Does fasting sixteen hours a day slim you down more than simply eating less every day? The most complete analysis to date on intermittent fasting, published in The BMJ, has an answer that isn’t the one social media trends promise: almost no difference at all.
The study, led by Zhila Semnani-Azad and an international team of researchers, was commissioned by the Diabetes and Nutrition Study Group of the European Association for the Study of Diabetes to update its clinical recommendations. It pools 99 randomized clinical trials with 6,582 adults, making it by far the largest comparison ever done between the three most popular forms of intermittent fasting (the 16:8 time-restricted eating window, alternate-day fasting, and 5:2 fasting), ongoing continuous calorie restriction, and eating with no restriction at all.
Every diet beats eating with no restriction, but by a small margin. Compared with unrestricted eating, alternate-day fasting cut weight by an average of 3.40 kg, 5:2 fasting by 2.36 kg, standard continuous calorie restriction by 2.11 kg, and the 16:8 eating window by just 1.72 kg. The authors themselves set the threshold for a truly meaningful change at 2 kg, so only two of the four strategies clearly cross that bar.
Alternate-day fasting is the only variant that beats calorie counting, and by a small margin. Against continuous calorie restriction, alternate-day fasting achieved 1.29 kg of additional weight loss, with moderate certainty of evidence. It also outperformed the 16:8 window (1.69 kg) and the 5:2 diet (1.05 kg), though both differences are considered trivial. Between the 16:8 window and 5:2, there was no difference at all.
These are controlled trials, not surveys about dietary habits. That matters because it allows for talk of causation, not just statistical association: one group was randomly assigned a fasting pattern and another a different diet, and researchers measured what happened. The real caution lies elsewhere: only three trials directly compared one type of intermittent fasting against another, so most comparisons between fasting strategies were made indirectly, through network analysis, not head-to-head.
Alternate-day fasting’s edge disappears in longer studies. In trials lasting less than 24 weeks (76 in total), the results above hold. But in the ones lasting 24 weeks or more (just 17 trials), no fasting strategy showed any difference anymore compared with standard calorie restriction. All still beat unrestricted eating, but among themselves it was a dead heat.
Cholesterol does distinguish between fasting types. Alternate-day fasting lowered total cholesterol, LDL cholesterol and triglycerides more than the 16:8 window, and 5:2 fasting also beat the 16:8 window on total cholesterol. None of the strategies, however, not even against unrestricted eating, showed differences in glycated hemoglobin or HDL cholesterol, the so-called “good” kind.
The real obstacle is sticking with it, not biology. The study found adherence collapses over time: in a one-year trial, compliance with 5:2 fasting fell from 74% at six weeks to 22% after 52 weeks. The most common side effects were mild (constipation, nausea, hunger, dizziness), though there was one serious case of hypoglycemia that ended in a fall.
Compared with weight-loss drugs, the difference between diets is tiny. The authors themselves flag this as context: GLP-1 receptor agonists, like semaglutide, produce weight loss of 10% to 15% of body weight (roughly 8 to 12 kg in an 80 kg person). Next to that figure, the 1 to 3 kg difference between the various fasting or calorie-restriction approaches in this study is almost an afterthought.
With 99 trials and more than 6,500 people behind it, the practical takeaway is simple: no intermittent fasting formula offers a magic advantage over simply eating less on a sustained basis. Anyone who prefers to skip breakfast and eat within an eight-hour window can keep doing so without guilt, just not because science says it will make them lose weight faster, but because it’s simply the strategy that particular person finds easiest to stick with over time.
This article is for informational purposes only and does not replace the advice of a healthcare professional. If you have diabetes, another medical condition, or take medication, consult your doctor before starting any fasting regimen.
Source: Semnani-Azad Z, Khan TA, Chiavaroli L, et al. “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, 2025. Open access (CC BY-NC). DOI: 10.1136/bmj-2024-082007. PMID: 40533200 | PMCID: PMC12175170
