Intermittent fasting, the name given to the practice of eating only between certain hours of the day (or, in more extreme cases, only on certain days), has received a good deal of praise in wellness and tech communities in recent years. Silicon Valley entrepreneurs like Jack Dorsey often present the practice as an intellectual endeavor; some studies, mostly done on mice, have shown that intermittent fasting can sharpen brain function. Other researchers have also suggested intermittent fasting initiates a process called autophagy, in which the body recycles “its own damaged cell bits and proteins,” thereby extending lifespan.
But intermittent fasting is ultimately a diet, and one that has promised to help people lose weight — which isn’t shocking, since food deprivation is baked into its design. What is surprising (and more than a little satisfying to critics) is a new study published in JAMA Internal Medicine, which finds that intermittent fasting typically results in only minimal weight loss, much of which is muscle.
Many followers of intermittent fasting skip breakfast in order to comply with the diet’s narrow food-consumption schedule, eating only between noon and 8 p.m. This results in a 16-hour fasting window, which is why intermittent fasting is also sometimes referred to as the 16:8 diet. Diet books have said this pattern results in weight loss because it encourages people to eat fewer calories overall, even if they eat whatever they want during non-fasting hours.
But according to the new study, most participants lost just two-to-three pounds over three months, and most of that weight was “lean mass,” which includes muscle. Losing some muscle is expected with weight loss, but the researchers note that the fasting group lost more than expected. This is worrisome because muscle is very important to overall health: It’s linked to lower mortality, helps prevent falls in old age, and increases metabolism, which can help prevent weight lost during a died from coming right back.
The study’s results were surprising to its author, Ethan Weiss, a cardiologist at the University of California, San Francisco, who’d practiced 16:8 intermittent fasting for years. As soon as he saw the data, he started eating breakfast again, for the first time since 2014 (!). I am so happy for him.