Research over the years has suggested that the practice spurs weight loss and improves metabolic health, although much of the data has come from animal experiments or small studies of relatively short duration in humans. Experts say the diet works because it allows people the freedom to eat what they want so long as they do it in a narrow window of time, which leads them to consume fewer calories over all.
But the new research found that overweight adults who were assigned to routinely fast for 16 hours daily, eating all their meals between noon and 8 p.m., popularly known as the 16:8 diet, gained almost no benefit from it. Over the course of the three-month study, they lost an average of just two to three and a half pounds — only slightly more than a control group — and most of the weight they shed was not body fat but “lean mass,” which includes muscle.
While it is normal to lose some muscle during weight loss, the fasting group lost more than expected. That is concerning because muscle provides many health benefits: It protects against falls and disability as people age, and it is linked to lower mortality. It also increases metabolism and can help prevent weight that is lost during dieting from returning later on. The researchers speculated that one reason for the muscle loss may have been that the fasting diet led people to consume less protein.