It’s cutting calories—not intermittent fasting—that drops weight, study suggests

Intermittent fasting, aka time-restricted eating, can help people lose weight—but the reason why may not be complicated hypotheses about changes from fasting metabolism or diurnal circadian rhythms. It may just be because restricting eating time means people eat fewer calories overall.

In a randomized-controlled trial, people who followed a time-restricted diet lost about the same amount of weight as people who ate the same diet without the time restriction, according to a study published Friday in Annals of Internal Medicine.

The finding offers a possible answer to a long-standing question for time-restricted eating (TRE) research, which has been consumed by small feeding studies of 15 people or fewer, with mixed results and imperfect designs.

The new study—led by Nisa Marisa Maruthur, an internal medicine expert at Johns Hopkins—has its own limitations and, like any one study, isn't the last word on the matter. But "it takes us one step closer to identifying the underlying mechanisms of TRE," nutrition experts Krista Varady and Vanessa Oddo of the University of Illinois wrote in an editorial accompanying the study. "Using a controlled feeding design, Maruthur and colleagues show that TRE is effective for weight loss, simply because it helps people eat less."

The study involved 41 people, 21 who followed a time-restricted diet for 12 weeks and 20 who ate a usual eating pattern (UEP). Most of the participants were Black women (93 percent) with obesity and either pre-diabetes or diet-controlled diabetes, limiting the generalizability of the findings. But the study carefully controlled what and when the participants ate; each participant got controlled meals (breakfast, lunch, dinner, and snack) with identical macro- and micro-nutrients. Each participant was assigned a calorie level for their meals based on an established, standardized equation that estimates baseline caloric need. They were told to maintain their current exercise level, which was monitored with a wrist-worn accelerometer.

No magic necessary

In the time-restricted group, people only ate in a 10-hour window between 8 am and 6 pm, with 80 percent of their total daily calories consumed before 1 pm. In the usual eating group, people ate between 8 am and midnight, with 55 percent of their calories eaten after 5 pm for dinner and a night-time snack. In each eating group, participants were given specific windows of a couple of hours in which they should eat each pre-made meal. The participants ate three meals each week at a research site, where dieticians addressed adherence issues, and their eating was carefully monitored with the use of food diaries and urine tests. Approximately 96 percent of people in both groups followed the schedules to within 30 minutes. Diet adherence—eating all their assigned food and not eating outside food—was also high, with 93 percent in the time-restricted group and 95 percent in the usual eating group.

At the end of the 12 weeks, both groups lost about the same amount of weight, an average of around 2.4 kg (5.3 pounds), with no statistically significant difference between the two groups. The researchers also found no differences between the two groups in their glucose homeostasis, waist circumference, blood pressure, or lipid levels.

"Our results indicate that when food intake is matched across groups and calories are held constant, TRE, as operationalized in our study, does not enhance weight loss," Maruthur and her colleagues concluded. The authors are upfront about the limitations of the study, though, noting that the results could have been different in different groups of people and potentially in shorter time-restricted windows, such as eight hours instead of 10. They called for more research to explore those questions.

Outside experts applauded the study while also adding that it's not surprising. "The headline finding that TRE does not magically lead to more weight loss sounds sensational but is also obvious," Adam Collins, a nutrition expert at the University of Surrey, said.

Naveed Sattar, a professor of cardiometabolic medicine at the University of Glasgow, called the study "well done." It "tells us what we expected—that there is nothing magical about time-restricted eating on weight change other than effects to reduce caloric intake," he said. "If time-restricted eating helps some people eat less calories than they would otherwise, great."

The experts Varady and Oddo, meanwhile, see it as a boon for anyone trying to lose weight. "Many patients stop following standard-care diets (such as daily calorie restriction) because they become frustrated with having to monitor food intake vigilantly each day," they wrote in their commentary. "Thus, TRE can bypass this requirement simply by allowing participants to 'watch the clock' instead of monitoring calories while still producing weight loss." It's a "simplified" and "accessible" dietary strategy that anyone can follow, including lower-resource populations, the researchers wrote.