Intermittent fasting is one of the most popular diets these days.
There are several different types, but what they have in common is fasts that last longer than a normal overnight fast.
While research has shown that this can help you lose fat, some worry that intermittent fasting may also cause muscle loss.
This article tells you everything you need to know about the effects of intermittent fasting on your muscles.
Although intermittent fasting is very popular, there is sometimes confusion about what it actually is.
Time-restricted eating (also known as time-restricted feeding) restricts all calories to a certain number of hours each day.
This may range from 4–12 hours, but an 8-hour eating period is common.
As the name implies, alternate-day fasting consists of alternating between fasting days and non-fasting days. This means you fast every other day.
While some people eat nothing on the fasting days (true fasting), it is more common to have one small meal on the fasting day (modified fasting).
Periodic fasting (also known as whole-day fasting) consists of occasional fasts, separated by days or weeks of normal eating.
While exact definitions vary, programs involving fasting for one or more days every 1–4 weeks are often considered periodic fasting.
The 5:2 Diet
The popular 5:2 diet is very similar to alternate-day and periodic fasting.
It involves eating normally for five days per week and eating about 25% of your normal amount of calories two days per week (2).
The very low-calorie days could be considered a form of modified fasting, particularly if you only consume one meal.
Many different religions have regular periods of fasting.
Examples include the month of Ramadan observed by Muslims and various fasts associated with Orthodox Christianity (3).
Summary There are several different types of intermittent fasting, such as time-restricted eating, alternate-day fasting, periodic fasting, the 5:2 diet and religious fasting. While they have some common features, the specific programs vary widely.
It is important to realize that without exercise, weight loss will usually come from a loss of both fat mass and lean mass. Lean mass is everything besides fat, including muscle (4).
This is true of weight loss caused by both intermittent fasting and other diets.
Because of this, some studies have shown that small amounts of lean mass (1 kg or 2 pounds) may be lost after several months of intermittent fasting (1).
In fact, some researchers believe that intermittent fasting may be more effective for maintaining lean mass during weight loss than non-fasting diets, but more research is needed on this topic (7).
Overall, it is likely that intermittent fasting will not cause you to lose more muscle than other weight loss diets.
Summary When you lose weight, you typically lose both fat mass and lean mass, especially if you do not perform regular exercise. Intermittent fasting does not appear to cause more muscle loss than other weight loss diets.
There is very limited research on whether or not it is possible to gain muscle during intermittent fasting.
This is likely because weight loss is the topic of interest in most studies on these diets.
However, one study of intermittent fasting and weight training provides some preliminary information about muscle gain (8).
In this study, 18 young men completed an 8-week weight training program. They had not previously performed weight training on a regular basis.
The men followed either a normal diet or a time-restricted eating program. The program required them to consume all their food in a 4-hour period on 4 days each week.
By the end of the study, the time-restricted eating group had maintained their lean body mass and increased their strength. However, the normal diet group gained 5 pounds (2.3 kg) of lean mass, while also increasing their strength.
This could mean that intermittent fasting is not best for muscle gain. This may be because the time-restricted eating group consumed less protein than the normal diet group.
There are a few other scientifically based reasons why intermittent fasting may not be optimal for gaining muscle.
Intermittent fasting could make it difficult to get enough calories to build muscle, especially if you are eating nutrient-dense foods that fill you up easily (12).
Additionally, you may have to make a bigger effort to get enough protein when eating less often than with a normal diet.
All of these reasons don’t necessarily mean that it is impossible to gain muscle with intermittent fasting, but that it may not be the easiest diet for gaining muscles.
Summary Intermittent fasting requires you to eat fewer calories and eat less frequently than a normal diet. Because of this, you may have trouble getting enough calories and protein to build muscle. Overall, this may not be the best diet for muscle gain.
Research has shown that weight training can help prevent muscle loss when you are losing weight (15).
One 8-week study examined the combination of intermittent fasting and weight training three days per week (16).
The researchers split 34 men who were very experienced with weight training into two groups: a time-restricted eating group (consuming all calories in 8 hours per day) and a normal diet group.
Both groups were assigned the same number of calories and amount of protein each day, and just the timing of the meals differed.
By the end of the study, neither group had lost lean mass or strength. However, the time-restricted group lost 3.5 pounds (1.6 kg) of fat, while there was no change in the normal diet group.
This shows that weight training three days per week may help maintain muscle during fat loss caused by intermittent fasting.
