liver.
It turns out that, in the first 16 to 24 hours of fasting, as glucose production goes down due to falling glycogen stores, the rate of gluconeogenesis does not increase. To be clear, gluconeogenesis is occurring at its normal baseline rate, but that rate doesn’t get any faster in the very early stages of fasting.
In other words, breaking down extra amounts of muscle to get amino acids for energy is not the first step when glycogen stores are low. Rather, what does clearly happen is that fat is metabolized to ketones, and ketones are used for energy.9
After an overnight fast, some amino acid release from skeletal muscle is normal. For example, a 154 lb (70 kg) man will release enough amino acids from muscle for the liver to make about 40 grams of glucose via gluconeogenesis. But early on in fasting, the liver will release somewhere between 150-400 grams of glucose per day. Hence, the contribution from muscle (40 grams) to overall fasting glucose production is low.10
The effect of intermittent fasting on muscle
In the previous section, we laid out evidence suggesting that the breakdown of muscle for energy does not substantially increase when fasting for 16 to 24 hours. While that research is intriguing, we really want to see controlled studies that measure what happens to lean body mass in subjects practicing intermittent fasting.
While there is a growing body of literature, it is difficult to generalize the studies’ conclusions to people engaging in intermittent fasting, as much of the evidence suffers from significant weaknesses.11
To the best of our knowledge, the first major review of medical literature examining the effect of intermittent fasting on body composition was a 2011 paper; it found intermittent fasting was superior to daily caloric restriction for preserving lean mass. Specifically, weight loss achieved with caloric restriction was 75% fat and 25% muscle, while weight loss via intermittent fasting was 90% fat and 10% muscle.12
The next paper to examine the literature was a large systematic review in 2015. Unfortunately, this large review broadened its scope to look at many forms of intermittent energy restriction that bear no resemblance to how intermittent fasting is practiced in real life. With that limitation in mind, the paper found that intermittent energy restriction had a roughly equal chance of preserving lean mass or leading to its loss.13
A 2018 review article was more relevant to the current practice of intermittent fasting; it looked more specifically at trials of time-restricted feeding and alternate-day fasting (including alternate-day modified fasting). The four time-restricted feeding studies showed no loss of lean mass, while three out of 10 alternate-day fasting trials showed some loss.14
Protein intake prevents muscle loss
Clinical trials with protocols resembling contemporary intermittent fasting mostly show that intermittent fasting is either the same or better than caloric restriction when it comes to preserving lean body mass. But there are some studies showing more lean-tissue loss with intermittent fasting, which begs the question: what can we do to counteract that possibility?
In the general medical literature, as well as the weight loss literature, it is well known that protein intake is critical when it comes to preserving or gaining muscle while losing fat.15 Diets that are moderate to high in protein will almost always preserve more muscle than low-protein diets.
People doing intermittent fasting may be concerned that it is too difficult to consume adequate protein within their eating window. Further, there is a myth that we can’t absorb or use more than 20 to 35 grams of protein in a meal and that any protein eaten beyond that amount will be oxidized (and unavailable for muscle protein synthesis).
To address those concerns, we should first point out that, with the right dietary approach, it is absolutely possible to eat a higher protein diet while doing intermittent fasting.
Next, we need to debunk the “muscle-full” hypothesis — the myth that there is a ceiling for how much protein we can use from any given meal. That hypothesis is based on several studies that have consistently shown a maximal amount of muscle protein synthesis after ingesting 20 to 35 grams of protein.16
There are two major flaws with the muscle-full hypothesis. The first is that it doesn’t make sense from an evolutionary perspective.17 Our ancestors likely ate large amounts of animal protein shortly after a kill, followed by an interval of less protein when food was scarce. They would not have been well-adapted to survival if only a small portion of the meat they ate could be used to build and preserve muscle.
Of course, the evolutionary argument is weak because we can’t prove it.
But the second flaw can be proven through a more careful examination of the medical literature.
The studies showing that muscle protein synthesis tops out after 20 to 35 grams of protein use a pure protein shake (usually whey), often in subjects who drink it on an empty stomach.18
But when we examine studies that feed their subjects whole foods, we see that greater protein consumption — well above 20 to 35 grams — leads to more significant muscle gains.19
Why does this happen? First, it’s necessary to understand that muscle is not the only tissue in the body that uses dietary amino acids to make proteins. The digestive system also makes proteins, which can be broken down and released into the bloodstream well after a meal, to be used by tissues like muscle to make protein.20
Second, studies looking only at muscle protein synthesis miss the fact that muscle protein breakdown is just as important as muscle protein synthesis when determining the overall effect on muscle tissue.21 (In other words, muscle is both lost and gained.)
