Does Intermittent Fasting Cause Muscle Loss?

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Is it true that intermittent fasting leads to muscle loss?

 Is it true that intermittent fasting leads to muscle loss?

The medical community is evaluating the potential drawbacks of intermittent fasting as a weight loss strategy. Intermittent fasting has raised questions about whether it causes a loss of lean body mass, specifically muscle tissue.

Why is the answer to this question so important to people? Even though the research on intermittent fasting is still in its early stages, we do know that people who lose weight by restricting calories for a long time lose one-fourth to one-third of their weight as lean tissue.

We all know that the best way to lose weight is to lose fat while keeping as much muscle as possible. As a result, if intermittent fasting causes equal or greater muscle loss than chronic caloric restriction, intermittent fasting would be a less useful tool in the weight loss toolkit.

This guide delves into the debate, examines what happens in the body during short-term fasting, and synthesizes clinical trial data on body composition and intermittent fasting. Finally, we'll discuss whether or not intermittent fasting is likely to result in muscle loss.


What is intermittent fasting, and how does it work?

Intermittent fasting is a broad term that refers to depriving oneself of calories for short periods of time. Check out our guides to time-restricted eating and one meal a day if you want to learn more about different types of intermittent fasting.

This guide focuses solely on short-term fasting, also known as time-restricted eating, for 16 to 24-hour windows several times per week. However, we will refer to intermittent fasting as "intermittent fasting." We will specifically avoid any discussion of fasting periods longer than 24 hours, as the majority of current research on intermittent fasting focuses on fasting periods of less than 24 hours.

While there is a body of literature on what happens to body composition with chronic starvation (days to weeks of calorie deprivation), we excluded it because longer fasts cause very different changes in the body than short, intermittent fasts. As a result, chronic starvation research is almost entirely irrelevant to the current debate.


What is the source of the debate over whether intermittent fasting causes muscle loss?

When it comes to intermittent fasting as a weight loss tool, there's debate over whether the effect is due solely to calorie restriction or whether short fasts flip a metabolic switch, shifting the body from sugar to fat burning mode.

If intermittent fasting is simply a clever way to cut calories without counting them, it could be a more convenient and long-term weight loss option than chronic caloric restriction, which is difficult to maintain and generally ineffective over time.

The disadvantage of this simple explanation is that intermittent fasting would have the same disadvantage as caloric restriction: one-third to one-quarter of the weight lost would be lean tissue.

Intermittent fasting, on the other hand, would be an ideal weight management strategy if it causes weight loss by preferentially encouraging the body to mobilize fat stores for energy while preserving lean tissue. Many people have lost sight of a third possibility while trying to figure out the "right" explanation for why intermittent fasting helps with weight loss: both explanations can be correct.

When we break a fast, we don't always "make up" for the calories we didn't get while fasting, resulting in calorie restriction. Also, when fasting — especially on a low-carb diet — the liver's glycogen stores are depleted within the first 24 hours, causing fatty acids from fat stores to be mobilized for use as energy.

To return to the question of whether intermittent fasting causes lean body mass loss, we need to learn more about how the body obtains energy when a person goes without food for 16 to 24 hours.

Is it true that the body uses mostly glycogen and fatty acids from fat stores, sparing muscle tissue? Or does it break down muscle to allow the liver to use amino acids to produce new glucose? Continue reading to find out.


Understanding the benefits of fasting for a short period of time

The debate over intermittent fasting and muscle loss has a major flaw in that it focuses too much on how the body reacts during long-term fasting. This is due to the fact that much of the literature in this field includes studies of long-term fasting. However, if you're only fasting for up to 24 hours at a time, you're only interested in what happens in the 24 hours or so leading up to the fast and during the fast itself, not what happens over several weeks without food.

We must extrapolate from the existing — and less specific — literature because current scientific evidence does not provide us with hour-by-hour details of how our bodies use and store energy in the first 24 hours of a fast.

