Intermittent Fasting Doesn’t Work, Research Says

Intermittent Fasting Doesn’t Work, Research Says

The nutrition world was rocked last week by the publication of a new study, which concluded that intermittent fasting (IF) is not very effective for weight loss. What’s worse, the results suggest that it may exacerbate the loss of lean muscle tissue.

Intermittent fasting has been a topic of intense public fascination for several years now. I give workshops, lectures, and interviews on a wide variety of nutrition topics, to a lot of different kinds of audiences: physicians, nutrition and fitness professionals, senior citizens, college students, parents, and the popular media. And no matter what I’m talking about or who I’m talking to, as soon as we open up the floor or the phone lines, we inevitably get questions about intermittent fasting. Is it effective? Is it safe? What does the research say?

Is intermittent fasting effective? Is it safe? What does the research say?

So it was not surprising that this study caused such a splash. Despite the breathless headlines, though, the study didn’t actually change what we already knew about intermittent fasting and weight loss. And the new finding on muscle loss confirmed a suspicion or concern that many had already raised.

Why is intermittent fasting so popular?

The allure of intermittent fasting is understandable. The premise is that we don’t actually have to change what we eat or even how much we eat. We can lose weight simply by changing when we eat it. 

There are a few different ways that IF can be practiced. One of the most popular protocols (and the one used in this latest study) is a restricted eating window. Instead of spreading your daily meals over the course of 12 or 14 hours, you shorten that eating window to 8 or 10 hours. For example, instead of eating breakfast at 7 a.m., lunch at 1 p.m., and dinner at 7 p.m., you might eat all your meals between the hours of noon and 8 p.m.

When researchers tried intermittent fasting in rats, it worked like gangbusters.

When they tried this in rats, it worked like gangbusters. Researchers gave the rats a high-fat diet and let them eat as much as they wanted. Not surprisingly, this led the rats to gain weight. But when they gave them the same diet and let them eat as much as the wanted for only 8 hours a day, they didn’t gain weight. In fact, the rats that started out overweight actually lost weight. 

It seemed that the extended fasting period did something to the rats’ metabolism or hormones that caused them to either burn more calories or store less fat. If the same were true for humans, it would mean that, as long as we kept our mouths shut for 12 to 16 hours a day, we could eat all the pizza, cheeseburgers, French fries, and ice cream we wanted and not gain weight. Sign me up!

Does it work in humans?

Unfortunately, as is so often the case, it didn’t seem to work as well in humans. Simply restricting food intake to a shorter window did not seem to change the rate at which humans burn calories or store fat. It did sometimes lead to modest weight loss but this was due to the fact that people following this schedule simply ended up eating fewer calories. No metabolic magic there.

In my experience, the benefits of a restricted eating window are purely behavioral. When you limit the number of hours a day that you eat, you often end up eating less, which leads to weight loss. If this turns out to be an easier or more comfortable way for you to limit your food intake, then this could be a very successful long-term strategy for you.  I’ve certainly heard that from many people that I work with. But if you don’t eat less, you probably won’t lose weight.

Simply restricting food intake to a shorter window did not seem to change the rate at which humans burn calories or store fat.

There is one possible exception. To the extent that there is any metabolic magic in our meal timing, it seems to hinge on eating our calories earlier in the day. So, instead of eating between noon and 8 p.m., you would eat from 8 a.m. to 4 p.m. and then fast until the next morning. This is significantly less popular, for obvious reasons.

And that’s why the authors of this latest study made their eating window from noon to 8 pm.

“We chose the 12P-8P because that is what seemed to fit best with how we live our lives…” writes researcher Ethan Weiss. “Dinner is a much more social meal than breakfast. We assumed that people would have a harder time fasting through dinner.”

The study involved 118 subjects over the course of 12 weeks. The subjects included both women and men, and they ranged from slightly overweight to obese. One group was told to eat three structured meals per day, following the typical breakfast, lunch, dinner pattern. The other was instructed to eat only between the hours of noon and 8 p.m. Importantly, neither group was instructed to change what or how much they were eating. 

“We wanted to test a real-world and simple prescription,” explained Weiss. “One of the most attractive things about [time-restricted eating] was that it was easy to do. Anyone can do it. There is nothing complicated about it.”

This was such a surprise to Dr. Weiss that, after several years of practicing time-restricted eating himself, he started eating breakfast again.

After 12 weeks, both groups had lost a very small amount of weight. But there was no significant difference between the groups. In other words, a restricted eating window, in and of itself, is not a terribly effective weight loss strategy. It offers no advantage over a more traditional eating pattern.

The other surprising finding was that, even though neither group lost much weight, the group following the time-restricted eating regimen lost mostly lean muscle.
This was such a surprise to Dr. Weiss that, after several years of practicing time-restricted eating himself, he started eating breakfast again.

How fasting affects lean muscle

I’ve talked quite a bit in previous episodes about muscle protein synthesis, the process by which dietary protein is used to build and maintain lean muscle tissue in our body.

We can maximize muscle protein synthesis by consuming 25-35 grams of protein at one meal. Here in the U.S., we are likely to ring that bell only once a day, at the evening meal. But by hitting that threshold two or even three times a day, we can potentially stimulate more muscle protein synthesis, and this can help us maintain more lean muscle. For most people, this means making a concerted effort to increase protein intake at breakfast and/or lunch.

RELATED: How to Build More Muscle with Less Protein

If your eating window is only 8 hours long, it may be harder to fit in multiple protein peaks and total muscle protein synthesis may suffer as a result. So there’s a trade-off here. A restricted eating window may make it easier for you to eat fewer calories but might make it harder to maximize muscle synthesis. 

If you find intermittent fasting to be a useful strategy for weight management, you can potentially mitigate this risk by paying a little extra attention to the amount and timing of your protein intake. If you’re only eating two meals (say, lunch and dinner) make sure that both of them include enough protein to give you a nice boost in muscle protein synthesis.

A restricted eating window may make it easier for you to eat fewer calories but might make it harder to maximize muscle synthesis.

The other key way to stave off the loss of lean muscle as a result of fasting and/or weight loss is to engage in some sort of resistance training. Protein alone does not lead to muscle maintenance. You also need to use those muscles if you don’t want to lose them.

The bottom line on this latest study: The benefits of a restricted eating window appear to be behavioral, not metabolic. If it makes it easier for you to limit your food intake, intermittent fasting may be a useful long-term strategy. To minimize the loss of lean muscle, try to include two separate and adequate servings of protein each day and don’t skimp on resistance training.

Share post

There are no comments

Leave a Reply

Start typing and press Enter to search

Shopping Cart

No products in the cart.