Time-restricted eating: what the trials show, and why the window's timing matters more than its length

What it means

Time-restricted eating (TRE) means consuming all calories within a fixed window each day — usually 8–10 hours — and fasting for the rest. Unlike other fasting patterns it does not prescribe how much to eat, only when. Early TRE (eTRE) places the window in the morning and early afternoon, aligned with the circadian peak in insulin sensitivity; late TRE starts at noon or later.

It is one of several intermittent-fasting patterns alongside alternate-day fasting, 5:2 and periodic longer fasts, and the easiest to sustain.

Why it matters for longevity

Most adults eat across 14–15 hours a day. Compressing that window gives the peripheral clocks a consistent signal, extends the overnight fast during which insulin falls and autophagy rises, and tends to reduce intake without counting. For metabolic health the question is whether it adds anything beyond the calories it removes — and the answer depends on when the window is.

What the evidence says

Sutton 2018: 8 men with prediabetes, a 6-hour early window (last meal by 3 pm) versus a 12-hour window, with calories matched so no weight was lost; insulin sensitivity, blood pressure and oxidative stress improved on eTRE. Wilkinson 2020 (TRE 10 hours, 19 adults with metabolic syndrome): 3% weight loss and improved blood pressure and LDL over twelve weeks. Jamshed 2022: eTRE plus calorie restriction lost 2.3 kg more than calorie restriction alone over 14 weeks.

The negative trial: Lowe 2020 (TREAT, 116 adults, 16:8 with a noon-to-8 pm window) found no significant weight or metabolic benefit versus controls and a hint of lean-mass loss. Liu 2022 (139 adults, one year) found TRE added nothing to calorie restriction. The pattern across trials: early windows help; late windows mostly do not; the main effect is eating less.

How to measure or use it

• A 10-hour window ending three hours before bed is the sustainable default; an 8-hour early window if glucose is the target.
• Keep protein at 1.6 g/kg inside the window, spread over three meals, to avoid the lean-mass loss seen in TREAT; a DEXA at baseline and three months checks it.
• Judge it on fasting insulin, HbA1c and a CGM trace over 12 weeks. Not for pregnancy, under-18s, or anyone with a history of eating difficulties.

Related

• Fasting and time-restricted eating — the full article, with all fasting patterns
• Metabolic health — the markers it moves
• The fat-loss protocol — TRE inside a fat-loss plan