Does black coffee break the fast?

Not in any sense that matters here. The mechanism on this page is calories, so this is a question of how many rather than a yes or no. Black coffee, plain tea and water sit outside the window on that basis, and a splash of milk is a small number you are choosing to spend.

What about autophagy? Isn't that the real reason to fast?

The trials cited on this page measured body weight, fat mass and fasting glucose. None of them measured autophagy, so nothing here either supports or refutes it. On the outcomes that were tested, the NEJM result says the window adds nothing once calories are matched.

Can I run a 16:8 window while I'm on a GLP-1?

The trials behind this page were run in people who were not taking weight-loss medication, so the effect sizes do not transfer. An eight-hour window already makes a 1.2-1.6 g/kg protein target easy to miss, and a suppressed appetite makes it harder still, which puts this in prescriber territory. Fasted training on these drugs is covered at GLP-1 and fasted training.

16:8 Time-Restricted Eating: Modest but Real

What this page establishes

A meta-analysis of eight RCTs found 8/16 time-restricted eating produced significant reductions in body weight and fat mass in adults with overweight or obesity.
In a randomised trial of calorie restriction with or without time-restricted eating (NEJM), adding the eating window to calorie restriction did not produce greater weight loss than calorie restriction alone.
A meta-analysis of TRE combined with calorie restriction found the combination effective for weight loss and cardiometabolic risk, consistent with the deficit rather than the window doing the work.

Quick answers

Does black coffee break the fast?
Not in any sense that matters here. The mechanism on this page is calories, so this is a question of how many rather than a yes or no.
Read the full answer
Black coffee, plain tea and water sit outside the window on that basis, and a splash of milk is a small number you are choosing to spend.
What about autophagy? Isn't that the real reason to fast?
The trials cited on this page measured body weight, fat mass and fasting glucose. None of them measured autophagy, so nothing here either supports or refutes it.
Read the full answer
On the outcomes that were tested, the NEJM result says the window adds nothing once calories are matched.
Can I run a 16:8 window while I'm on a GLP-1?
The trials behind this page were run in people who were not taking weight-loss medication, so the effect sizes do not transfer.
Read the full answer
An eight-hour window already makes a 1.2-1.6 g/kg protein target easy to miss, and a suppressed appetite makes it harder still, which puts this in prescriber territory. Fasted training on these drugs is covered at GLP-1 and fasted training.

16:8 is the most searched and best studied fasting protocol, and it is the one where the honest answer is least exciting. It produces modest weight loss, reliably, in people who were not otherwise restricting. It produces nothing extra in people who already are. Both of those statements come from the same body of evidence, and holding them together is the whole point of this page.

What 16:8 is

All food inside an eight-hour daily window, water and other zero-calorie drinks permitted outside it. A common pattern is noon to 8pm, which removes breakfast. An early pattern runs 8am to 4pm.

The mechanism is calorie restriction. Removing an eating occasion removes the calories in it, and most people do not fully make those calories up inside the window. That is the whole engine. Nothing about the window changes how the food is processed.

The meta-analysis picture

A meta-analysis of eight randomised trials found 8/16 time-restricted eating produced significant reductions in body weight and fat mass in adults with overweight or obesity. A 2025 meta-analysis of 14 RCTs found the protocol effective particularly in adults aged 40-50 and in interventions of eight weeks or less, with reduced adherence in older adults and weaker effects in longer studies.

That pattern, strong in short trials and weaker in long ones, is the standard shape of behavioural weight-loss data. It usually reflects adherence decay rather than the intervention losing potency. Read the eight-week numbers as the best case.

On glucose, a 2025 meta-analysis of 23 RCTs found 16/8 produced a slight reduction in fasting glucose. Slight is the operative word, and it is a separate outcome from weight.

The trial that matched calories

This is the result that settles the mechanism question. In a randomised trial published in the New England Journal of Medicine, both arms received the same calorie prescription and only one also had to eat inside a restricted window. Adding the window produced no greater weight loss than calorie restriction alone.

A separate meta-analysis of time-restricted eating combined with calorie restriction found the combination effective for weight loss and cardiometabolic risk, which is consistent with the deficit doing the work and the window doing none of it independently.

So the practical rule is simple. If you are not currently tracking or restricting, 16:8 is a low-effort way to create a deficit. If you already are, the incremental value of adding a window is approximately zero, and you should not expect the two to add up.

