Move over keto, there’s a new eating plan creating controversy: Intermittent fasting (IF), which Forbes named one of the hottest diets of 2020. With touted benefits from weight loss to improved insulin resistance—without a complicated regimen involving lots of calorie counting— it’s no wonder many people have gravitated towards the eating plan.

Ethan J. Weiss, MD, a cardiologist and associate professor at the Cardiovascular Research Institute at the University of California, San Francisco, tried IF after reading a few studies on it. “I lost some weight and thought it could be a great tool that would be simple and easy to do for weight loss,” he says.

But the scientist in Dr. Weiss needed more evidence. “There were scant few trials in humans, and the ones that were done were small and or uncontrolled. It was hard to know what to think,” he says. So he took matters into his own hands. His resulting study, which was published this September in JAMA Internal Medicine, is a randomized controlled trial (RCT), making it one of the most rigorous studies on IF to date. RCTs are considered the gold standard in research because participants are randomly selected to be in the test group (in this case, to follow the IF diet) to reduce the risk that participants’ or researchers’ biases impact the results.

What did his team find? IF may not actually be all that good for weight loss or your metabolism. In fact, it may even cause you to lose muscle mass.

Remind me: What is intermittent fasting?

“Intermittent fasting has created quite the buzz, although it has been around for many years,” says Lisa Moskovitz, RD, the founder and CEO of New York Nutrition Group. IF is relatively flexible with one big caveat: You can’t eat for an extended period of time. That can be for 12 or 16 hours in a day, or one or more days in a week.

The most popular type of IF is 16:8 fasting, says Moskovitz, which involves eating within an eight-hour period and then fasting for 16 hours. People often eat from 12 p.m. to 8 p.m.—which really just means they skip breakfast and don’t snack after dinner. “For some, this ‘circadian rhythm’-aligning strategy proves effective, especially if a majority of their empty-calorie eating occurs after dinner,” says Moskovitz.

Fans of IF say putting your body into a temporary fasting state revs your metabolism, increases insulin sensitivity, and boosts fat burning. Moskovitz notes there is some research, mostly in animals, suggesting these benefits. “This is largely due to the impact fasting has on insulin response in the body,” she says. Basically, during periods when you don’t eat, the body’s natural levels of insulin (the hormone that regulates blood sugar levels) drop, which forces the body to release sugar stored in its cells to use for energy.

Moskovitz adds that IF isn’t safe for everyone: People with certain serious medical conditions, those who are pregnant or breastfeeding, kids, teens, and people with a history of disordered eating patterns should avoid IF.

Here’s the full 101 on the many forms of intermittent fasting, straight from a top RD: 

How did this new study work?

Dr. Weiss’s study randomly put a total of 116 overweight or obese adult participants into one of two groups. About half were instructed to eat three meals per day, with snacks allowed, between 7 a.m. and 11 p.m. “We did not tell them how much to eat or what to eat, but it was pitched as a weight-loss study,” says Dr. Weiss. The other group was on a 16:8 IF plan, where they ate as much as they wanted from 12:00 p.m. to 8:00 p.m., then fasted until noon the next day.

A total of 105 people completed the study for a full 12 weeks. All participants received a scale to use at home and weighed themselves twice a day. Another 46 participants received four rounds of metabolic testing to check biomarkers including their insulin and fasting glucose levels.

One limitation: The participants didn’t record their exercise or the foods they ate, including their calories or macronutrients like protein. Dr. Weiss says that his team intended to include food logs but ran into a technical issue. He adds that food surveys are “notoriously inaccurate,” because they rely on the accuracy of people’s memory in order to capture what they ate (and can you even remember what you ate for breakfast last Tuesday?).  Instead, his team used a mathematical model to determine participants’ energy intake.

What did the study find?

Dr. Weiss says he was “very surprised” to find there wasn’t a big difference between the IF group and the control group, especially in terms of weight loss.

His team noted that roughly the same number of people in each group followed their plan as instructed (92 percent of the control group and 84 percent of the IF group). People on the IF eating plan had minimal weight loss that wasn’t significantly different from the control group (1.17 percent versus 0.75 percent). There also wasn’t any significant difference in other health markers between groups, including blood pressure, triglyceride levels, total cholesterol levels, fasting glucose levels, or insulin levels.

What’s more, most of the weight loss in the IF group was “lean mass” (muscle, not fat), which wasn’t the case for the regular diet group. That may be because IF participants ate less protein, says Moskovitz, or because they exercised less than the control group. Since we don’t know exactly what the participants ate, it’s not entirely clear why the IF group had these results. But considering that losing muscle mass is associated with decreased bone density, altered metabolism, and increased risk of falls and fractures (especially as you age), this is a concerning find.

“This form [of IF] as it is, without calorie restriction or a different diet, does not appear to work,” says Dr. Weiss. “Lots more research needs to be done here.”

What does this mean for IF?

While it’s unclear if everyone will encounter muscle mass loss on a 16:8 diet, the study’s findings do throw some cold water on the hype surrounding IF. “It could be that the benefits of time-restricted eating are smaller than we thought, or that you just get better results when you eat earlier in the day,” Courtney Peterson, a researcher at the University of Alabama at Birmingham, told the New York Times. “The jury is still out.” As Dr. Weiss says, more research is needed to better understand the full effects of intermittent fasting on the body.

What is clear right now: IF may not work for everyone, says Moskovitz. “ The best approach is one that fits the individual’s needs and lifestyles,” she says. “Balanced, customized, and intuitive eating prevails when it comes to long-term health and well-being. Research has proven this time after time.”

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