Why Intermittent Fasting May Not Help When You’re Already Cutting Calories

Weight loss may be similar when people focus only on calorie reduction and when they combine this with intermittent fasting, a new study suggests.

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For the study, researchers randomly assigned 118 people with obesity to limit how much they ate for one year.Tetiana Kreminska/Adobe Stock

When it comes to weight loss, two diets may not be better than one. In a new study of people who cut calories to shed excess pounds, participants got similar results when they limited meals to certain hours of the day and when they ate anytime they wanted.

For the study, researchers randomly assigned 118 people with obesity to limit how much they ate for one year — 1,500 to 1,800 calories for men and 1,200 to 1,500 calories for women. Half the participants were also asked to eat only between 8 a.m. and 4 p.m., while the rest of them could eat whenever they liked.

People who ate only during certain hours, a practice known as intermittent fasting, lost an average of 8.0 kilograms (17.6 pounds) after one year, compared with 6.3 kilograms (13.9 pounds) for people who only cut calories, the researchers reported April 21 in the New England Journal of Medicine. But that difference was too small to rule out the possibility that it was due to chance.

While weight loss was the main goal of the experiment, researchers also looked at several other outcomes that can be indicators of health problems for people with obesity, including waist circumference, BMI, proportions of body fat and lean body mass, blood pressure, and metabolic risk factors like blood sugar levels. Outcomes for all these indicators were similar with and without intermittent fasting, the study found.

“The two weight loss regimens had similar results,” says the senior study author,?Huijie Zhang, MD, PhD, a chief physician, professor, and deputy director at Nanfang Hospital of Southern Medical University in Guangzhou, China. “People can choose the most suitable weight loss approach according to their own preference and needs,” Dr. Zhang says.

There are several caveats to these findings, however.

For one thing, the participants were generally healthy, making it harder to see dramatic changes in outcomes like blood pressure or metabolic risk factors after one year of new eating habits, says Blandine Laferrère, MD, PhD, the coauthor of an editorial?accompanying the study and a professor of medicine at Columbia University Irving Medical Center in New York City.

Beyond this, the participants already tended to eat over a fairly narrow window of about 10 hours and 23 minutes per day at the start of the study. This may have made it hard to see dramatic changes in weight loss when they were put on an intermittent fasting plan because it reduced their food intake period by only about two hours, Dr. Laferrère says. People who eat during more hours of the day at baseline are more likely to benefit from intermittent fasting.

In addition, the study participants got unusually good results from cutting calories, losing an average of 9 percent of their body weight. This may have blunted any effect from intermittent fasting, Laferrère notes.

“The study?shows that the main driver of the weight loss was the calorie restricted diet, and not the time-restricted eating diet,” says Krista Varady, PhD, a professor of nutrition at the University of Illinois in Chicago who wasn’t involved in the study.

But this doesn’t necessarily mean intermittent fasting is a bad approach to weight loss, particularly for people who struggle with typical means of cutting calories, such as by tracking everything they eat.

“The main benefit of time-restricted eating is that you don't need to count calories in order to?lose weight," Dr. Varady says. "Just by limiting the eating window to eight hours per day, people naturally cut out 300 to 500 calories per day.”

And not all calories are created equal, says Lon Ben-Asher, RD, a nutritionist at Pritikin Longevity Center who wasn’t involved in the study.

“We should aim our focus on the quality of the food we consume by following our hunger and satiety cues, not the time of the day when we eat,” Ben-Asher advises. “In addition, we should concentrate our efforts on consuming more foods that create greater satiety per calorie, such as vegetables, fruits, whole grains and unrefined carbohydrates, beans, lentils, and other legumes which contain a high level of dietary fiber and water content.”