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.
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.
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For those who prefer to know our conclusions in advance, to give context to the rest of the article, we've got you covered:
Conclusions
- 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."
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.
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.
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.
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.
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.
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.
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.
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.
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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.
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.
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.
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.
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. 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.
There are two major flaws with the muscle-full hypothesis. The first is that it doesn't make sense from an evolutionary perspective. 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.
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.
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.
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. (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.
To recap, the body can "use" larger amounts of protein when the source is a whole-food meal rather than a pure protein shake. 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.
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.
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.
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. 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.
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.