Recent diet trends have promised that clocks are as important for weight loss as scales. One such diet is known as intermittent fasting, which entails a schedule of alternating fasting and eating. A popular intermittent fasting schedule is time-restricted eating. By restricting eating to a limited number of hours a day, some proponents of this diet argue people can harness their bodies’ natural rhythms to shed pounds.
But according to new research, most recently a study published in the Journal of the American Heart Association on Jan. 18, the time when you eat, in and of itself, doesn’t appear to be helpful for weight loss.
In the study, 547 participants used a phone application to track their daily meals over a six-month period, which the researchers used to determine, on average, what times each person ate every day; how many meals they ate; whether the participants described each meal as small, medium, or large; and how much weight they gained or lost. Ultimately, they found that the time between participants’ first and last meal, and when they ate relative to the time they woke up or went to sleep didn’t impact weight. What did matter was the size of the participants’ meals: people who ate more large- or medium-sized meals were more likely to gain weight, while people who ate small meals were more likely to lose weight.
Study co-author Dr. Wendy Bennett, a primary care doctor and associate professor at Johns Hopkins School of Medicine, emphasized that this isn’t the final word on meal timing, in part because it’s an observational study—meaning the researchers didn’t control the conditions while they were happening, such as the calories they consumed. Krista Varady, a nutrition researcher who studies intermittent fasting at the University of Illinois, Chicago, who did not participate in the study says that there are several factors that may cloud the results, including the small sample size, its reliance on participants describing the size of their meals as large or small—and not log specific calorie counts—and the use of different scales to weigh the participants at doctors’ offices.
Nevertheless, Varady agrees that there doesn’t seem to be any magic to eating at a certain time. But that doesn’t mean it’s an entirely useless concept. Restricting eating to certain times can help people to lose weight, Varady says, so long as it causes you to eat less. And for some, she notes, it can be easier to stick to than other diets that require you to count calories. For example, she says, previous research shows that it can help if you only eat within a six-to-eight hour window—say, 10 a.m. to 4 p.m. (six hours) or 9 a.m. to 5 p.m. (eight hours).
Dr. Nisa Maruthur, an associate professor at Johns Hopkins School of Medicine, agrees. “If your calories are the same, regardless of when you eat them, there’s not an impact on weight,” says Maruthur. However, establishing temporal boundaries can help. “If you decide you’re only eating between 10 a.m. and 4 p.m., the fact is that you may eat fewer calories, just because you only fit [in] so many [meals] in that time.” Maruthur, who did not participate in this particular study, but is involved in a broader initiative at Johns Hopkins to examine meal timing, says that while eating at a particular time isn’t beneficial, she’d still recommend timed eating if it helps a particular person eat more healthfully. “The best diet for anyone is probably the diet that they can follow,” she says. “If some people find it easier to eat healthier foods because they’re planning a bit more,” she says, time restricted eating could be beneficial.