Alternate-day fasting

What is alternate-day fasting?

Alternate-day fasting, often shortened to ADF, swaps the daily eating window for a two-day rhythm: a "feast day" when you eat freely, then a fast day. In most trials the fast day wasn't a full fast: people ate about a quarter of their usual needs, often around 500 calories, in one meal ^1,2. A strict version, with nothing to eat on fast days, is harder and much less studied.

What the research shows

Shorter trials look a little better. In 32 adults with normal weight or overweight, 12 weeks of alternate-day fasting led to about 5 kg of weight loss compared with a control group, almost all of it fat ³. A 2025 review of 99 randomized trials found alternate-day fasting was the only fasting approach that beat daily calorie cutting, by about 1.3 kg, but only in trials shorter than six months; in longer ones the advantage disappeared ⁴.

It seems to work similarly for different people. Pooling three 12-week trials, premenopausal women, postmenopausal women and men lost a similar share of their weight ².

Can you stick to it?

This is where alternate-day fasting is most demanding. Across the trials, people ate more than planned on fast days: in the pooled analysis, about 400 to 500 calories more ². In the year-long trial, more people gave up on it than on daily calorie cutting ¹.

Hunger, though, isn't always the problem people expect. After 8 weeks of alternate-day fasting, 59 adults with obesity lost almost 4 kg and reported no increase in hunger, and they felt fuller after meals ⁵. Bodies adapt. Whether a plan fits your life is often the bigger question.

Side effects

In an 8-week study of 59 adults with obesity, some people reported constipation (17%), dizziness (under 20%) and general weakness (under 15%), and bad breath became more common ⁶. Binge eating and low mood decreased rather than increased. That was a short study in volunteers, though: if you have or have had an eating disorder, alternate-day fasting isn't for you without professional support.

• Stop and eat if you feel faint, confused or very weak, have palpitations, or can't keep fluids down.
• Mild headache, tiredness and feeling cold are common on fast days. Water, and a pinch of salt in it, often help ⁷.

Alternate-day fasting in Fasting Tracker

Fasting Tracker times fasts; it doesn't log food or count calories, so it can't track a 500-calorie fast day. What it can time is a fast without food. A full fast day, from dinner to breakfast two days later, is a 36-hour fast (Monk Fast in the app), and a dinner-to-dinner fast is 24 hours (Full Day). Both come with Pro, along with custom fasts of up to 72 hours. The ring shows the stage you may be in as the hours pass:

1. 0 hBlood Sugar RisingYour body is digesting your last meal and using that energy. Insulin is elevated.
2. 4 hBlood Sugar FallingBlood sugar settles back to normal and insulin starts to drop.
3. 12 hFuel SwitchYour liver's glycogen is getting low, so your body starts turning to fat for fuel.
4. 16 hFat BurningYour body starts burning stored fat for energy. Stay hydrated!
5. 24 hKetosisFat is now your main fuel, and your liver turns more of it into ketones.
6. 36 hAutophagyCells may start clearing out old components, a natural recycling process.

Repeating 36-hour fasts every other day is far harder than the modified version tested in most trials. If you want the evidence-based version, plan a small meal on fast days and use a daily plan, such as 16:8, on the days between, or treat longer fasts as an occasional 24-hour fast rather than a schedule.

Doing it safely

Sources

1. Trepanowski JF, Kroeger CM, Barnosky A, et al. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial. JAMA Intern Med. 2017;177(7):930-938. doi.org/10.1001/jamainternmed.2017.0936
2. Lin S, Lima Oliveira M, Gabel K, et al. Does the weight loss efficacy of alternate day fasting differ according to sex and menopausal status? Nutr Metab Cardiovasc Dis. 2021;31(2):641-649. doi.org/10.1016/j.numecd.2020.10.018
3. Varady KA, Bhutani S, Klempel MC, et al. Alternate day fasting for weight loss in normal weight and overweight subjects: a randomized controlled trial. Nutr J. 2013;12(1):146. doi.org/10.1186/1475-2891-12-146
4. Semnani-Azad Z, Khan TA, Chiavaroli L, et al. Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trials. BMJ. 2025;389:e082007. doi.org/10.1136/bmj-2024-082007
5. Hoddy KK, Gibbons C, Kroeger CM, et al. Changes in hunger and fullness in relation to gut peptides before and after 8 weeks of alternate day fasting. Clin Nutr. 2016;35(6):1380-1385. doi.org/10.1016/j.clnu.2016.03.011
6. Hoddy KK, Kroeger CM, Trepanowski JF, et al. Safety of alternate day fasting and effect on disordered eating behaviors. Nutr J. 2015;14:44. doi.org/10.1186/s12937-015-0029-9
7. Sigler MH. The mechanism of the natriuresis of fasting. J Clin Invest. 1975;55(2):377-87. doi.org/10.1172/JCI107941
8. Leidy HJ, Clifton PM, Astrup A, et al. The role of protein in weight loss and maintenance. Am J Clin Nutr. 2015;101(6):1320S-1329S. doi.org/10.3945/ajcn.114.084038