Eating larger amounts of protein leads to suppression of muscle protein breakdown, to an even greater extent than the increase in muscle protein synthesis. The net effect of the increase in making muscle and the even larger decrease in breaking down muscle is that muscle mass will increase when dietary protein intake increases.22
To recap, the body can “use” larger amounts of protein when the source is a whole-food meal rather than a pure protein shake.23 And, to accurately predict what happens to muscle mass when doing intermittent fasting, we must consider the net effect of muscle protein synthesis and muscle protein breakdown.
While the rate of muscle protein synthesis will begin to slow at higher protein intakes, the rate of muscle protein breakdown will decrease to a much larger extent, leading to net positive protein balance.
Longer studies on protein intake
We have additional data to help us answer the more clinically relevant questions: does eating more protein help us preserve muscle over the long term, and can we get this benefit by consuming all that protein in just one or two meals during periods of intermittent fasting?
One study reported that older women retained their muscle mass when eating 80% of their daily protein at lunch and the remaining 20% spread over breakfast and dinner. However, those who ate equal amounts of protein over all three meals lost muscle mass.24
Looking more specifically at intermittent fasting, one eight-week trial studied subjects doing time-restricted feeding (20-hour fast, 4-hour eating window) four days per week and compared them to subjects eating a normal diet. All subjects did resistance training three days per week. Despite eating 650 fewer calories per day, the time-restricted feeding group did not show a significant loss of lean mass.25
A similar eight-week study looked at time-restricted feeding (16-hour fast, 8-hour eating window) compared to a normal diet in resistance-trained males, but this time the groups were matched for calories and macronutrient intake. The time-restricted feeding group lost more fat and showed no difference in lean body mass compared to the normal-diet group.26
Exercise prevents muscle loss
We’ve now seen that moderate-to-high protein intake helps preserve or build muscle during intermittent fasting. And it is common knowledge that multiple types of exercise improve body composition.27 But are there data showing that combining exercise with intermittent fasting is effective for preserving lean body mass?
Before we get to the clinical data, it’s important to understand that priming our muscles with exercise makes them more receptive to taking up amino acids, and more protein can mean more gains in muscle mass.
One study gave 20 grams or 40 grams of whey to subjects after whole-body resistance training and found 20% higher muscle protein synthesis with the 40-gram dose.28
A systematic review of studies examining the effect of intermittent fasting plus resistance training on lean body mass found that lean tissue was either preserved or increased in all studies. This finding suggests that adding resistance training to intermittent fasting might be better for muscle retention than increasing protein intake alone.29
It would be helpful to have studies of intermittent fasting that look at subjects who exercise compared to subjects who don’t. Hopefully these trials are in the works and will soon be added to the body of medical literature.
Intermittent fasting doesn’t cause muscle loss
As long as you eat a diet with adequate or high protein, you likely don’t need to be concerned about losing muscle with short-term intermittent fasting. Further, eating plenty of protein and adding resistance training to intermittent fasting seems to help preserve muscle.
However, we acknowledge that long-term data in this area are sparse. We also have concerns about whether doing intermittent fasting “too often” could ultimately lead to muscle loss, given that longer duration and higher frequency fasting have similarities to chronic calorie restriction.
And, because the weight loss with chronic caloric restriction tends to be one-quarter to one-third muscle, it is reasonable to question whether intermittent fasting, done for too long or done too often, could have this effect as well.
Moderation is wise: if practicing intermittent fasting for less than 24 hours several times per week, you’re likely to experience a favorable effect on body composition without significant loss of muscle mass.
Does intermittent fasting cause muscle loss? - the evidence
The guide contains scientific references. You can find these in the notes throughout the text, and click the links to read the peer-reviewed scientific papers. When appropriate we include a grading of the strength of the evidence, with a link to our policy on this. Our evidence-based guides are updated at least once per year to reflect and reference the latest science on the topic.
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Advances in Nutrition 2021: Perspective: time-restricted eating compared with caloric restriction: potential facilitators and barriers of long-term weight loss maintenance[review article; ungraded] ↩
Clinical trial data are conflicting with respect to: whether intermittent fasting is superior to caloric restriction as a weight loss tool, intermittent fasting’s magnitude of effect when it is helpful, and whether intermittent fasting exerts its potential effect mainly through caloric restriction or mainly by flipping the metabolic switch.
This systematic review found that intermittent fasting was associated with weight loss in all 27 trials, regardless of changes in calorie intake, and most of the weight lost was fat mass. But intermittent fasting could not clearly be shown to be superior to caloric restriction.
Nonetheless, many patients and physicians find intermittent fasting useful.
