16:8 vs 18:6 vs OMAD vs 5:2 — which fits you.

Four protocols. Thousands of Reddit threads. Very little head-to-head clinical evidence. Here is what the peer-reviewed research actually supports — and, more importantly, what it does not support.

The four protocols, head to head

The four protocols below are listed in rough order of evidence strength and accessibility. The first has the best human RCT data and the lowest cost; the last is best understood as a calendar-flexibility tool, not a stronger stimulus. Read across, then drop down to the table at the end for the side-by-side.

16:8 — the default, and the one with the best evidence

Eat within an 8-hour window; fast for 16. Both Sutton (2018) and the TIMET trial (2024) support 16:8 or the slightly wider 10-hour window for improvements in insulin sensitivity, blood pressure, and waist circumference. This is where the cleanest human evidence sits.

Best for: almost everyone. If you are choosing one protocol and not optimizing for anything specific, this is the answer.

18:6 — moderate, if 16:8 feels too easy

Eat within a 6-hour window; fast for 18. Less research directly on the 6-hour window, but evidence from Sutton's early-TRE work (which used a 6-hour window ending at 3pm) is favorable. Slightly higher adherence cost — social meals and restaurants become harder.

Best for: people who have been doing 16:8 successfully for a month and want to tighten. Not for beginners.

OMAD — one meal a day (23:1)

One meal within a ~1-hour window. Zero head-to-head RCTs comparing OMAD to 16:8. Anecdotal evidence is loud. Clinical evidence is thin. As the window shrinks, muscle preservation, adherence, and side-effect load get worse.

Best for: a specific situation — rare. Most people who try OMAD eventually regress to 16:8 and get better metabolic results.

5:2 — five normal days, two restricted days

Five days of normal eating, two non-consecutive days at ~500–600 calories. Easier social calendar than daily protocols. Evidence for weight loss and insulin sensitivity is present but modest relative to daily TRE.

Best for: people who cannot commit to a daily eating window (busy social lives, shift workers, food-focused travel).

Who should avoid each protocol entirely

Women may be more sensitive to aggressive energy restriction. The tighter the protocol, the higher the risk of menstrual cycle disruption, low bone density, and hormonal dysregulation. OMAD and modified alternate-day fasting are highest risk.

Anyone with: pregnancy, type 1 diabetes, eating disorder history, GLP-1 medications, or active recovery from illness should not fast without medical supervision, regardless of protocol.

The protocol comparison table

• 16:8 — strongest evidence, most accessible, best for beginners.
• 18:6 — moderate tighten, evidence supportive, not first-line.
• OMAD — minimal RCT evidence, highest adherence cost, highest risk.
• 5:2 — useful for lifestyle fit, modest benefits.
• Modified ADF — 2025 meta-analysis of 56 RCTs showed largest weight effect (−5.18 kg) but at significant cost — not recommended for most.

What's safe to believe vs. what to flag

The honest bottom line

Start at 16:8. If you have been there a month and feel stable, consider 18:6 briefly. Do not reach for OMAD unless you have a specific reason. Remember: the metabolic benefits of fasting compound over months, not hours. Tighter is not always better.

Sources

• TIMET trial (2024), Annals of Internal Medicine. PubMed 39348690
• Sutton et al. (2018), Cell Metabolism. PubMed 29754952
• Trepanowski et al. (2017), JAMA Internal Medicine — IF ≈ CR when matched. PubMed 28459931
• Kibret et al. (2025), Current Nutrition Reports — network meta-analysis of 56 RCTs, modified ADF weight/waist results. PubMed 40705196
• Sun et al. (2024), EClinicalMedicine — 23 meta-analyses. PubMed 38500840
• de Cabo & Mattson (2019), NEJM. PubMed 31881139