That's quite difficult to do; it's like trying to navigate Manhattan using a general map of the state of New York. You'll end up making a lot of "best-guess" navigational decisions, mostly based on your position on the water.

Given the current state of science, we must focus on the most relevant data and extrapolate where necessary to describe what happens in the body during a 16- to 24-hour fast.

The first point to discuss is how the body uses energy during a fasting period of 16 to 24 hours. During that time, the liver is converting a significant portion of its glycogen to glucose, which the rest of the body can use for energy. As glycogen stores deplete, the body must seek alternative energy sources.

The body's options for obtaining energy at this point are:

  • Gluconeogenesis: The liver can produce glucose from amino acids, lactate, or glycerol, which can then be used for energy.
  • Fatty acids can be released from fat tissue and metabolized into ketones, which can be used for energy immediately.
  • Muscle: Muscle can be broken down into amino acids and sent to the liver if there aren't enough amino acids available for gluconeogenesis.

It turns out that as glucose production decreases due to decreased glycogen stores, the rate of gluconeogenesis does not increase in the first 16 to 24 hours of fasting. To be clear, gluconeogenesis is continuing at its normal rate, but it is not increasing.

In other words, when glycogen stores are low, breaking down extra muscle to obtain amino acids for energy is not the first step. Rather, fat is metabolized into ketones, and ketones are used for energy.

Some amino acid release from skeletal muscle is normal after an overnight fast. A 154-pound (70-kg) man, for example, will release enough amino acids from muscle to allow the liver to produce about 40 grams of glucose through gluconeogenesis. However, the liver will release 150-400 grams of glucose per day during the first few days of fasting. As a result, muscle's contribution to overall fasting glucose production (40 grams) is minimal.


Intermittent fasting's effect on muscle

We presented evidence in the previous section that suggests muscle breakdown for energy does not increase significantly when fasting for 16 to 24 hours. While that study is intriguing, we'd like to see more controlled studies that look at what happens to lean body mass in people who fast intermittently.

While there is a growing body of evidence, it is difficult to apply the findings of the studies to people who fast intermittently because much of the evidence has significant flaws.

The first major review of medical literature examining the effect of intermittent fasting on body composition, to our knowledge, was a 2011 paper, which found intermittent fasting was superior to daily caloric restriction for preserving lean mass. Weight loss achieved through caloric restriction was 75% fat and 25% muscle, whereas weight loss achieved through intermittent fasting was 90% fat and 10% muscle.

In 2015, a large systematic review was published to examine the literature. Unfortunately, the scope of this large review was expanded to include a variety of forms of intermittent energy restriction that bear no resemblance to how intermittent fasting is actually done. With that limitation in mind, the researchers discovered that intermittent energy restriction had roughly equal chances of preserving or losing lean mass.

A 2018 review article that looked more specifically at trials of time-restricted feeding and alternate-day fasting was more relevant to the current practice of intermittent fasting (including alternate-day modified fasting). Three out of ten alternate-day fasting trials showed some loss of lean mass, while the four time-restricted feeding studies showed no loss.


Muscle loss can be avoided by eating enough protein.

When it comes to preserving lean body mass, clinical trials with protocols similar to contemporary intermittent fasting show that intermittent fasting is either the same as or better than caloric restriction. However, some studies have found that intermittent fasting causes more lean-tissue loss, raising the question of what can be done to mitigate this risk.

Protein intake is well known in the general medical literature, as well as the weight loss literature, when it comes to preserving or gaining muscle while losing fat. Diets with a moderate to high protein content will almost always preserve more muscle mass than diets with a low protein content.

Intermittent fasters may be concerned that getting enough protein during their eating window will be too difficult. Furthermore, there is a misconception that we can't absorb or use more than 20 to 35 grams of protein per meal, and that any protein consumed above that amount is oxidized (and unavailable for muscle protein synthesis).

To address those concerns, we should first point out that eating a higher protein diet while doing intermittent fasting is absolutely possible with the right dietary approach.