Glucose is stored in the liver as glycogen. When not eating, glycogen is broken down to glucose and released into the blood to be used by all cells for energy. The liver can also make new glucose through a process called gluconeogenesis, but it can’t make enough glucose to meet all the body’s energy needs. Fat then gets used for energy.
A reasonable critique of this statement would be: what about people fasting for 24 hours at a time, every other day, for long periods of time without taking a break? Could they be at greater risk for muscle loss than people who fast less often or intersperse their every-other-day schedule with periods of regular eating? We acknowledge that the longer and more frequently one fasts, the more possible it becomes that the medical literature about long-term fasting could be relevant. In that case, the risk of muscle loss might be higher, but we do not have data to support nor refute it. ↩
This is a critical point to underscore, as it makes it much less likely that excessive amounts of muscle will be broken down so that its amino acids can be used for energy. Why use protein, an inefficient source of energy, when fatty acids — a better source of energy — are readily available?
The following study also found that the amount of glycerol, lactate, and amino acids used to make glucose during fasting remained constant, suggesting that the rate of muscle breakdown did not increase.
The following two studies also show that the rate of gluconeogenesis stays constant during early fasting. Note that gluconeogenesis accounts for an increasing percentage of overall glucose production over the course of a fast. This is because glycogen breakdown decreases as the stores are depleted, making gluconeogenesis responsible for a higher percentage of overall glucose production. But the rate of gluconeogenesis stays the same.
The weaknesses in the intermittent fasting literature are:
- Study designs and populations are markedly different from one another, making systematic reviews and meta-analyses difficult to perform.
- Fasting protocols often do not correspond to commonly practiced methods of intermittent fasting.
- Studies are often small and of short duration.
- Dropout rates are high.
- Some studies provide food, while others offer only dietary counseling and rely on self-reported food intake at just a few time points.
- Methods of body composition assessment often differ.
- Protein intakes may be inadequate (more on this later).
This is a review article, not a systematic review or meta-analysis, meaning that no statistical evaluation was performed by the author. The article is intended to summarize the clinical data qualitatively.
Note that most of the caloric restriction trials in this review used MRI to assess body composition. MRI is more accurate than DEXA, which is used by most intermittent fasting trials. This weakens the magnitude of the finding that intermittent fasting is superior to caloric restriction for muscle preservation.
This systematic review included 32 independent clinical trials over 40 publications. 17 out of 32 trials reported on fat-free mass which, for our purposes, is synonymous with lean mass. Of those 17 trials, 9 reported a decrease in fat-free mass with intermittent energy restriction, and 8 reported no change.
Further, of the 12 publications that directly compared intermittent energy restriction with continuous energy restriction, five reported on fat-free mass. Four publications showed no change in fat-free mass, while one showed a decrease with intermittent energy restriction compared to continuous energy restriction.
Although very few trials looked at the effect of exercise, the few that did showed no loss of fat-free mass with intermittent energy restriction when also exercising.
Although we usually grade systematic reviews of clinical trials as strong evidence, we are downgrading this one to moderate, given its significant weaknesses: 14/40 papers in the review were by the same researchers; many trials were uncontrolled or not compared to caloric restriction; study designs were markedly different from one another and from intermittent fasting as it is practiced today; many trials were short and small; and many trials were deficient in protein as part of a low-calorie weight loss diet.
Molecular and cellular endocrinology 2015: Do intermittent diets provide physiological benefits over continuous diets for weight loss? A systematic review of clinical trials [moderate evidence] ↩
A 2021 meta-analysis, along with a few other meta-analyses we haven’t mentioned, didn’t add much new information to what we’ve already described, finding little difference in lean-mass changes between intermittent fasting and caloric restriction.
European Journal of Clinical Nutrition 2021: Effectiveness of an intermittent fasting diet versus continuous energy restriction on anthropometric measurements, body composition and lipid profile in overweight and obese adults: a meta-analysis [strong evidence] ↩
Journal of Nutrition 2020: The role of protein intake and its timing on body composition and muscle function in healthy adults: A systematic review and meta-analysis of randomized controlled trials
American Journal of Clinical Nutrition 2010: Muscle full effect after oral protein: time-dependent concordance and discordance between human muscle protein synthesis and mTORC1 signaling[non-controlled study; weak evidence]
American Journal of Clinical Nutrition 2014: Myofibrillar muscle protein synthesis rates subsequent to a meal in response to increasing doses of whey protein at rest and after resistance exercise[non-controlled study; weak evidence] ↩
We are not the first to argue that the muscle-full hypothesis is incorrect. Martin Berkhan from leangains.com has previously made this point. ↩
Liquid protein — particularly whey — in the absence of other macronutrients (carbs and fat) will digest very quickly, leading to both sharply increased availability and oxidation of those amino acids.
