Intermittent fasting (IF) is an approach to eating based on timing. The idea is that fasting for long enough allows insulin levels to fall low enough that our body will use fat for fuel. Growing evidence in animals and humans shows that this approach leads to significant weight loss. When combined with a nutritious, plant-based diet and regular physical activity, IF can be part of a healthy weight loss or maintenance plan, as I described in an earlier blog post.
Now, a randomized controlled trial published in JAMA claims that IF has no significant weight loss benefit and a substantial negative effect on muscle mass. News outlets picked up the story and ran headlines like A Potential Downside of Intermittent Fasting and An Unintended Side Effect of Intermittent Fasting.
But what did this study actually look at and find?
In the study, 141 patients were randomly assigned to 12 weeks of either a time-restricted eating plan (TRE) that involved fasting for 16 hours and eating only during an eight-hour window of the day, or a consistent meal timing (CMT) eating plan, with three structured meals a day plus snacks.
Neither group received any nutrition education or behavioral counseling, nor was physical activity recommended. There was no true control group (meaning a group that did not receive any instructions about meal timing).
Interestingly, both groups lost weight. Given the headlines, I had to read and reread the results several times, because they show that the IF group lost a statistically significant amount of weight from beginning to end — which wasn’t true in the CMT group. The researchers reported: “There was a significant decrease in weight in the TRE group (−0.94 kg; 95% CI, −1.68 kg to −0.20 kg; P = .01) and a nonsignificant decrease in weight in