Spend ten minutes in any fasting forum and you'll find a quiet rivalry. The 16:8 crowd eat within an eight-hour window every day. The 5:2 crowd eat very little on two days a week. Some go further: three low days a week (4:3), or every other day. Each camp has its success stories, and each is sure its numbers are the right ones.
So I looked for the trials that compared them. The answer turned out to be less about the numbers and more about people.
What the evidence shows
The big picture. A 2025 analysis in the BMJ pulled together 99 randomised trials with 6,582 adults (Semnani-Azad et al., BMJ, 2025). It used a method that can compare schedules even when they were never tested head to head. Every schedule beat eating freely. Only alternate-day fasting beat ordinary daily calorie cutting, by about 1.3 kg on average — a gap the authors called small. And in the 17 trials that lasted six months or more, even that edge didn't show up.
Alternate days, a year on. A one-year trial in Chicago put 100 adults with obesity on alternate-day fasting, daily calorie cutting, or no diet (Trepanowski et al., JAMA Internal Medicine, 2017). It's an older study, but still one of the few that ran this long. Weight loss was almost identical: about 6% on alternate days and 5% on daily dieting. The telling detail was elsewhere. More people dropped out of the alternate-day group — 38%, against 29% — and those who stayed tended to eat more than planned on fast days and less than planned on the others. Without quite meaning to, many of them drifted towards ordinary daily dieting.
4:3, with plenty of support. A 2025 trial in Colorado followed 165 adults for a year (Catenacci et al., Annals of Internal Medicine, 2025). One group cut their food by 80% on three non-consecutive days a week and ate freely on the other four. The other cut back by about a third every day — the same weekly total on paper. Both groups got 34 group coaching sessions and advice to be active.
The 4:3 group lost more: 7.6% of their body weight against 5.0%, a difference of about 2.9 kg. Fewer of them dropped out, too: 19% against 30%. Here's the revealing part: when the researchers measured what people actually ate, neither group came close to the planned cut. Over the year, the 4:3 group cut about 10% of their calories and the daily group about 6%. The authors' explanation for the extra weight loss was simple: people stuck to 4:3 better.
5:2 in early type 2 diabetes. One more trial is worth knowing about. In China, 405 adults with recently diagnosed type 2 diabetes were given either a 5:2 plan using meal-replacement products on the two low days, or one of two standard diabetes tablets (Guo et al., JAMA Network Open, 2024). After 16 weeks, the 5:2 group had the biggest drop in long-term blood sugar and lost the most weight — about 9.7 kg, against roughly 5.5 to 5.8 kg on the tablets. It was short, used specific products, and involved one population. It's a promising result, not a reason to change your treatment on your own; we look at it properly in Fasting with type 2 diabetes: can it work as well as a tablet? (coming soon).
Put together, the pattern is hard to miss. The schedule that "won" in each trial was the one people could keep doing.
Why scientists still disagree
Part of the problem is that trials don't compare like with like. A schedule plus 34 coaching sessions is not the same thing as a schedule from an app. A 5:2 plan with meal-replacement shakes is not the same as a 5:2 plan you design yourself.
Then there's time. Most trials last weeks or months, and the small edges seen in short studies tend to shrink in longer ones. And the analyses that rank every schedule against every other often rely on indirect comparisons, because head-to-head trials of 16:8 against 5:2 against 4:3 are rare.
What you'll hear online
"16:8 is the only schedule that works." Every schedule in the big analysis beat eating freely, and none was a clear, lasting winner.
"Alternate days is the most powerful." It had a small edge in short trials. Over a year, in the trial above, it matched daily dieting and lost more people along the way.
Before believing any fasting claim, it helps to ask five quick questions:
Five questions before you believe a fasting claim
- Was this seen in people, or in mice?
- How many people, and for how long?
- Compared to what?
- Did they measure a real outcome, or a number in the blood?
- Does the person telling me this sell something related? (Not an accusation — just useful to know.)
For fasting schedules, add one more to question two: how many people were still doing it at the end?
What would actually settle it
A head-to-head trial of 16:8, 5:2, 4:3 and ordinary daily dieting, with the same level of support for everyone, running for at least two years and reporting not just weight but how many people were still following their plan. It would be a big, unglamorous study. It's also the one that would actually help people choose, and it's worth funding.
What you can do
Choose a schedule by looking at your diary, not at a chart. If your weekends are busy with meals out, low days on quiet weekdays might suit you. If evenings are when the snacking happens, an eating window might fit better. Then give it an honest few weeks and ask yourself one question: could I still be doing this in a year?
If you take medication for diabetes or blood pressure, talk to your doctor before you add fast days — Before you fast on medication: the questions to ask your doctor covers what to ask.
The best schedule isn't the cleverest one. It's the one that quietly becomes part of your week.
Curiosity first.
— Dr. Brugal
Voices in the debate
If you'd like to hear the case for fasting from doctors who champion it, these are some of the most influential voices. Their views often go beyond what published human trials have shown so far — which is exactly why it's worth hearing them and checking the studies side by side.
- Dr Michael Mosley (1957–2024) — trained in medicine at the Royal Free Hospital and became a BBC science journalist; he popularised the 5:2 diet. Book: The Fast Diet. Obituary
- Dr Steven Gundry — MD, former cardiac surgeon, who favours a daily eating window of six to eight hours for longevity and cell health. NBC California Live
- Dr Jason Fung — MD, nephrologist (Canada), who sees fasting as a central tool against obesity and type 2 diabetes. Book: The Complete Guide to Fasting.
For the wider debate, see our series guide: Why scientists still can't agree on fasting (coming soon).
References
All studies retrieved via PubMed.
- 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. BMJ. 2025;389:e082007. Network meta-analysis of 99 randomised trials, 6,582 adults. doi:10.1136/bmj-2024-082007 (PMID 40533200)
- Trepanowski JF, Kroeger CM, Barnosky A, et al. Effect of alternate-day fasting on weight loss, weight maintenance, and cardioprotection among metabolically healthy obese adults: a randomized clinical trial. JAMA Internal Medicine. 2017;177(7):930-938. Randomised trial, 100 adults, 12 months. doi:10.1001/jamainternmed.2017.0936 (PMID 28459931)
- Catenacci VA, Ostendorf DM, Pan Z, et al. The effect of 4:3 intermittent fasting on weight loss at 12 months: a randomized clinical trial. Annals of Internal Medicine. 2025;178(5):634-644. Randomised trial, 165 adults, 12 months. doi:10.7326/ANNALS-24-01631 (PMID 40163873)
- Guo L, Xi Y, Jin W, et al. A 5:2 intermittent fasting meal replacement diet and glycemic control for adults with diabetes: the EARLY randomized clinical trial. JAMA Network Open. 2024;7(6):e2416786. Randomised trial, 405 adults with early type 2 diabetes, China, 16 weeks. doi:10.1001/jamanetworkopen.2024.16786 (PMID 38904963)
Awareness content — not medical advice. Always talk to your doctor or pharmacist before making changes.