In one of the longest trials of its type ever conducted researchers have found limiting food intake to a short period of time every day is no more effective a dietary strategy than simple calorie counting. The year-long experiment ultimately found calorie restriction to be the most important part of any weight loss plan.
Over the last decade fasting diets have increased in popularity, with advocates claiming they are the most effective way to lose weight and improve metabolic health. This dietary strategy can take a variety of forms, from alternate-day fasting (such as eating every other day, or fasting for a whole day once or twice a week) to time-restricted feeding (where eating is limited to a span of a few hours every day).
This new research focused specifically on a type of time-restricted feeding informally known as the 16:8 diet, which involves limiting all food intake to an eight-hour period every day. In this case the trial limited its fasting cohort to eating only between 8 am and 4 pm.
The trial recruited 139 subjects, all either overweight or obese but otherwise healthy. The cohort was randomly split into two groups: one group tasked with a time-restricted feeding strategy and another that could eat at any time. Both groups were required to limit their caloric intake each day: men no more than 1,800 calories and women no more than 1,500 calories. The trial lasted 12 months.
Restricting the calorie intake of both groups allowed the researchers to home in on the particular weight loss or metabolic outcomes of time-restricted feeding. Prior studies had floated the possibility that some of the benefits of fasting diets were due to the regime simply reducing a person’s overall calorie intake, so this trial looked to focus on whether fasting 16 hours every day had any specific benefits.
From baseline to 12 months the study found those in the time-restricted feeding group lost 9 percent of their body weight. This compared to a 7.2 percent drop in the all-day eating group. The researchers point out this difference is not statistically significant and concluded weight loss seems to be primarily driven by calorie restriction, and is not affected by time-restricted feeding patterns.
“In addition, time-restricted eating and daily calorie restriction produced similar effects with respect to reductions in body fat, visceral fat, blood pressure, glucose levels, and lipid levels over the 12-month intervention period,” the researchers write in the new study. “These results indicate that caloric intake restriction explained most of the beneficial effects seen with the time-restricted-eating regimen.”
The researchers are cautious to note these findings are subject to a number of limitations, so this study should not be interpreted as saying all kinds of intermittent fasting methods are useless. There may still be benefits in time-restricted feeding strategies for individuals with habitual eating patterns that span large stretches of a 24-hour period. And, time-restricted eating could be helpful in those with pre-existing metabolic or cardiovascular problems.
One strength of time-restricted feeding often raised by advocates is that it is an easy dietary strategy to maintain over long periods of time. While this trial showed similar high retention rates in both groups it is important to note all participants were strongly monitored and coached throughout the 12-month stretch.
So it may be true that in real-world conditions it is easier for people to follow a 16:8 plan compared to more general calorie restrictions. However, the findings do affirm that those engaging in time-restricted feeding strategies should still limit their calorie intake. Eating whatever you want for eight hours every day and then fasting for 16 will not help you lose weight.
The new study was published in The New England Journal of Medicine.
