Scientists see no benefit in intermittent fasting

The idea of ​​losing weight is quite appealing: restrict your eating to six to eight hours a day, during which you can eat whatever you want.

Studies in mice appeared to support so-called time-restricted eating, a form of the popular intermittent fasting diet. Small studies of obese people suggested that it helped them lose weight.

But now a rigorous study over a year in which people followed a low-calorie diet between 8 am and 4 pm, or consumed the same number of calories at any time of day, has not found a direct effect.

The conclusion, according to Dr. Ethan Weiss, a diet researcher at the University of California at San Francisco: “There is no benefit to eating in a narrow window of time.”

The study, published Wednesday in the New England Journal of Medicine, was conducted by researchers at the University of Medicine of the South in Guangzhou, China, and included 139 people with obesity. Women ate between 1,200 and 1,500 calories a day, and men ate between 1,500 and 1,800 calories a day. To ensure validity, participants had to photograph all the foods they ate and keep food diaries.

The two groups lost weight — an average of 6.3 kg to 8.1 kg — but there was no significant difference in the amount of weight lost with each dietary strategy. There were also no significant differences between groups in measurements of waist circumference, body fat, and lean body mass.

Scientists also found no differences in risk factors such as blood glucose levels, insulin sensitivity, blood lipids and blood pressure.

“These results indicate that restriction of caloric intake explained most of the beneficial effects seen with the intermittent eating regimen,” concluded Weiss and colleagues.

The new study is not the first to test time-restricted feeding, but previous studies were generally smaller, or of shorter duration, and without control groups. These surveys tended to conclude that people lost weight if they ate only for a limited time of day.
Weiss used to believe in intermittent fasting and said that for seven years he only ate between 12pm and 8pm.

In previous research, he and his colleagues asked about 116 adult participants to eat three meals a day, with snacks if they felt hungry, and others were told to eat whatever they wanted between 12pm and 8pm. Participants lost little weight — an average of 0.900 kg in the intermittent fasting group and 0.680 kg in the control group, a difference that is not statistically significant.

Weiss recalled that he could hardly believe the results. He asked statisticians to analyze the data four times, until he was told that would not change the results.

“I was a devotee,” he said. “It was a hard thing to accept.”

The experiment lasted 12 weeks. Now, it appears that even a year-long study found no benefit from intermittent fasting.

Christopher Gardner, director of nutrition studies at Stanford’s Prevention Research Center, said he wouldn’t be surprised if time-restricted eating occasionally worked.

“Almost every type of diet out there works for some people,” he said. “But the upshot of this new research is that when subjected to a properly designed and conducted study — scientific investigation — it is no more effective than simply reducing caloric intake for weight loss and health factors.”

Weight loss experts say intermittent diets are unlikely to go away. “We don’t have a clear answer yet” on whether the strategy helps people lose weight, said Courtney Peterson, a researcher at the University of Alabama at Birmingham who studies time-restricted diets.

She suspects the diet could benefit people by limiting the number of calories they have the opportunity to consume daily. “We just need to do larger studies,” Peterson said.

Dr. Louis J. Aronne, director of the Center for Comprehensive Weight Management at Weill Cornell Medicine in New York, said that in his experience some people who struggle with calorie-counting diets do better when told to eat only during limited time each day.

“While this approach hasn’t been confirmed as better, it doesn’t appear to be any worse” than calorie counting, he said. “It gives patients more options for success.”

The hypothesis behind intermittent fasting is that circadian genes that would increase metabolism are turned on during the daytime hours, said Dr. Caroline Apovian, co-director of the Center for Weight Management and Wellness at Brigham and Women’s Hospital in Boston.

The question for researchers, she added, is, “If you eat a little too much during the daytime, are you better able to burn those calories than store them?” Apovian said she would like to see a study that compares a group of subjects who overeat all day with a group of subjects who also overeat, but with a time restriction.

She said she would still recommend time-restricted feeding to patients, although “we have no proof.”

Weiss said he was convinced by his own study that intermittent feeding was of no benefit, and that the new data reinforced his conviction.

“I started having breakfast again,” he said. “My family says I’m much nicer.”

^{Translated by Luiz Roberto M. Gonçalves}