Do Fasting Diets Work? Study Finds Little Benefit

In the end, those participants lost about 2 pounds, on average. But people who stuck with their usual routine also lost a bit of weight — which meant there was no statistically meaningful difference between the two groups.

The researchers, led by Dr. Ethan Weiss of the University of California, San Francisco, acknowledged that their findings contradict some previous research. A potential reason, they speculated, is that the 8-hour time window for eating was “not optimal.”

In her study, Varady tested both a 4- and 6-hour window. Both were similarly effective for weight loss.

She noted that participants in the new study did not keep track of what they ate — which, in a study setting, leaves unanswered questions. There is no way of knowing whether people made any changes in calorie or nutrient intake.

Jennifer Bruning is a registered dietitian and spokeswoman for the Academy of Nutrition and Dietetics. She was not surprised by the findings.

Simply telling people to eat between noon and 8 p.m. — without any portion control or other diet advice — “may not be the best way to induce weight loss,” Bruning said.

And in a clinical trial, people are randomly assigned to a diet they may or may not like. In the real world, Bruning pointed out, people would generally choose intermittent fasting because it appeals to them, and would adjust the eating time window to fit their lifestyle.

“If you find intermittent fasting appealing for its simplicity,” Bruning said, “it can be safely undertaken by most people.”

But, she added, some people should take extra care and consult their doctors first — including those on time-sensitive medications that need to be taken with food; people with diabetes; and people with a history of eating disorders.

And no matter what eating plan a person chooses, Bruning said, weight-loss goals should not come at the expense of good nutrition.

Varady agreed that for the sake of overall health, people should pay attention to what they eat, not just when.

She also said that like any diet, time restriction is not for everyone, or without downsides.

In the first week or two, she said, people can get headaches, dizziness and nausea — and some give up.