Does Intermittent Fasting and Time-Restricted Eating Cause Muscle Loss?

Intermittent Fasting

It’s a growing trend among health-minded people. More guys and gals are fasting for short periods to give their body and metabolism a break from processing food. When you eat a meal, your body has to work harder and expend more energy. Just as your muscles need a break from working out, your body needs rest time too. Fasting gives your body a respite.

In fact, preliminary studies suggest that intermittent fasting, fasting for periods of 16-24 hours, may offer health benefits. Studies show intermittent fasting helps with weight loss but also may be beneficial for cellular health. When you don’t eat for periods of time, it activates cellular mechanisms that help cells clean up the damage, a process called autophagy. Some people even refer to autophagy as a cell’s internal recycling center. It helps cells break down and reassemble damaged parts.

A variation on fasting is called time-restricted eating, where you limit the number of hours during the day when you eat. You can fast on various schedules and it doesn’t have to be a long fast.  For example, you might only eat during a window period of 10 hours and fast for the other 14. Studies suggest that this type of short-term fasting may have health benefits too. Some studies also suggest that intermittent fasting helps with blood sugar control and may lower the risk of type 2 diabetes.

One concern about fasting is that it might increase the risk of muscle loss. If you strength train and work hard to build muscle mass, you might hesitate to jeopardize your hard-earned muscle by fasting. Is there evidence that fasting leads to muscle loss?

Fasting and Muscle Loss

Whether you lose muscle when you fast may depend on how long you fast. Most research suggests that for short fasts of a few days, people maintain muscle mass. One reason is your body produces ketone bodies when you fast or restrict carbohydrates and the ketone bodies reduce muscle protein breakdown. They also supply the body with energy.

In fact, a study of people who fasted on alternate days found that they lost body fat without significant loss of muscle mass. Researchers also noted that the fasters had reduced markers of inflammation. Another 2012 study published in Nutrition Journal found that fasting one day each week reduced cardiovascular risk factors and body weight without the loss of muscle mass.

Based on current research, it doesn’t appear that intermittent fasting leads to significant muscle loss. Plus, if you’re consuming enough protein when you aren’t fasting and strength training, you’re giving your muscles a signal to grow. What would happen if you fasted for a week, as some religious groups do? You might assume the risk of muscle is higher since the stress of fasting for that long would raise cortisol and lead to greater muscle breakdown.

However, during Ramadan, people fast for 11 to 15 hours per day for an entire month. Do they experience muscle loss? A study followed 43 medical personnel during a Ramadan fast and again 4-5 weeks after. They found that the subjects lost weight during the fast but didn’t lose a significant amount of muscle. However, most regained the weight they lost within a month of the fast.

Overall, it doesn’t appear that fasting for short periods of time increases muscle loss, but can it reduce muscle gains if you strength train?

Can Fasting Reduce Muscle Gains When You Strength Train?

It’s reassuring that you won’t lose the muscle you gained from strength training from intermittent fasting. In one study, researchers asked a group of men to consume all of their calories within a 4-hour period for 4 days each week. The guys, who were untrained, also lifted weights 3 days per week, alternating upper and lower body, for 8 weeks.

The results? Subjects who restricted their calorie intake to a 4-hour window period retained their muscle mass, but the guys in the non-restricting group gained 5 pounds of muscle and gained more strength. Therefore, intermittent fasting and time-restricted eating may not be optimal for muscle and strength gains.

It’s hard to generalize from this study since they tested only a single fasting schedule–eating during a 4-hour window period. Would the results be the same if they had restricted their calorie intake to a wider window period such as 8 to 10 hours? Studies suggest that this fasting schedule also has potential health benefits. But based on the studies available, fasting may not be the optimal approach for making gaining muscle mass and strength.

Should You Fast?

The potential health benefits of fasting are compelling, although we need more humans studies. Fasting and time-restricted eating helps with weight loss, although there’s little evidence that it’s superior to calorie restriction without fasting. However, fasting appears to have benefits for heart health and may reduce inflammation based on some studies.

If you fast for the health benefits, make sure you’re consuming enough protein between fasts and keep the periods you fast short. Time-restricted eating places less stress on the body than fasting for a day or more at a time. Continue weight training. Doing so is vital for preventing muscle loss. The combination of consuming enough protein and weight training should prevent muscle loss and help you continue to make gains, although you may not gain strength or muscle mass as quickly.

The Bottom Line

The easiest way to fast is to restrict your meals to an 8 to 10- hour window period each day. This form of intermittent fasting called circadian rhythm fasting may offer the same benefits as other forms of fasting. It can be as simple as eating an early dinner and not eating again until the next day. But as encouraging as early studies are regarding the benefits of intermittent fasting, we still need more research to know whether the positive results in the laboratory and in animals translate to humans.



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