Skip to content
Fasting Index

16 hours daily · beginner

16:8 intermittent fasting

Sixteen hours fasting, an eight-hour eating window, every day. The most common entry point and the one most people mean by "intermittent fasting". The largest randomised trial of it found no weight advantage over three ordinary meals a day.

Who should not do a 16:8 fast

  • Anyone taking insulin or a sulfonylurea, unless the schedule and the doses are managed with the prescribing clinician. Skipping meals on these drugs risks hypoglycaemia, and this is the most serious interaction on the page.
  • Anyone who is pregnant or breastfeeding.
  • Anyone with a current or past eating disorder. Rule-based eating windows are a known trigger for relapse.
  • Children and adolescents.
  • Anyone underweight, or losing weight without intending to.
  • Anyone on medication that must be taken with food.

This is not medical advice and no clinician has reviewed it. If any of the above applies to you, speak to a doctor before fasting rather than after.

A schematic timeline of a fast from 0 to 72 hours. A tone band along the top darkens as the body moves from fed to deeply fasted. Beneath it, liver glycogen is drawn as a stipple field that is largely spent within the first 24 hours. Below that, three traces on independent scales: glucose falls then levels, insulin falls steeply and stays low, ketones rise from about 12 hours and climb through the second and third day. The pattern is well described but individuals vary widely, and the horizontal scale breaks at 24 hours.
The hours of a fast. Above, the phases as a tone that darkens as the body moves from fed to deeply fasted. In the middle, liver glycogen as stipple, largely spent inside the first day. Below, three traces on independent scales: glucose, insulin, ketones. A schematic of a well-described pattern. Individuals vary widely, and the scale breaks at 24 hours.

What is actually happening

A 16:8 schedule extends the overnight fast by moving or skipping one meal, usually breakfast. Through the fasting hours insulin falls and the liver supplies blood glucose, first by breaking down its glycogen store and then increasingly by making glucose from other substrates. Liver glycogen is not typically exhausted at 16 hours, so a daily 16:8 fast does not reliably reach deep ketosis, which is the metabolic state most 16:8 pages imply it produces. The mechanism that does the work in practice is simpler and less interesting: an eight-hour window removes eating occasions, and people who eat fewer times often eat less. That is a behavioural effect, not a metabolic one, and it only appears if intake actually falls. In the largest trial of the protocol it did not.

What the evidence shows

  • In the TREAT randomised trial, 16:8 time-restricted eating produced no significant weight difference against three structured meals a day over 12 weeks: a between-group difference of −0.26 kg, P = 0.63, in 116 adults whose estimated energy intake did not differ between groups.
  • Appendicular lean mass index fell significantly further in the 16:8 group than in the control group in the TREAT trial, a −0.16 kg/m² between-group difference. This is the result most often omitted when that trial is cited approvingly.
  • In a 2025 BMJ network meta-analysis of 99 randomised trials and 6,582 adults, time-restricted eating showed no weight benefit over continuous calorie restriction. Alternate-day fasting was the only fasting schedule that did, by 1.29 kg.
  • Among the 17 trials in that network meta-analysis lasting 24 weeks or longer, no diet strategy outperformed any other. Every separation between fasting schedules appeared only in trials shorter than 24 weeks.
  • When meals were supplied and body weight deliberately held constant, a six-hour early eating window improved insulin sensitivity, blood pressure and oxidative stress markers in men with prediabetes, showing that meal timing can act independently of weight loss. That trial enrolled eight men.

What the evidence does not support

  • The evidence does not support the claim that 16:8 burns more fat than eating the same number of calories on an ordinary schedule. The best-designed trials directly tested that and found no advantage.
  • The widely repeated claim that autophagy switches on at 16 hours is not established in humans. No human trial has demonstrated a meaningful autophagy threshold at 16 hours, and the human studies that exist measured muscle rather than liver and did not move as the popular claim predicts.
  • The insulin-sensitivity findings come from very small, short, mostly male samples with food supplied. They are a reason to take meal timing seriously as a research question, not a demonstrated benefit for a person running 16:8 on their own food.
  • Nothing here establishes long-term outcomes. The trial base is dominated by studies of 12 weeks or less, and the longer trials show the differences disappearing.

How it is run

  • Pick an eight-hour window and keep it roughly constant. Noon to 8pm and 10am to 6pm are the usual choices.
  • Water, black coffee and plain tea through the fasting hours. Anything with calories ends the fast, and the threshold is stricter if the goal is autophagy rather than weight loss.
  • The window can be moved earlier. The only trial to isolate timing from weight used a window finishing in mid-afternoon, and earlier windows are the ones with mechanistic support.
  • If weight loss is the goal, the window is not the intervention. What is eaten inside it is.

What breaks a 16:8 fast

The rules differ by protocol. Check any food, drink or supplement against this protocol below.

See the full index

Sources

No clinician reviews this site. Sources on this page were verified against PubMed by the author, who is not a doctor or a dietitian. What that means.

Last updated 2026-08-16.

The phases of a fast covers the physiology behind every protocol on this site, hour by hour.