Question 1. What added value does the Cochrane review provide to existing knowledge on intermittent fasting for weight loss in overweight individuals?

"There have been dozens of meta-analyses on this topic, and they have had similar results. Intermittent fasting (IF) produces similar weight losses to other moderate calorie restriction type weight loss interventions – typically around 7% after 6-12 months in both groups. This weight loss average is very common over 6-12 months. More can be achieved with a very low calorie diet (10-15%), pharmacotherapy (15-20%) and surgery (>20%).

This review combines many forms of intermittent fasting, which is a mistake. There are many types of ‘intermittent fasting’, and they are not equivalent for weight loss and should not be mashed together.

Time-restricted eating is a milder intervention than intermittent fasting - it implements a 2-3 hour time restriction each day, resulting in ~300kcal/d restriction and 2-3 kg of weight loss over 6 -12 months (unless added on top of other diets). Time-restricted eating trials typically compare against no intervention control or a change in diet quality. There is some evidence that health is improved despite modest weight loss by Time-restricted eating due to better alignment of circadian rhythms (but the jury is out – more trials required).

Intermittent fasting, or alternate-day fasting, is a 75-100% energy restriction for 2 or 3 days per week and regular eating practice on other days. Most trials compare it against moderate 20-30% calorie restriction over 6-12 months, some include a third arm ‘standard care or control’. Most of the trials comparing calorie restriction against intermittent fasting have around 5-7% weight loss in both groups and similar improvements in body weight and in diabetes and cardiovascular disease risk factors."

Question 2. The press release states: "Intermittent fasting did not appear to have a clinically meaningful effect on weight loss compared to standard dietary advice (Claim 1) or doing nothing (Claim 2)." To what extent can this statement be substantiated by the review?

"Claim 1: In their first claim, they show that intermittent fasting produces similar outcomes to weight loss interventions with 21 studies included.

Claim 2: Intermittent fasting produces statistically significant weight loss. Across the six studies included in claim 2, intermittent fasting resulted in about 3.4% greater weight loss than ‘control’. That’s arguably clinically meaningful on its own—for example, NICE guidelines consider 3% meaningful, but US guidelines do use 5% as a benchmark. But many of the control participants in those studies still lost about 2%, so the actual weight loss with Intermittent fasting is closer to 5%.

Very few studies were included. Claim 2 draws on only six studies compared with 21 in claim 1. Of these six, two used time-restricted eating, which generally produces smaller weight loss, and one involved fasting just one day per week, which would also be expected to have less impact. That leaves only three intermittent fasting trials. In one of these, only the 18-month follow-up was reported; they left out the primary 6-month time point when participants were undertaking the diet. In that study, the difference between Intermittent fasting and control was around 5% weight loss versus the control.

Type of control matters. Some of the control groups were not completely inactive or waitlist—they received some information and guidance for weight loss. This reduces the apparent difference between intermittent fasting and control."

Question 3. The authors of the review emphasise that the scientific evidence is very uncertain. To what extent does this statement correspond with existing evidence syntheses/reviews?

"Completely agree, they have heterogeneous results because they are lumping together diets that should not be put together. Which will result in heterogeneity… some also include those with diabetes who are typically refractory to weight loss (even with semaglutide)."

Question 4. To what extent is it possible to recommend intermittent fasting as a means of weight loss based on scientific evidence?

"You can either look at the results of individual trials or combine like in a meta-analysis. Intermittent fasting is a proven alternative to continuous calorie restriction over 6–12 months. Both intermittent fasting/alternate day fasting and calorie restriction typically produce around 7% weight loss when dietary support is provided. Time-restricted eating is a more modest approach, usually resulting in about 3 kg of weight loss—unless the eating window is further shortened or combined with other dietary strategies.

Many people find it very difficult to keep weight off beyond 12 months, no matter what approach they use. This is because the body has biological mechanisms that encourage weight regain after weight loss. For example, levels of satiety peptides and other hormones that make you feel full drop, leptin also drops, decreasing your energy expenditure, and hunger hormones like ghrelin increase. As a result, weight often returns after a year, whether someone followed a calorie-restricted diet or intermittent fasting. Exercise and weight loss medications can help maintain the loss by counteracting these biological signals."