Other research on alternate-day fasting has shown that 25–40 minutes of exercise on a bike or elliptical three times per week can help maintain lean mass during weight loss (17).
Summary Weight training during intermittent fasting may help you maintain muscle, even when losing fat. Other forms of exercise, such as using a stationary bike or elliptical, may also be beneficial.
Even among those who use intermittent fasting, there is debate about whether or not to exercise when you are fasted. Several studies have also looked into this.
One 4-week study followed 20 women performing fasted versus non-fasted exercise on a treadmill. Participants exercised three days per week for one hour per session (18).
Both groups lost the same amount of weight and fat, and neither group had a change in lean mass. Based on these results, it may not matter whether or not you exercise fasted if your goal is weight loss.
However, it is possible that training fasted could impair your exercise performance, particularly for serious athletes (19).
Overall, it seems that exercising while fasted may be a matter of personal preference.
It will probably not make your exercise more effective, and it’s even possible that exercising fasted will decrease your performance.
However, some people enjoy exercising fasted. If you choose to do this, it is recommended that you get 20+ grams of protein soon after exercising to support muscle recovery (19).
Summary Exercising while fasted is probably not more beneficial than exercising at other times. In fact, it is possible that it could decrease your performance. For most people, whether or not to exercise fasted is a matter of personal preference.
If you choose to use intermittent fasting as a tool for weight loss and health, there are several things you can do to maintain as much muscle as possible.
As discussed, exercising — especially weight training — can help maintain muscle. A slow and steady rate of weight loss may also help.
Research has shown that you are more likely to lose lean mass, including muscle, when you lose weight quickly (20).
This means that if you are performing intermittent fasting, you should try to not drastically reduce your calorie intake all at once.
While the ideal rate of weight loss may vary, many experts recommend 1–2 pounds (0.45–0.9 kg) per week. However, if preserving muscle is your top priority, you may want to shoot for the lower end of this range (21, 22).
In addition to the rate of weight loss, the composition of your diet can play an important role in maintaining muscle during intermittent fasting.
Regardless of which type of diet you follow, getting enough protein is important. This is especially true if you are trying to lose fat.
It is possible that adequate protein intake is particularly important when using intermittent fasting, since your body will go for longer periods of time without receiving nutrients (1).
Summary Important nutritional strategies that may help you maintain muscle during intermittent fasting are attempting a slower rate of weight loss and ensuring adequate protein intake. Choosing nutritious foods is also recommended.
If you are trying to maintain or gain muscle during intermittent fasting, some dietary supplements may be helpful.
However, you need to consider when you want to take the supplements, as this could interfere with the results of your fasting.
Supplements During Your Feeding Periods
Two of the most important supplements to consider are protein and creatine.
While protein supplements aren’t necessary if you get enough protein from foods, they can be a convenient way to ensure that you get enough.
Especially if you are physically active, protein supplements may help improve muscle size and exercise performance (26).
In addition to protein, creatine supplements can support your muscles.
Creatine is a molecule that is found naturally in your body. You can increase the amount of creatine in your cells via dietary supplements (27).
Supplements During Your Fasting Periods
You may wonder if you should take protein, creatine or other supplements such as BCAAs during your fasting periods. This is primarily due to a concern that these periods will negatively affect your muscles.
What’s more, some of the health benefits of intermittent fasting are probably due to the fact that your body is not receiving any nutrients (30).
This mild stress on your body may strengthen it to fight off larger threats, such as disease, in the future (31).
Additionally, if you get enough protein in your feeding period, fasting for 16 hours does not appear to be detrimental to your muscles, compared to a normal diet (16).
Overall, it is unlikely that you need to take dietary supplements during your fasting periods. Some supplements, like creatine, may even be more beneficial when taken with food (33).
Summary Taking dietary supplements during your fasting period is not necessary. However, protein and creatine supplements may support muscle mass. These can be taken during the feeding periods of your intermittent fasting diet.
Intermittent fasting is a popular dietary strategy that uses fasting periods longer than a typical overnight fast.
There are several different types of intermittent fasting, including time-restricted eating, alternate-day fasting, periodic fasting, the 5:2 diet and religious fasting.
Intermittent fasting probably does not cause more muscle loss than other weight loss diets.
Nevertheless, adding exercise — especially weight training — to your intermittent fasting program can help you maintain muscle.
However, whether or not you exercise during fasting periods is up to you. Fasting probably does not add benefits, and it could compromise your optimal exercise performance.
Aiming for a slow rate of weight loss and consuming enough protein can help you maintain muscle during intermittent fasting.