16/8, 5:2 or Alternate Day: The Fasting Protocols Compared

What the controlled trials actually found

The question that matters is not whether fasting causes weight loss — it does, because it reduces intake — but whether it does anything a matched calorie deficit does not. Trials designed to answer that keep the calories equal between groups and vary only the timing. They consistently find differences too small to matter, and a twelve-month randomised trial comparing time-restricted eating with daily calorie restriction found no significant advantage for the fasting group.

One finding is worth flagging because it cuts against the popular framing: in a trial of 16/8 without any instruction to count calories, a substantial share of the weight lost was lean mass rather than fat. Whatever protocol you pick, keeping protein high and continuing to train is what protects muscle — the fasting window does not do it for you.

Why it works for the people it works for

The advantage is behavioural, and it is a real one. A window is a single rule you either keep or break, whereas counting calories is a decision repeated at every meal. For people who find tracking exhausting or who eat most of their surplus late at night, closing the kitchen at eight o'clock removes the problem without any arithmetic at all.

It fails for the mirror-image reasons. Compressing the day into a short window can produce genuinely large single meals, and appetite after a fast is not always proportionate to what was skipped. If a 20/4 window ends in a 1,800 kcal dinner, the protocol has not created a deficit — it has moved the calories.

Who should not do this without asking first

Fasting is not a neutral experiment for everyone. It is not appropriate during pregnancy or breastfeeding, in adolescence, or for anyone with a history of disordered eating, where rules about when not to eat can restart a pattern that took years to leave behind.

It also interacts with medication. Anything taken with food, and above all insulin or sulfonylureas, needs a plan before the eating window moves, because a dose timed for a meal that no longer happens is how hypoglycaemia occurs. That conversation belongs with your prescriber, before the first fasted day rather than after it.

Frequently asked questions

Does coffee break a fast?

Black coffee, plain tea and water contain almost no calories and are accepted on every protocol. Milk, sugar and anything sweetened are calories, so they end the fast in the only sense that matters for weight — the intake one. Claims about specific thresholds for other effects rest on much thinner evidence.

Is 16/8 better than 5:2?

Not in the trial data, which shows them producing similar results when the calorie total lands in the same place. The useful question is which one fits your week: 16/8 asks for a small change every day, 5:2 asks for a large change twice. People who cannot face a daily rule often manage two hard days, and vice versa.

Will I lose muscle?

Some, unless you actively defend it. Any weight loss removes a share of lean tissue, and at least one fasting trial found that share unusually high. Keeping protein at the upper end of the usual range and continuing resistance training are what shift the balance towards fat, and they matter more than which protocol you chose.