Fasting: what intermittent fasting can and can't do, and how to do it sensibly

Fasting is sold as a reset button: burn fat, clear your head, slow ageing, prevent disease. Some of that is supported, some is from animal studies, and some is wishful. This guide covers the main types of fasting, what the human trials actually found, who should not fast, and how to do it sensibly if you want to try.

The main types of fasting

• Time-restricted eating (16:8 and similar). You eat within a set window each day, often 8 hours, and fast for the rest. The most popular version, and the easiest to keep up.
• 5:2. You eat normally on 5 days and cut right down (around 500 to 600 calories) on 2 non-consecutive days.
• Alternate-day fasting. A very low-calorie day every other day. Effective on paper, hard in practice.
• Longer fasts (24 hours or more). Water-only fasts of a day or longer. Anything beyond 24 hours is best done with medical supervision.
• Religious fasting. Ramadan, Lent, Yom Kippur and other traditions. These have their own rules and purposes, and this article isn't advice about them.

What happens in your body when you fast

A few hours after your last meal, insulin falls and your body starts drawing on stored glycogen. As that runs down, usually somewhere after 12 hours, it shifts towards burning fat and making ketones. Researchers call this the "metabolic switch", and a 2019 review in the New England Journal of Medicine argues that flipping it regularly is where many of fasting's possible benefits come from.

You'll also read a lot about autophagy, the process cells use to break down and recycle worn-out parts. It's real, and fasting switches it on in animal studies. What nobody can yet tell you is how long a person needs to fast for a meaningful effect, or whether it changes health outcomes in people. Treat any "fast for X hours to trigger autophagy" claim as a guess.

What the human trials found

Weight loss: it works about as well as ordinary calorie cutting, not better. This is the most consistent finding. A 1-year trial of 100 adults found alternate-day fasting lost about the same weight as cutting calories every day, but 38% of the fasting group dropped out, the highest of any group. A 12-month trial in China of 139 adults found that adding an 8-hour eating window to a calorie-controlled diet made no significant difference to weight loss. A 2021 Cochrane review reached the same conclusion: fasting beats doing nothing, but isn't clinically better than a normal calorie-controlled diet.

The catch for women over 40: muscle. In a 2020 US trial of 116 adults, 16:8 eating without other changes produced very little weight loss, and a noticeable share of what was lost was lean mass. Holding on to muscle gets harder from your 40s, so if you fast, protein at your meals matters more, not less.

Blood sugar: promising, still early. A 2011 trial of 107 women found 5:2 lowered fasting insulin and insulin resistance a little more than daily calorie cutting, for the same weight loss. A small 2018 study of 8 men with prediabetes found that eating within 6 hours, finishing mid-afternoon, improved insulin sensitivity and blood pressure even without weight loss. Interesting, but 8 people for 5 weeks isn't a recommendation.

Heart health, ageing, brain health and cancer: not shown in people. Improvements in cholesterol and blood pressure track the weight lost, and match ordinary dieting. The lifespan and brain-protection findings come from animals. No human trial has shown that fasting prevents or treats heart disease, dementia or cancer, and anyone who tells you otherwise is ahead of the evidence.

Who should not fast without a doctor

• Anyone taking insulin or other diabetes medicines that lower blood sugar: fasting can cause dangerous lows.
• Pregnant or breastfeeding women.
• Anyone with a past or present eating disorder.
• Under-18s, and frail older people at risk of losing muscle.
• Anyone on medicines that must be taken with food, or with low blood pressure, kidney disease, or a history of gout or gallstones.

If you want to try it: the sensible version

• Start with 12 hours. Finishing dinner at 7 pm and eating breakfast at 7 am is already a 12-hour fast. Move to 14 or 16 hours only if that feels easy.
• Eat earlier rather than later. The early-eating window is where the blood-sugar results looked best, and it suits sleep better than a late dinner.
• Put protein first in your eating window. Eggs, fish, meat, dairy, legumes, and a collagen scoop if you use one. It protects muscle and keeps you full.
• Drink. Water, black coffee and plain tea are fine during a fast. Headaches in the first week are often dehydration.
• Judge it by how you feel and what you eat. If fasting leaves you binging in the evening, sleeping badly or obsessing over food, it isn't your tool. A regular, calorie-sensible way of eating does the same job.

Frequently asked questions

Does coffee break a fast?
Black coffee and plain tea, no: they have almost no calories. Milk, sugar or "bulletproof" butter coffee do.

Does collagen break a fast?
Yes, strictly speaking. Any protein has calories and raises insulin a little. If you're doing time-restricted eating, take it in your eating window.

Is 16:8 better than just eating less?
For weight, the trials say no: the results are similar. Its advantage is practical. Some people find a time rule easier to stick to than counting.

Can fasting reverse diabetes?
Weight loss can put type 2 diabetes into remission for some people, and fasting is 1 way to lose weight. But if you're on diabetes medicine, fasting without your doctor adjusting your doses can be dangerous. Talk to them first.

Is fasting safe for women in menopause?
A 12-hour overnight fast is fine for most. Longer fasts are where muscle loss and poor sleep become a risk, so keep protein up and stop if you feel worse.

The sources

• de Cabo R, Mattson MP (2019). Effects of intermittent fasting on health, aging, and disease. New England Journal of Medicine 381:2541–2551. View →
• Trepanowski JF et al. (2017). Effect of alternate-day fasting on weight loss, weight maintenance, and cardioprotection among metabolically healthy obese adults. JAMA Internal Medicine 177:930–938. View →
• Liu D et al. (2022). Calorie restriction with or without time-restricted eating in weight loss. New England Journal of Medicine 386:1495–1504. View →
• Allaf M et al. (2021). Intermittent fasting for the prevention of cardiovascular disease. Cochrane Database of Systematic Reviews. View →
• Lowe DA et al. (2020). Effects of time-restricted eating on weight loss and other metabolic parameters: the TREAT randomized clinical trial. JAMA Internal Medicine 180:1491–1499. View →
• Harvie MN et al. (2011). The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. International Journal of Obesity 35:714–727. View →
• Sutton EF et al. (2018). Early time-restricted feeding improves insulin sensitivity, blood pressure, and oxidative stress even without weight loss in men with prediabetes. Cell Metabolism 27:1212–1221. View →

Find out more

• Can collagen help with weight loss?
• The benefits of collagen for a keto diet
• 8 ways women's bodies can change after 40, and what to do about it
• How sleep improves your health
• The collagen library: every guide, in order

This article is general information, not medical advice. If you take medicine for diabetes or blood pressure, are pregnant, or have a history of disordered eating, speak to your doctor before changing when you eat.

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