How Long Can I Fast Before I Start Losing Muscle?

Food is fuel — it's how you power through your workouts and then recover from them. So when you hear about this trend of intermittent fasting for weight loss and other health benefits, you might be suspicious. After all, isn't there some connection between fasting and muscle loss?

If you've avoided intermittent fasting because you think it will negatively affect your muscle gains or your workout performance, have no fear. There are healthy ways to fast intermittently that can offer great benefits; however, it may not be for everybody. There are a few important points to keep in mind.

Fasting is the practice of not eating or eating very little for a set period of time, sometimes done for religious reasons or for medical purposes. Then there's what's known as intermittent fasting, which Johns Hopkins Medicine explains is a popular weight-loss method that focuses less on what a person eats and more on when the person eats.

In most cases, a person will eat during a set period of hours and then fast for the rest of the day. In other cases, a person might eat normally on most days but then have two days a week when they have only one meal.

It is important to differentiate intermittent fasting from other methods of fasting for weight loss, which Beaumont Health notes will often have attributes of a fad diet. For example, a person might consume only fruit or cabbage soup, or they'll restrict themselves to fewer than 800 calories a day, often with the hopes that this extremely limited diet will help them drop pounds quickly or will somehow cleanse the body of toxins.

This kind of fasting is extremely bad for you and for your fitness or wellness efforts. Beaumont Health emphasizes that eating too few calories can make you hungrier and set you up for gaining fat in the long term. More important, prolonged fasting with few nutrients can pose such risks as electrolyte imbalance, heart arrhythmia, dizziness, dehydration, constipation, cold intolerance, decreased metabolic rate and, yes, even muscle loss.

As the American Academy of Family Physicians points out, fad diets — including very low-calorie diets — are so detrimental not only because they limit the nutrients you consume but also because they're not sustainable in the long term. People follow them for a short time and then go back to their regular lifestyle.

Intermittent fasting is different because it simply limits the times of the day that you're eating and encourages fasting for the other hours of the day — and this is a lifestyle that Johns Hopkins Medicine notes is much more in tune with the way humans have evolved to live, going for long periods of time between eating.

During this fasting stage, the body undergoes what's referred to as metabolic switching, where it runs out of carbohydrate stores to use for energy and instead starts burning fat. But with the average modern lifestyle, where most people eat throughout their waking hours, the body never has a chance to undergo this metabolic switch.

The SCNM Medical Center adds that restricting the hours when you're eating often means you'll end up eating less overall, and, provided that the foods you are consuming during your eating period are sufficiently healthy, you will see fat loss and muscle gain.

Harvard Health Publishing encourages those who are trying intermittent fasting to restrict their eating to eight to 10 hours a day and to avoid eating too late in the evening before bed. This could mean breaking their fast at 7 a.m. when they wake up and finishing their last meal by 3 p.m.

If this makes them too hungry later in the day, they could hold off on breaking their fast until 10 a.m. and finish eating their last meal at 6 p.m. This leaves a 16-hour window of time for their body to enter a fasted state. (Note that if 16 hours is too long for you to fast, you could shorten your fasting period to 12 hours or even 10 hours.)

In short, intermittent fasting isn't about starving yourself. It's about getting sufficient calories and nutrients every day but still giving your body a period of time where it can fast and not take in food.

Read more: How Intermittent Fasting Can Get You Lean

But if you're trying to eat correctly during that window of eating while doing intermittent fasting, what does that entail? What does a healthy diet for building muscle include?

One of the primary reasons good nutrition is vital to strength gains is that it's during those fed periods, as opposed to during those fasting periods, that the body has the insulin and amino acids it needs to build muscle, according to University of Utah Health.

But just because these feeding periods are when your body builds muscle, that doesn't mean short-term fasting and muscle loss are as cause-and-effect related as you might think. According to both University of Utah Health and the SCNM Medical Center, you don't need to worry about losing muscle during fasting as long as you are getting proper nutrition during your periods of eating.

