99 trials of intermittent fasting: the differences between methods are small

Intermittent fasting has been studied intensively enough that the useful question is no longer whether it works but whether the version matters. A network meta-analysis published in The BMJ on 18 June pools 99 randomised trials to answer that, and the answer is: barely.

What was pooled

The review searched Medline, Embase and Central from inception to 14 November 2024 for randomised clinical trials comparing intermittent fasting strategies — alternate day fasting, time restricted eating and whole day fasting — against continuous energy restriction and against ad-libitum diets [s1].

Ninety-nine trials involving 6,582 adults met the criteria, of whom 720 were healthy and 5,862 had existing health conditions [s1]. The primary outcome was body weight; secondary outcomes covered anthropometry, glucose metabolism, lipid profiles, blood pressure, C-reactive protein and markers of liver disease [s1]. Analysis used a frequentist network meta-analysis, with certainty assessed by GRADE [s1]. The review is registered as NCT05309057 [s1].

A network meta-analysis is worth understanding before reading the result. It compares interventions that were often never tested head to head, by chaining them through shared comparators. That is a legitimate and powerful method, and it also means some of the comparisons below rest on indirect evidence rather than on any trial that actually randomised people between those two diets.

The results

All intermittent fasting strategies and continuous energy restriction reduced body weight compared with an ad-libitum diet [s1]. That is the unsurprising finding and the one with the most consistent support.

Against continuous energy restriction, alternate day fasting was the only intermittent strategy to show a benefit: a mean difference of -1.29 kg (95% CI -1.99 to -0.59), with moderate certainty of evidence [s1].

Alternate day fasting also showed what the authors describe as a trivial reduction against the other two intermittent methods — -1.69 kg (-2.49 to -0.88) versus time restricted eating and -1.05 kg (-1.90 to -0.19) versus whole day fasting, both with moderate certainty [s1].

The word "trivial" is the authors', and it is doing honest work. Roughly one to two kilograms, in trials that mostly ran under six months, is not a difference most people would notice as a change in health.

The duration finding is the important one

Estimates were similar in the 76 trials with less than 24 weeks of follow-up [s1]. But in the 17 trials running 24 weeks or longer, the only benefits observed were of diet strategies against an ad-libitum diet — the differences between fasting methods and continuous restriction did not persist [s1].

Seventeen trials is a thin evidence base for the question people actually care about, which is what happens after half a year. The review states the limitation directly: longer duration trials are needed [s1].

The lipid results cut both ways

Among the intermittent strategies, alternate day fasting lowered total cholesterol, triglycerides and non-HDL cholesterol compared with time restricted eating [s1]. But compared with whole day fasting, time restricted eating produced a small increase in total cholesterol, LDL cholesterol and non-HDL cholesterol [s1].

No differences were found between intermittent fasting, continuous energy restriction and ad-libitum diets for HbA1c or HDL cholesterol [s1].

The authors' overall conclusion is that the current evidence gives some indication that intermittent fasting diets have similar benefits to continuous energy restriction for weight loss and cardiometabolic risk factors [s1]. That is a statement about equivalence, not about superiority.

A different question about diet, from the same month

A cohort study published in Nature Food on 2 June asks about composition rather than timing [s2]. Using 124,805 UK Biobank participants, it reports that those consuming the widest diversity of dietary flavonoids, flavonoid-rich foods or specific flavonoid subclasses had a 6-20% lower risk of all-cause mortality and of incident cardiovascular disease, type 2 diabetes, cancer, respiratory disease and neurodegenerative disease [s2].

The novel part is the separation of diversity from quantity: the authors report that both are independent predictors of mortality and several chronic diseases, suggesting that a higher quantity and wider variety together beat either alone [s2]. The flavonoid-rich foods named are tea, berries, apples, oranges and grapes [s2].

This is observational, in a cohort known to be healthier and more affluent than the general UK population, with diet measured by self-report. A 6-20% range across six different outcomes is a wide band, and people who eat many different fruits and drink tea differ from people who do not in ways no model fully adjusts away. The paper frames its finding as an association and a suggestion, not as a recommendation [s2].

Reading the two together

Neither study tells a reader what to do, and neither is written to. Between them they describe a body of evidence in which the timing of eating produces small differences that mostly disappear in longer trials, while the variety of what is eaten shows associations that are larger but cannot be attributed to diet alone [s1][s2].

What to watch

Whether trials of 24 weeks or longer accumulate — the current 17 are the weakest link in the fasting evidence [s1]. And whether flavonoid diversity is ever tested experimentally rather than observed [s2].

Sources

• [s1] Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trials. The BMJ, 18 June 2025. https://doi.org/10.1136/bmj-2024-082007
• [s2] High diversity of dietary flavonoid intake is associated with a lower risk of all-cause mortality and major chronic diseases. Nature Food, 2 June 2025. https://doi.org/10.1038/s43016-025-01176-1