Intermittent fasting (IF) diets, which involve restricting calorie intake to specific time intervals during the day, have become a popular weight-loss strategy. As with any diet, the fundamental question is do they work? Fortunately, a number of recent trials have evaluated the effects of IF diets on weight loss, so there is a significant amount of scientific evidence from which to draw a conclusion.

Over the past five years, six trials have been published that compared IF diets to usual eating. In some trials, participants were instructed to consume a calorie-restricted diet, while in others, no calorie restriction was assigned. Below is a summary of the trials and their findings.

• IF Diet vs Usual Eating with Calorie-restricted diet - In three trials, participants were given intensive counseling on consuming a calorie-restricted diet and then randomized to IF or usual eating. Two of the trials demonstrated significantly more weight loss in the IF group compared to the usual eating group: a 14-week study showed weight loss of 13.9 lbs in the IF group compared to 8.8 pounds in the usual eating group, while a 12-month study showed weight loss of 17 lbs and 10.5 lbs, respectively. [Study one abstract, Study two abstract] The third trial, which lasted 12 months, did not find a significant difference in weight loss between IF and usual eating (17.6 lbs vs 13.9 lbs) [Study abstract]
• IF Diet vs Usual Eating without calorie restriction - A 12-week trial, in which participants did not receive dietary counseling, compared an IF diet where no calories were consumed from 8 PM to 12 PM the following day to usual eating. At the end of the trial, there was no significant difference in weight loss between groups (2 lbs vs 1.5 lbs). [Study abstract]
• IF Diet vs Usual Eating with Meals Provided - A 12-week trial enrolling obese adults with prediabetes or diabetes randomized participants to IF or usual eating. All meals were prepared for the participants and consumed onsite or taken home. Participants were instructed not to eat non-study food. Subjects were given diets containing calories that matched their activity levels. At the end of the trial, there was no significant difference in weight loss between the IF and usual eating groups (5.1 lbs vs 5.7 lbs) [Study abstract]
• IF Diet vs Usual Eating vs Calorie-restricted diet - A 12-month study compared three different diets: (1) IF diet (eating between noon and 8 PM), (2) Calorie-restricted diet (25% energy restriction daily), and (3) Usual eating (no IF or calorie restriction). At the end of the study, the IF group lost 5 lbs, the calorie-restricted group lost 5.7 lbs, and the usual eating group gained weight. [Study abstract]

Conclusion: In the studies above, IF did not significantly enhance weight loss in four trials and showed only a modest effect in two. The most revealing study is the one where all food was provided to participants. In this trial, any difference in weight loss between the IF and usual eating groups would be directly attributable to IF itself, rather than variations in calorie intake. This particular study found no significant effect of IF on weight loss.

In summary, IF can enhance weight loss if it leads to an overall reduction in calorie intake. However, there's no evidence that it increases metabolism, and individuals who consume their full daily calorie expenditure during the feeding window won't lose weight. Long-term adherence to the time constraints seems impractical, making it a fad diet for most.