And it's not all about protein. The Academy of Nutrition and Dietetics notes that while protein is important, it's not necessary to guzzle protein shakes the way some people think you must. Instead, protein should make up about 10 to 35 percent of your total calories. If you're not building muscle, then maintaining muscle may take as little as 0.37 grams of protein per pound of body weight.

USDA ChooseMyPlate recommends getting between two and six and a half 1-ounce servings of lean protein a day. A 1-ounce serving could entail 1 ounce of lean beef, chicken, turkey, fish or shellfish, or it could entail one egg, a half-ounce of nuts or seeds, 1 tablespoon of peanut butter or almond butter, a quarter-cup of cooked beans or peas, 1 ounce of tempeh or 2 tablespoons of hummus.

But as the Academy of Nutrition and Dietetics points out, you shouldn't neglect your other two macronutrients: carbohydrates and fat. Carbohydrates are important for fuel, as they are stored in your muscles as glycogen and will help power you through your workout. Half your calories should come from carbohydrates.

Finally, fat is another great source of energy, and you should get about 20 to 35 percent of your calories from fat. Just be careful about watching your portion sizes with fat, as it contains more than twice the number of calories as carbohydrates and protein.

Read more: 12 Dos and Don'ts of Intermittent Fasting

What you eat is only a small part of the process to build or maintain muscle. The Academy of Nutrition and Dietetics emphasizes that you need to exercise. You should do muscle-strengthening activities that work all your major muscle groups — your legs, hips, back, chest, abdomen, shoulders and arms — twice a week through strength-training exercises like lifting weights, using resistance bands or doing calisthenics such as push-ups, pull-ups or sit-ups.

Remember that you'll need proper nutrition to get you through these types of workouts. People who exercise in a fasted state risk worsening performance, particularly during high-intensity training, according to a January 2020 review published in the Open Access Journal of Sports Medicine.

However, the review also notes that exercising in a fasted state will increase fat breakdown in untrained people and may be good for weight loss. Still, those who exercise in a fasted state could also lose lean body mass along with fat. In this review, there is still a lot of unknowns regarding how fasting affects performance in regard to gender, trained status and performance outcomes.

However, significant lean mass loss, formally known as muscle atrophy, is more often caused by a sedentary lifestyle and not using your muscles enough, notes the Mount Sinai Health Library. In some cases, muscle atrophy can be linked to nutrition, but this is when it is due to starvation or malnutrition —that is, insufficient eating in the long term rather than for a few hours.

If intermittent fasting sounds like a lifestyle you could adhere to, the first decision you have to make is whether you're going to fast every day or twice a week. Johns Hopkins Medicine encourages people to go for the former, as eating for eight hours and fasting for 16 hours is easier to stick with over a long period of time.

If you eat regularly five days a week and then reserve two days for eating only one meal, you could get far too hungry during those two days. Remember, during your fasting periods, you can still have water or calorie-free beverages.

Read more: How to Fast One Day a Week for Weight Loss

Those who are doing a daily fast likely won't feel overwhelming hunger as long as they time their non-eating periods out properly. As University of Utah Health points out, most people can go about five or six hours after a meal before they feel significantly hungry; therefore, if you finish your last meal by 5 or 6 p.m. you might feel hungry only shortly before you go to bed.

Johns Hopkins Medicine reminds people not to binge during their eating periods and instead focus on whole grains, vegetables, fruits and lean protein. Determine how many calories your body needs every day and aim to meet that number during your period of feeding. Intermittent fasting can sometimes take a while to acclimate to, but after two weeks, a person following this lifestyle should overcome any hungriness or crankiness they feel in the beginning.

Finally, it's also important to remember that intermittent fasting is not for everyone. Children or teens under 18 should not be going for prolonged periods without nourishment, nor should women who are pregnant or breastfeeding.

Fasting may not be ideal for athletes depending on their goals, trained status and gender. And last, intermittent fasting is also not an ideal choice for people with diabetes or blood sugar problems, and it should not be undertaken by anyone with a history of eating disorders.

Read more: You Are When You Eat? How Intermittent Fasting Affects Weight Loss