Fasting diets vs. cutting calories: Which works best?

A trendy form of intermittent fasting does seem to help people lose some weight—though it may be no better than old-fashioned calorie counting, a new clinical trial suggests.

Researchers found that the tactic—called time-restricted eating—helped people with obesity drop around 8 pounds, on average, over one year. That was right on par with a second study group who went the traditional route of calorie counting and portion control.

Time-restricted eating is a form of intermittent fasting where people limit themselves to eating within a certain time window each day. Outside that window, they swear off everything other than calorie-free drinks.

The main selling point of time-restricted eating is its simplicity: Instead of laboriously counting calories, people only have to watch the clock. And small studies have shown that limited eating windows—6 hours being a popular one—can help people eat less and shed some pounds.

The new trial adds to the story because it aimed to be as real-world as possible, said senior researcher Krista Varady, a professor of nutrition at the University of Illinois Chicago.

Participants were allowed to eat across an 8-hour time span, from noon to 8 p.m., which let them have dinner with family or go out to a restaurant with friends. And they merely stuck with that time window, without having to cut out particular foods or measure strict portion sizes.

Past studies of time restriction have had people take additional steps, like tracking calories.

But that misses the point, Varady said, since people try time-restricted eating to keep things easy.

“I think it became so popular because it’s simple, it’s accessible and it’s free,” she said.

So for their trial, Varady and her colleagues recruited 90 adults with obesity who were looking to lose weight. The majority were Black or Hispanic—two groups often underrepresented in clinical trials.

Each participant was randomly assigned to one of three groups: One started the time-restriction plan, a second committed to calorie-counting, and the third made no changes and served as a control group.

The calorie counters aimed to trim their daily calories by 25% and met with a dietitian to come up with healthy food choices. The time-restriction group, meanwhile, went by the clock: For the first six months, they limited their eating window to noon to 8 p.m.; for the next six months, they were allowed to expand it to 10 a.m. to 8 p.m.

Both diet groups also had periodic phone or video calls with a dietitian to talk about generally healthy eating.

In the end, the two groups fared similarly. After one year, people in the control group had gained weight—about 2.5 pounds, on average. But those in the diet groups had lost an average of about 8 to 9.5 pounds.

The findings were published June 26 in the Annals of Internal Medicine.

“I don’t think intermittent fasting works better than other diets,” Varady stressed. But the findings show that if people stick with the tactic, they can maintain weight loss.

Dr. Adam Gilden, an associate professor at the University of Colorado Anschutz Medical Campus, co-wrote an editorial published with the study.

He said that in his experience, patients ask about time-restricted eating “all the time.” But it’s important, Gilden said, not to oversell the ease of the approach.

“There’s nothing magical about it,” Gilden said. “You still have to put some work into it.”

He noted that the trial participants did have help, in the form of regular dietary counseling sessions—and that may have been a key ingredient in their weight loss.

“I think that support is the important part,” Gilden said.

Varady said that while people can certainly do time restriction on their own, they might meet with more success if they have support. That could be from a dietitian, she said, or from a group like Weight Watchers.

One concern with time restriction is that it focuses on limiting food intake, and not on making healthier food choices.

“What we saw is that people’s diet quality doesn’t get better, but it doesn’t get worse, either,” Varady said.

She added that changing diet quality is “the hardest part”—since people’s food preferences are so ingrained, and factors like income and access to healthy food come into play.

Ideally, though, healthier eating should be the goal, Gilden said.

He suggested that people looking to lose weight start with some kind of support, like talking to their doctor (who might refer them to a dietician).

Time restriction is not for everyone. According to Varady, people can initially have headaches, nausea or dizziness as the body adjusts to the new eating schedule—and some give up.

It’s also considered inappropriate for certain groups, like pregnant women, people with a history of eating disorders and those taking medications that have to be timed with food.

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