There’s Nothing Magical About Intermittent Fasting

Photo: erdikocak/Getty Images

In a blow to the latest dietary darling, a recent study found that people who practiced time-restricted eating (also known as intermittent fasting) didn’t experience any significant weight loss compared to a control group. The paper undermines one of the most popular and seemingly simplest diet and optimization fads of the past decade — eat whatever you want during a specific time window, and you’ll lose weight, achieve mental clarity, and simplify your life.

“It seemed like the ideal intervention,” says Ethan Weiss, MD, lead author of the new study and an associate professor at the University of California, San Francisco Cardiovascular Research Institute. “It was relatively easy to do, it didn’t require any fancy adherence to specific diets, it didn’t require calorie counting, it didn’t require you to track macros — just change the time you eat.”

The problem is, according to Weiss’s research, it doesn’t work.

The study was conducted in 116 people who were overweight or obese based on their BMI and wanted to lose weight. Half of the participants were instructed to eat three square meals a day but could snack in between (the control group), while the other half practiced the 16:8 time-restricted eating regimen, meaning they fasted for 16 hours a day and ate as much as they needed between 12:00 p.m. and 8:00 p.m. Both groups could eat whatever they wanted during the trial, the only thing that differed was the time at which they ate. At the end of the three-month study, the intermittent fasters lost an average of two pounds, while the control group lost roughly a pound and a half.

“There wasn’t really much weight loss at all,” says Weiss, who was a time-restricted eater himself before the results came out. “We were very surprised and, frankly, somewhat disappointed.”

Other intermittent fasting researchers say the study’s findings should be taken with a grain of salt, and there’s still reason to believe in the diet’s benefit.

“This study really sticks out as not working for some reason,” says Krista Varady, PhD, a professor of nutrition at the University of Illinois, Chicago who’s published multiple papers on intermittent fasting, including several showing positive results.

Some earlier trials by Varady and others demonstrated significant weight loss — as much as two to four percent of a person’s body weight — while other studies show improvements in body composition or metabolic markers such as cholesterol and blood glucose but no change in weight.

Varady says that one possible reason for the discrepancy is that in the new trial, the participants did the intervention at home without coming into a study center for regular checkups. “This study is kind of showing if somebody just buys a book or reads something on the internet, can they do it on their own,” she says.

Intermittent fasting is no more effective for weight loss than eating throughout the day

Weiss says the key difference in his study is a strong control group. A sticky question in dietary research is whether being in a nutrition study in and of itself is motivating for people to lose weight, regardless of what the diet is. Most of the previous intermittent fasting trials that resulted in significant weight loss weren’t run against a control condition to show that fasting was superior to other interventions (or lack thereof). Consistent with this theory, in the new study, the intermittent fasters did lose some weight, but they didn’t lose significantly more weight than the people eating three meals a day.

“We [tried] to isolate the one variable that we thought was important here, which was the window of time that people ate,” says Weiss. “We said, ‘Eat what you normally eat, don’t change anything else except when you eat.’ What we learned is that intervention in, I think, a very rigorously designed study was found to be ineffective.”

However, Varady and others say the instructions to the control group were too strict and could have ended up being a dietary intervention on its own.

“The control group also lost weight, and I think that’s because the control group was given a regimen where they were told to have three meals a day,” Varady says. “Research shows that people don’t eat in three meals a day; people tend to eat every two hours. There’s no set meals. So I think by forcing the control group to eat three meals a day, they were probably eating less food, and that’s why they lost weight.”

“My view of it is that it’s all the weight loss. I think people lose weight with intermittent fasting because it fools the body into eating less food.”

The study participants also received text messages to remind and encourage them throughout the trial. For the intermittent fasting group, these were about the times they should eat or not eat during the day. For the control group, they were more generalized messages about healthy eating, like “fruits and vegetables are healthy snacks” or “start your day with a healthy breakfast.”

“It may not have been a perfect control group because it looks like the control group not only got advice to change their meal timing habits, but also inadvertently got some advice to eat healthier,” says Courtney Peterson, PhD, an associate professor in the Department of Nutrition Sciences at the University of Alabama at Birmingham. “So, unfortunately, although they didn’t intend it, the control group did get advice to eat healthier.”

It’s unclear whether eating regular meals was a deviation for the control group, or if receiving healthy eating prompts changed their behavior. Weiss didn’t collect information on what or how much both groups ate during the study or what their eating patterns were before joining, only whether or not they adhered to their assigned diet in the trial. Even if the control group was less of a control and more of a general healthy eating intervention, though, there still was no additional weight loss from intermittent fasting. And, really, there was not much weight loss from either way of eating. In other words, practicing how doctors and dietitians have recommended people eat for years — real meals consisting of real, healthy foods — was as effective as intermittent fasting, a fairly substantial change in behavior.

Positive metabolic effects from fasting are probably due to weight loss

What about the other metabolic benefits often attributed to intermittent fasting? Much of the prior research has suggested that intermittent fasting improves people’s glucose control and cholesterol levels and can even help manage metabolic diseases, although a few studies have found negative effects on people’s insulin sensitivity and blood pressure. The current study, however, saw virtually no changes at all.

To assess this, 50 people enrolled in the trial came into the lab for further testing, including body composition, energy expenditure, blood lipids, glucose, insulin, and cholesterol levels. There were no changes either within or between the groups on any of the measurements, except for a loss of lean mass in the intermittent fasters, which could be a concerning sign of muscle loss. (Weiss says he wouldn’t make too much of this finding.)

Despite the disappointing results, at the end of the day, none of the scientists, including Weiss, are ready to give up on intermittent fasting just yet, although they say the new study has provided a needed reality check.

Varady says she isn’t surprised by these lack of changes because the metabolic benefits that often accompany intermittent fasting are typically the result of weight loss, of which there wasn’t much. “My view of it is that it’s all the weight loss. I think people lose weight with intermittent fasting because it fools the body into eating less food,” she says. “And then, if you lose weight, you’re going to see decreases in blood pressure and cholesterol levels. So my opinion is that we just see those metabolic effects because of the weight loss.”

Another possibility is that the time of day when someone eats or fasts matters. Other studies looking at intermittent fasting see metabolic benefits in people who eat breakfast and lunch but skip dinner. For example, in a cross-over control study (meaning all participants took part in both the experimental and the control arm at different times), Peterson found that time-restricted eating improved insulin sensitivity, blood pressure, and other metabolic markers in men with pre-diabetes, even without any weight loss. However, the men only ate between the hours of 7:00 a.m. and 3:00 p.m.

“We’ve actually known for about 50 years that in most people, their blood sugar control is better in the morning and it’s worse later in the day,” she says. “Your body is better at producing and releasing insulin early in the morning. Your cells are better at taking up the glucose that’s in your bloodstream. There are a bunch of things that are all lining up pointing to morning is better” for eating and evening is better for fasting.

Despite the disappointing results, at the end of the day, none of the scientists, including Weiss, are ready to give up on intermittent fasting just yet, although they say the new study has provided a needed reality check.

“I wouldn’t say that it killed the field or that we think it’s definitive. I would say it raises some uncertainties,” Peterson says. “But there are enough other studies that don’t find such negative results, and I don’t think this study just negates them.”

“The good thing about the study,” she adds, “is I think it tempered some of the unrealistic enthusiasm.”

Health and science writer • PhD in 🧠 • Words in Scientific American, STAT, The Atlantic, The Guardian • Award-winning Covid-19 coverage for Elemental

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