Whey is digested at a rate of about 10 grams per hour. Therefore, 20 grams of whey will be digested over two hours. This may spike muscle protein synthesis, but evidence shows that this is also associated with simultaneously increased protein oxidation, presumably because muscle has a hard time using all the amino acids that rise so quickly in the blood. This may result in lower net-protein balance compared to a slower-digesting protein, like egg.
Cooked egg protein is absorbed at about 3 grams per hour, meaning that the same 20 grams of protein in an omelet would take about seven hours to digest. This might help blunt oxidation of amino acids and allow for higher net-protein balance in the body.
Journal of the International Society of Sports Nutrition 2018: How much protein can the body use in a single meal for muscle-building? Implications for daily protein distribution[review article; ungraded] ↩
This study showed that “whole body net anabolic response” was higher with 70 grams of protein than with 40 grams of protein, consumed in the context of a mixed meal.
American Journal of Physiology. Endocrinology and Metabolism 2016: The anabolic response to a meal containing different amounts of protein is not limited by the maximal stimulation of protein synthesis in healthy young adults[randomized trial; moderate evidence] ↩
Muscle protein breakdown is part of our normal physiology, typically increasing in the fasted state and decreasing in the fed state.
American Journal of Physiology. Endocrinology and Metabolism 2016: The anabolic response to a meal containing different amounts of protein is not limited by the maximal stimulation of protein synthesis in healthy young adults[randomized trial; moderate evidence] ↩
It’s not entirely clear why muscle protein breakdown decreases with larger amounts of ingested protein. Some postulate that it’s mainly the insulin response to dietary carbohydrate that would suppress muscle protein breakdown, because insulin is known to cause uptake of amino acids by lean tissue and suppress protein breakdown. However, the study below showed similar insulin responses between the moderate-protein (40g) and high-protein (70g) groups, and the high-protein group clearly showed a greater decrease in muscle protein breakdown.
The authors suggest that, as blood–amino acid levels rise after a high-protein meal, these amino acids will be driven into cells. Once muscle protein synthesis is maxed out, these high intracellular–amino acid levels will then serve to suppress muscle protein breakdown. This is a plausible mechanism, but we don’t know if it is the correct one.
American Journal of Physiology. Endocrinology and Metabolism 2016: The anabolic response to a meal containing different amounts of protein is not limited by the maximal stimulation of protein synthesis in healthy young adults
[randomized trial; moderate evidence] ↩
Although, as you will see in the next section, there are ways to increase the amount of protein that can be used for muscle protein synthesis when consumed as a shake. Read on to learn more. ↩
It is also worth noting that nitrogen balance was more positive in the “pulse” group that ate most of their protein in one meal. In this study, protein turnover, synthesis, and breakdown were all higher in the pulse group. The significantly higher protein balance was determined to be mostly due to higher muscle protein synthesis. The target protein amount for both groups was 1.05 g/kg/day.
A similar study done with young women by the same authors showed no difference in protein retention between the two groups, suggesting that pulse feeding is at least no worse than spread feeding.
Both the time-restricted feeding and normal-diet groups showed similar increases in cross-sectional area of leg and arm muscles. Interestingly, effect-size data suggested greater improvements in strength with time-restricted feeding.
The time-restricted feeding group wound up consuming only 1.0 g/kg/day of protein, compared to 1.4 g/kg/day in the normal diet group. This was likely due to lower overall calorie intake with time-restricted feeding. It’s possible that lower protein intake influenced the effect-size data, which showed a trend toward greater lean-mass gains with a normal diet.
Both groups ate around 150 grams of protein per day (1.9 g/kg/day), but the time-restricted feeding group consumed all that protein within 8 hours, further calling into question concerns about exceeding 20 to 35 grams per meal for the purpose of muscle retention.
Journal of Translational Medicine 2016: Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males[randomized trial; moderate evidence] ↩
Physiological Reports 2016: The response of muscle protein synthesis following whole-body resistance exercise is greater following 40 g than 20 g of ingested whey protein[randomized trial; moderate evidence]
A similar study found a non-significant 10% increase in muscle protein synthesis with 40 grams vs. 20 grams of egg protein. But that study performed leg-only exercise, suggesting that the amount of muscle activated in the body may determine how much of an ingested protein load can be used for muscle protein synthesis.
American Journal of Clinical Nutrition 2009: Ingested protein dose response of muscle and albumin protein synthesis after resistance exercise in young men[randomized trial; moderate evidence] ↩
Without comparing an intermittent fasting group that exercises with an intermittent fasting group that doesn’t exercise, we can’t know if exercise will have an additive effect with a higher protein diet. Given the other potential benefits of exercise, though, we strongly recommend it.