A popular dieting trend, intermittent fasting, may not offer much over traditional dieting for losing weight, according to a new randomized trial published this week. The study found that people living in China who were told to fast while dieting did lose weight over a year’s time, but not significantly more than those who were told to diet as usual. There were also no significant differences in other relevant measurements, such as changes in body fat or metabolic risk factors.
Intermittent fasting has emerged as a prominent style of dieting in recent years. Proponents claim that by restricting when you eat to certain times, you can better burn off the body’s stores of fat. Some also say that it makes it easier for people to limit the calories they eat on a daily basis. There are various methods of fasting, but popular ones include eating very little food for two days a week, then normally the rest of the week, or only eating during the daytime hours.
There has been some research suggesting an added benefit from intermittent fasting over traditional dieting. But much of the data has come from animal studies or from observational or short-lasting studies in people. This new trial, published in the New England Journal of Medicine, appears to provide a more rigorous test of its effectiveness.
The study involved 139 volunteers with a body mass index between 28 and 45 (a BMI over 25 is considered overweight, and a BMI over 30 is obese). They were randomized into two groups. Each group was told to restrict their calories for the next year, with 1,500 to 1800 calories a day recommended for men and 1,200 to 1,500 calories a day for women. But one group was further instructed to only eat between 8:00 a.m. and 4:00 p.m. Both also received nutritional shakes for the first six months and counseling for the entire year to help them adhere to the diet.
By the study’s end, 118 volunteers had completed the trial. Those who were intermittent fasting lost an average of 8 kilograms from baseline (about 17 pounds), compared to the 6.3 kilograms (just under 14 pounds) lost by the control dieting group—a difference that wasn’t statistically significant. Similarly, there were no major differences in how other factors changed over the year, including people’s waist circumference, their level of body fat, blood pressure, and levels of glucose and lipids (all of which had improved from baseline). Mild adverse events, like fatigue, dizziness, and headaches, were just as common between the groups as well.
“In this 12-month trial, we found that the 8-hour time-restricted–eating regimen did not produce greater weight loss than the regimen of daily calorie restriction, with both regimens resulting in similar caloric deficits,” the authors wrote.
Longer-term randomized controlled trials of different dieting techniques are often hard to come by, in part due to the resources and effort it can take to run them. So these results may turn out to be especially influential. They also seem to line up with another larger-scale randomized trial published in 2020, involving over 100 participants, which similarly found no difference in the amount of weight loss between groups during a 12-week period.
At the same time, the authors do caution that their findings have some limitations. For one, the study involved members of the general population, and it’s possible that fasting could have added benefits for people with certain conditions, such as diabetes or cardiovascular disease. The researchers also didn’t try to specifically measure how many total calories people were burning on average a day or their amount of physical activity. And perhaps most importantly, the findings may not necessarily be generalizable to other populations or ethnicities.
At the very least, the authors noted, intermittent fasting doesn’t seem to be any worse than traditional dieting. So for those who might prefer it, they say, their findings “suggest that the time-restricted-eating regimen worked as an alternative option for weight management.”