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Does Intermittent Fasting Make You Lose Muscle or Fat?

Two new studies shed light on the fear that people will end up losing more muscle than they would on a regular diet.

by Christian Finn
Aug 22 2018, 5:00pm

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On paper, the benefits of intermittent fasting for weight loss sound great: You don’t need to count calories, cut out entire food groups, give up your favorite meals, or stick to a set of complicated diet rules. However, one of the things that puts people off intermittent fasting is the fear they’ll end up losing more muscle than they would on a regular diet.

Can you fast without losing muscle? Or are you going to burn through muscle tissue faster than Samson losing his strength after a trip to the barber? Here’s what the science has to say on the subject.

The story starts back in 2009, when researchers looked at the effect of alternate day fasting in a group of obese men and women. As the name suggests, alternate day fasting involves a fast day followed by a feed day. On the fast day, you don’t go without food completely. Rather, you eat lunch— 400 to 500 calories for women and 500 to 600 calories for men—between noon and 2pm. Then on feed day, you get to eat (within reason) whatever you want.

After eight weeks, participants lost an average of 12 pounds in weight. In most diet studies, you expect to see a small amount of muscle loss. But not this time, as all of the lost weight came from fat. That’s despite the fact that subjects taking part in the study made no special effort to hold on to their muscle.

Nobody lifted any weights. Nor was there any emphasis on maintaining a high protein intake, which averaged just 80 grams per day on the feed days and 24 grams on the fast days. And this wasn’t just a one off. In fact, there are plenty of other studies showing much the same thing.

A follow-up trial, published in the International Journal of Obesity, assigned a group of overweight and obese young women to either a standard or an intermittent fasting diet. After six months, there was no significant difference in the rate of fat loss between the two groups. Women who fasted lost an average of 14 pounds of fat, compared to 12 pounds in the normal dieters.


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More interesting still, the proportion of lost weight that came from fat was exactly the same in both groups. For every 10 pounds of weight lost, roughly eight pounds came from fat. The remaining two pounds came from muscle. In other words, intermittent fasting was no better or no worse than continuous calorie restriction as a way to preserve muscle while dropping fat.

There’s more. In 2016, a team of scientists from Johns Hopkins Bloomberg School of Public Health published a meta-analysis on the subject of alternate day fasting and weight loss.

A meta-analysis involves pooling the results from multiple trials on the same subject. Instead of lots of small experiments, you end up with one big experiment, conducted on lots of people.

As a result, you’re left with a conclusion that’s more reliable than anything that could have been drawn from each of the smaller studies. The researchers found no evidence to support the idea that intermittent fasting increases the risk of muscle loss. In fact, the retention of muscle mass with alternate day fasting was slightly better than it was with continuous dieting.

The main drawback with most of these studies is that they’ve used overweight and obese subjects with a relatively large amount of fat to lose. Why does that matter? For obese individuals, muscle loss doesn’t tend to be much of a problem unless they severely restrict their calorie intake. It’s only as you get leaner that the proportion of lost weight coming from muscle tissue starts to rise. That’s why we can’t extend the results from studies of overweight and obese participants to people with less fat to lose.

However, we do have some data on leaner, fitter subjects using another popular form of intermittent fasting known as the 16:8 Diet. This involves fasting for 16 hours each day, usually from 8pm to noon the following day. All your food is eaten during an eight-hour feeding window.

In short, the 16:8 method involves skipping breakfast, making lunch your first meal of the day, and eating dinner no later than 8pm.

Although this protocol was popularized by Swedish trainer Martin Berkhan back in 2007, it was almost a decade before a team of Italian researchers put the 16:8 method to the test. The men taking part in the study were mostly in their late 20s or early 30s, had been lifting weights for at least five years, and were all in reasonably good shape.

Those in the fasting group ate three meals over an eight-hour period (1pm, 4pm and 8pm). Subjects in the normal group divided their calorie intake into three meals consumed at 8am, 1pm and 8pm. Men in both groups were eating plenty of protein, as well as lifting weights three times a week.

The result? The intermittent fasting group lost around three pounds more fat than subjects in the normal group, mainly because they ended up eating fewer calories. While both groups gained a small amount of muscle, there was no significant difference in muscle growth between the two diets. Intermittent fasting didn’t slow down the rate at which muscle was built.

The bottom line? Most research shows that you don’t need to worry about losing muscle on an intermittent fasting protocol. They’re no worse, nor significantly better, than a regular diet when it comes to maintaining muscle mass while you drop fat.

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