A randomized trial published today in the Annals of Internal Medicine will be reported as a win for intermittent fasting, and in a narrow sense it is. One hundred sixty-five adults with overweight or obesity were assigned either to 4:3 fasting — three non-consecutive days a week at 80% energy restriction — or to a daily 34% cut in calories. At twelve months the fasting group had lost 7.6% of body weight against 5% for the daily group, a difference of 2.89 kilograms, with somewhat better blood pressure, cholesterol and fasting glucose. That is the headline. The rest of the paper is the story.
What both groups did. Both arms received comprehensive behavioral support for a year and were encouraged to get 300 minutes of moderate activity a week. About three-quarters of participants finished, with the fasting group finishing at a higher rate. In other words, two groups of people who were coached weekly and told to move five hours a week both lost clinically meaningful weight, and the group that found its structure easier to keep lost a bit more. The trial does not show that fasting does something biologically special. It shows that a rule people can follow beats a rule they abandon, which is the oldest finding in weight loss and the one the diet industry least wants to hear.
Loree’s takeThe patients who will bring me this paper are not the ones it was run on. The trial enrolled adults aged 18 to 60; it was not designed to tell a 68-year-old anything. And it says nothing about women's hormones, where the evidence on fasting is thin and in places worrying. What I take from it is the part the headlines will skip: a year of support plus movement produced results in both arms. The structure is the medicine. Pick the one you can keep.
Ben’s takeBy Friday someone will be selling a 4:3 program with an app, a meal kit and a monthly fee. The thing that produced the weight loss in this trial — weekly coaching, a simple rule, five hours of movement — cannot be put in a box, so the box will contain the rule and leave out the rest. The 2.89 kilograms belong to the people who showed up for a year. They are not for sale.
Why it matters to a recovering optimizer. The pull of fasting in longevity culture is mechanistic — autophagy, insulin, the idea that the clock does the work. This trial, which is better than almost anything that literature offers, found a modest difference explained by adherence. If you fast and you like it, this is good news. If you have been fasting because you were told it is a longevity intervention, note that the trial measured weight, not lifespan, and that the control group did fine.
Pick the eating pattern you can keep for a decade. The kilograms came from showing up, not from the clock.
This post reports on evidence and expert disagreement. It is not medical advice; decisions about treatment belong with you and your clinician.