The idea of losing weight is quite appealing: limit your meal to a period of six to eight hours each day, during which you can eat whatever you want.
Studies in mice seemed to support the so-called time-limited diet, a form of popular fasting diet. Small studies on people with obesity suggest that this could help with weight loss.
But now, a rigorous one-year study in which people followed a low-calorie diet between 8 a.m. and 4 p.m., or consumed the same number of calories at any time during the day failed to find an effect.
Conclusion, said Dr. Ethan Weiss, a nutrition researcher at the University of California, San Francisco: “There is no benefit to eating in a narrow window.”
The study, published Wednesday in the New England Journal of Medicine, was conducted by researchers from the Southern Medical University in Guangzhou, China, and involved 139 people with obesity. Women ate 1,200 to 1,500 calories a day, and men 1,500 to 1,800 calories a day. To ensure compliance, participants were required to photograph each piece of food they ate and keep food diaries.
Both groups lost weight – averaging about 14 to 18 pounds – but there was no significant difference in the amount of weight lost with either diet strategy. There were also no significant differences between groups in measures of waist circumference, body fat, and lean body mass.
The researchers also found no differences in risk factors such as blood glucose levels, insulin sensitivity, blood lipids or blood pressure.
“These results show that caloric restriction explained most of the beneficial effects seen with a time-limited diet,” concluded Dr. Weiss and colleagues.
The new study is not the first to test a time-limited diet, but previous studies were often smaller, of shorter duration, and without control groups. This research showed a tendency to conclude that people lost weight by eating only for a limited period of time during the day.
Dr. Weiss himself was a true believer in the time-limited diet and said he ate only between noon and 8pm for seven years.
In a previous study, he and his colleagues asked some of the 116 adult participants to eat three meals a day, with snacks if they were hungry, and others were instructed to eat what they wanted between noon and 8 p.m. Participants lost a small amount of pounds. – an average of two pounds in the time-limited group, and 1.5 pounds in the control group, a difference that was not statistically significant.
In one interview, Dr. Weiss recalled that he could hardly believe the results. He asked statisticians to analyze the data four times, until they told him that further work would not change the results.
“I was a fan,” he said. “This was hard to accept.”
That experiment lasted only 12 weeks. Now it seems that even a one-year study has failed to find the benefit of a time-limited diet.
Dr. Christopher Gardner, director of nutrition studies at the Stanford Center for Prevention Research, said he would not be surprised if a time-limited diet still works occasionally.
“Almost every type of diet works for some people,” he said. “But what supports this new research is that when subjected to a properly designed and conducted study – scientific research – there is nothing more beneficial than simply reducing daily calorie intake for weight loss and health factors.”
Weight loss experts say a time-limited diet is unlikely to go away. “We don’t have a clear answer yet” about whether this strategy helps people lose weight, said Courtney Peterson, a researcher at the University of Alabama at Birmingham who studies time-limited nutrition.
She suspects that the diet could benefit people by limiting the number of calories they have the opportunity to ingest each day. “We just need to do bigger studies,” Dr. Peterson said.
Dr. Louis J. Aronne, director of the Comprehensive Weight Control Center at Weill Cornell Medicine in New York City, said that in his experience, some people who have problems with calorie-counting diets do better if they are told to simply eat only for a limited time. every day.
“While that approach hasn’t proven better, it doesn’t seem to be worse than counting calories,” he said. “It gives patients more opportunities to succeed.”
The hypothesis behind the time-limited diet is that circadian genes thought to increase metabolism are involved during the day, said Dr. Caroline Apovian, co-director of the Center for Weight Management and Wellness at Brigham and Women’s Hospital in Boston.
Asked by researchers, she added: “If you overeat a little during the day, are you more able to burn those calories than store them?” Dr. Apovian said she would love to see a study that compares a group of respondents who overeat all day with a time-limited group of respondents who also overeat.
She said she would continue to recommend time-limited diets to patients, she said, although “we have no evidence”.
For Dr. Weiss, he said he was convinced by his own research and said the new data strengthened his belief that a time-limited diet offers no benefit.
“I started eating breakfast,” he said. “My family says I’m much prettier.”