Protocols · 4 min read
16:8 vs 18:6 vs OMAD: which one actually suits your day
No trial has shown one fasting window beats another for fat loss. A head-to-head of 4-hour and 6-hour eating windows produced the same 3 percent weight loss and the same metabolic changes. What does appear to matter is when your window sits: eating earlier in the day outperformed eating later, even at the same length.
The comparison nobody wants to hear
When researchers put a 4-hour eating window against a 6-hour eating window, both groups lost about 3 percent of body weight. Both improved insulin resistance similarly. Both spontaneously ate around 550 fewer calories a day without being asked to count anything.
The narrower window did not win. It just made people eat a bit less, which the wider window also did.
Zoom out further and the pattern holds. A twelve-month randomised trial of 139 adults found time-restricted eating plus calorie restriction was no better than calorie restriction alone. A network meta-analysis of 99 trials and 6,582 adults found only alternate-day fasting beat continuous restriction at all, by 1.29 kg, which its authors call a trivial difference, and even that disappeared in the trials lasting six months or more.
So if you are choosing a protocol hoping one is metabolically superior, the honest answer is that the evidence does not support that hope. Choose on whether you can live with it.
| Comparison | Result | Study |
|---|---|---|
| 4-hour vs 6-hour window | No difference, ~3% loss in both | 2020, randomised |
| TRE + restriction vs restriction | No difference at 12 months | 139 adults, NEJM |
| Fasting vs continuous restriction | No significant difference | Meta-analysis, 11 trials |
| Alternate-day vs continuous | 1.29 kg, called trivial, gone by 6 months | 99 trials, 6,582 adults |
What does seem to matter: when, not how long
This is the finding that deserves more attention than it gets. In a controlled five-week crossover, eight men with prediabetes ate within a 6-hour window that finished before 3pm. Their insulin sensitivity, beta-cell responsiveness, blood pressure and oxidative stress all improved, and their weight was deliberately held constant throughout. Eight people is a very small trial, so treat the direction as more reliable than the size.
That is an effect independent of weight loss, which is rare in this field. A separate three-arm trial found an early window improved insulin sensitivity more than a mid-day window of the same length. No late window was tested.
Practically: shifting your existing window two hours earlier may do more than making it two hours narrower. And it is usually easier.
Choosing by your day, not by the numbers
16:8 is the default for good reasons. It usually means skipping breakfast and eating between noon and 8pm, it survives a normal social life, and it is the most studied. If you have no strong reason to do otherwise, start here.
18:6 tightens things by two hours at each end. Worth trying if 16:8 has stopped producing results and you want a lever, but be aware that a narrower window makes adequate protein harder, which is the main risk to lean mass.
OMAD is one meal a day. It suits some people who genuinely prefer one large meal. It is the hardest window in which to eat enough protein, the hardest to meet micronutrient needs in, and the hardest to do socially. No trial shows it outperforms wider windows. Two things worth saying plainly: if the appeal is that it stops you thinking about food, or if one meal has a way of becoming a very large one, OMAD is the wrong tool and a wider window is both safer and works as well.
12:12 and 14:10 are underrated. If you are starting from three meals and snacks, 12:12 is a real change and costs almost nothing socially. Beginning here and progressing is more likely to stick than starting at OMAD and quitting in week two.
| Your situation | Try | Why |
|---|---|---|
| Starting out | 12:12 then 14:10 | Real change, no social cost |
| Standard weekday, family dinner | 16:8, noon to 8pm | Most studied, most liveable |
| Training seriously | 16:8 with protein spread | Narrower windows make protein harder |
| Prefer one big meal | OMAD, but watch protein | Works if it suits you, not if it is a way to eat as little as possible |
| Want the metabolic edge | Move the window earlier | The only timing effect with weight-independent evidence, though from 8 men |
The reason any of them work
Every protocol on this page works through the same mechanism: a shorter window makes you eat less without counting. In the 4-versus-6-hour study, participants dropped roughly 550 calories a day without being given a calorie target at all.
There is nothing lesser about that explanation. Most people who lose weight and keep it off do so with an approach they can maintain without constant arithmetic, and this is one of the few that qualifies.
The same mechanism is also the risk. Eating 550 fewer calories a day without noticing is useful if you have some to spare and a problem if you do not. Energy, sleep and training are the check on that, rather than the scale.
So pick the window you will still be doing in three months. The trials say that decision matters more than the number attached to it.
Related tool: Calorie deficit calculator
Sources
- Cienfuegos S et al. Effects of 4- and 6-h time-restricted feeding on weight and cardiometabolic health: a randomized controlled trial in adults with obesity. Cell Metab. 2020;32(3):366-378.
- Liu D et al. Calorie restriction with or without time-restricted eating in weight loss. N Engl J Med. 2022;386(16):1495-1504.
- Cioffi I et al. Intermittent versus continuous energy restriction on weight loss and cardiometabolic outcomes: a systematic review and meta-analysis of randomized controlled trials. J Transl Med. 2018;16(1):371.
- Sutton EF et al. Early time-restricted feeding improves insulin sensitivity, blood pressure, and oxidative stress even without weight loss in men with prediabetes. Cell Metab. 2018;27(6):1212-1221.
- Semnani-Azad Z 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.
- Xie Z et al. Randomized controlled trial for time-restricted eating in healthy volunteers without obesity. Nat Commun. 2022;13(1):1003.
- Lowe DA et al. Effects of time-restricted eating on weight loss and other metabolic parameters: the TREAT randomized clinical trial. JAMA Intern Med. 2020;180(11):1491-1499.
Every source here has been checked to exist and to say what this article says it says. Findings from animal or laboratory studies are labelled, because they often do not transfer to people.