A number of studies have suggested that eating only during a limited window of time can help some people lose weight. But it’s unclear why: Does the strategy just help people eat less, or is there something beneficial about keeping meals within a shorter time frame?
New research falls in the former camp, suggesting that the amount of calories people consume matters more than the timing.
A small, randomized trial at Johns Hopkins University School of Medicine — the results of which were published Friday in the journal Annals of Internal Medicine — found that when researchers controlled the number of calories people consumed, both time-restricted and regular eating schedules resulted in similar degrees of weight loss.
“It makes us think that people who benefit from time-restricted eating — meaning they lose weight — it’s probably from them eating fewer calories because their time window’s shorter and not something else,” said the study’s lead author, Dr. Nisa Maruthur, an associate professor of medicine at Johns Hopkins University.
Time-restricted eating regimens vary, and some are similar to intermittent fasting, a type of diet that involves alternating between periods of fasting and eating. The new trial looked at a 10-hour eating window, which is longer than what would typically qualify as intermittent fasting.
The researchers supplied 41 people with prepared meals for 12 weeks. The participants, most of them Black women, had obesity and were either prediabetic or diabetic.
Their meals (breakfast, lunch, dinner and a snack) were designed to contain the same number of calories they ate in their daily lives, based on their age, sex, weight, height and level of physical activity. The food had a healthy balance of fat, carbohydrates and protein.
On a given day, participants might have cereal and a fruit cup for breakfast, a kale salad with white beans and lentils for lunch, peanuts or mandarin oranges for a snack and a beef stew for dinner.
Half the group ate meals over a 10-hour window, from 8 a.m. to 6 p.m. Around 80% of their calories were consumed at breakfast and lunch, because some research has suggested that eating most of your calories early in the day could be beneficial for weight loss.
The other half ate from 8 a.m. to midnight and consumed half of their daily calories at dinner — a schedule meant to mimic many people’s eating patterns outside a research setting.
During the eating windows, the trial didn’t limit beverages if they were calorie- and caffeine-free. Participants were also each allowed one cup of coffee, diet soda and alcoholic beverage per day. Outside the designated time periods, only water was allowed.
At the end of the study, participants lost roughly the same amount of weight regardless of which regimen they followed. The average in the time-restricted eating group was around 5.1 pounds lost, compared to 5.7 pounds for the other group. There were no significant differences in blood sugar, blood pressure, waist circumference or lipid levels.
The results were similar to the findings of a randomized trial last year, which found that intermittent fasting was similar to calorie counting as a weight loss strategy.
However, research overall on time-restricted eating is mixed: A six-year study didn’t find a link between weight change and limiting food intake to a specific time window.
The new trial was distinctive in that the researchers controlled what all participants ate — a rarer and more complex experiment design.
But its results come with a few caveats, said Dr. Lisa Chow, a professor of medicine at the University of Minnesota who wasn’t involved in the study.
The trial may not have captured the long-term benefits of time-restricted eating, she said. One of her own studies, published last year, found that six months of time-restricted eating — from noon to 8 p.m. daily — was more effective for weight loss than calorie restriction in people with Type 2 diabetes.
What’s more, Chow added, the trial didn’t reflect what time-restricted eating looks like in real life, because it controlled people’s calories and meals.
“When you’re talking about the doctor talking to a patient about weight loss, it’s not the same thing, because the doctor isn’t going to provide all the food,” she said.
Other researchers also think meal timing matters for weight loss.
According to Satchin Panda, a professor at the Salk Institute for Biological Studies, the ideal time-restricted eating schedule involves waiting an hour or two after you wake up to have your first meal, then consuming your last meal three hours before bedtime.
“It doesn’t depend on the world clock — it depends on your internal clock,” Panda said. “A person who wakes up at 6 a.m., it may be OK for them to start at 8 a.m. But the person who wakes up at 8 a.m. may start eating at 9 or 10 a.m.”
That’s because the stress hormone cortisol is high after you wake up, and melatonin — a hormone that regulates sleep — is still declining at that time. So it may be more difficult for people to properly digest soon after they arise, Panda said.
Eating close to bedtime, meanwhile, can disturb sleep, and insufficient sleep may increase cravings, Panda added. Some research also suggests that late-night eating could cause people to store more energy as fat.
Time-restricted eating may offer benefits other than weight loss, as well.
Panda’s research has found that the approach may reduce so-called “bad cholesterol” and improve blood pressure and blood sugar in people with certain chronic health conditions.
On the other hand, a study published last month found that restricting food consumption to less than eight hours per day may increase the risk of cardiovascular death long-term.
At the very least, Panda said, time-restricted eating may offer a simpler alternative to calorie counting.
“It’s easier to count time than count calories,” he said. “I’d guess that most people cannot remember how many calories they’ve consumed, but timing is easy to remember.”