Do not stack this on an existing deficit and expect more 16:8 and calorie counting are two routes to the same place. In the stack builder, selecting both counts the larger of the two rather than summing them, which is what the NEJM trial says should happen.

Protein density inside eight hours

An eight-hour window is wide enough for a 1.2-1.6 g/kg/day protein target across two or three feedings, and narrow enough that it will not happen by accident. For an 85 kg adult that is roughly 100-135 g of protein, which splits comfortably into three servings of 35-45 g.

The people who lose more muscle than they intended on 16:8 are usually the ones who used the window to eat two ordinary meals and a snack without ever counting protein. The window does not care what is in it, and that is exactly the weakness of a protocol that removes the need to track.

Why the effect shrinks in longer studies

Short trials report bigger results than long ones across this literature, and it is worth knowing which explanation applies. Adherence is the main one. An eight-week trial captures people during the period when a new rule still feels novel and the research team is still calling. A twelve-month trial captures what happens after that.

The second explanation is arithmetic. Weight loss slows as you get lighter, because a smaller body costs less to run. A protocol producing 0.15% per week at the start produces less than that six months in at the same level of effort, and that is true of every method on this site rather than anything specific to fasting.

Neither explanation means the protocol stopped working. It means the honest expectation for month nine is lower than the number you read in an eight-week paper.

Early window or late window

Evidence mildly favours earlier windows for glucose markers. The weight-loss difference between an early and a late window is small, and the adherence difference is not small at all.

Choose by your schedule. If your household eats dinner together at 7pm, an 8am to 4pm window is a protocol you will abandon within a fortnight, and a protocol you abandon has an effect size of zero regardless of what the glucose data says.

Who should be careful

16:8 is the gentlest protocol in this cluster and it still has a contraindication list.

Pregnancy and breastfeeding.
A current or past eating disorder.
Under 18.
Type 1 diabetes, or treatment with insulin or a sulfonylurea, because of hypoglycaemia risk during the fasted hours.
Medication requiring food, or requiring stable intake for safe dosing.

If restriction is already loaded for you Fasting schedules can escalate disordered eating fast, and the risk rises with how aggressive the protocol is. In the US, the National Alliance for Eating Disorders helpline is 1-866-662-1235. In the UK, Beat runs a helpline on 0808 801 0677. Talking to someone is a better first step than a stricter protocol.

The evidence, one row per claim

Claim	Tier	Source
A meta-analysis of eight RCTs found 8/16 time-restricted eating produced significant reductions in body weight and fat mass in adults with overweight or obesity.	established	PMC — TRE (8/16) systematic review and meta-analysis
In a randomised trial of calorie restriction with or without time-restricted eating (NEJM), adding the eating window to calorie restriction did not produce greater weight loss than calorie restriction alone.	established	Liu et al., New England Journal of Medicine
A 2025 meta-analysis of 14 RCTs found 16:8 significantly effective particularly in adults aged 40-50 and in short interventions (8 weeks or less), with reduced adherence in older adults and weaker effects in longer interventions.	emerging	Digital Health (Sage) — TRE systematic reviews with meta-analysis
A 2025 meta-analysis of 23 RCTs found 16/8 TRE produced a slight reduction in fasting glucose.	emerging	Nutrition Reviews, Oxford Academic
A meta-analysis of TRE combined with calorie restriction found the combination effective for weight loss and cardiometabolic risk, consistent with the deficit rather than the window doing the work.	established	European Journal of Clinical Nutrition

Sources

Where this comes from

Researched and edited by Thomas Prommer — biohacker, performance athlete, data enthusiast and health coach.

Every number on this page traces to a named source. There are 5 sourced claims below the fold, each with the document it came from.

Sources consulted: PMC — TRE, Liu et al., Digital Health, Nutrition Reviews, European Journal of Clinical Nutrition.

Not clinically reviewed. This page was researched and written against primary regulatory and trial sources, and no clinician has checked it.

Sources last read 2026-08-23.

Keep reading

Educational content, reviewed 2026-08-23. Not medical advice, not a prescription, and not a substitute for a clinician who knows your history. Doses named here are label schedules or doses used in named trials, never a recommendation to you.

16:8 Time-Restricted Eating: Modest, Real, and Mostly Just Calorie Restriction