Next, we must disprove the "muscle-full" hypothesis, which holds that there is a limit to how much protein we can consume in a single meal. Several studies have consistently shown a maximum amount of muscle protein synthesis after ingesting 20 to 35 grams of protein, supporting this hypothesis.

The muscle-full hypothesis has two significant flaws. The first is that it is illogical from an evolutionary standpoint. Our forefathers and mothers most likely ate a lot of animal protein right after a kill, followed by a period of less protein when food was scarce. If only a small portion of the meat they ate could be used to build and preserve muscle, they would not have been well-adapted to survive.

Of course, we can't prove evolution, so it's a weak argument.

The second flaw, on the other hand, can be demonstrated through a more thorough review of the medical literature.

The studies that show that muscle protein synthesis peaks at 20 to 35 grams of protein use a pure protein shake (usually whey), which is often consumed on an empty stomach by the subjects. When we look at studies where the subjects are fed whole foods, we see that higher protein intake — well above 20 to 35 grams — leads to more significant muscle gains.

What causes this to happen? To begin, it's important to realize that muscle isn't the only tissue in the body that makes proteins from dietary amino acids. Proteins are produced by the digestive system and can be used by the body.

What causes this to happen? To begin, it's important to realize that muscle isn't the only tissue in the body that makes proteins from dietary amino acids. Proteins are made by the digestive system and can be broken down and released into the bloodstream long after a meal, where they can be used by tissues like muscle to make protein.

Second, studies that focus solely on muscle protein synthesis overlook the fact that muscle protein breakdown is just as important in determining the overall effect on muscle tissue as muscle protein synthesis. (In other words, muscle is lost and gained at the same time.)

The suppression of muscle protein breakdown is even greater than the increase in muscle protein synthesis when you eat more protein. When dietary protein intake rises, muscle mass rises as a result of the increased ability to build muscle and the even greater ability to break down muscle.

To summarize, the body can "use" more protein when it comes from a whole-food meal rather than a protein shake. Furthermore, we must consider the net effect of muscle protein synthesis and muscle protein breakdown when predicting what happens to muscle mass during intermittent fasting.

While the rate of muscle protein synthesis slows as protein intake increases, the rate of muscle protein breakdown slows even more, resulting in a net positive protein balance.


Muscle loss is prevented by exercise.

We now know that eating a moderate-to-high protein diet during intermittent fasting helps to maintain or build muscle. It's also common knowledge that a variety of exercises can help you lose weight and improve your body composition. Is there any evidence that combining exercise and intermittent fasting helps to maintain lean body mass?

Before we get into the clinical evidence, it's important to understand that exercising our muscles makes them more receptive to amino acid absorption, and that more protein means more muscle mass gains. After whole-body resistance training, subjects were given either 20 grams or 40 grams of whey, and the 40-gram dose resulted in a 20% increase in muscle protein synthesis.

In every study that looked at the effect of intermittent fasting plus resistance training on lean body mass, lean tissue was either preserved or increased. This finding suggests that combining resistance training with intermittent fasting may be more effective at retaining muscle mass than simply increasing protein intake.

Studies of intermittent fasting that compare subjects who exercise to subjects who don't would be beneficial. These trials are, hopefully, in the works and will soon be added to the body of medical knowledge.


Muscle loss is not caused by intermittent fasting.

You shouldn't worry about losing muscle with short-term intermittent fasting if you eat a high-protein diet. Furthermore, eating a high-protein diet and combining intermittent fasting with resistance training appears to help muscle preservation.

We acknowledge, however, that long-term data in this area is limited. We're also worried that doing intermittent fasting "too often" could lead to muscle loss, because longer duration and higher frequency fasting are similar to chronic calorie restriction.

Because one-quarter to one-third of the weight lost with chronic caloric restriction is muscle, it's reasonable to wonder if intermittent fasting, done for too long or too frequently, might have the same effect. Moderation is key: if you practice intermittent fasting for less than 24 hours several times per week, you'll likely see a positive change in body composition without losing significant muscle mass.






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