How much weight can you lose with intermittent fasting?

What the trials found

Trial	Who and how long	What they did	Result
TREAT 1	116 adults with overweight or obesity, 12 weeks	16:8, noon to 8 p.m., no other advice	About 0.9 kg, no more than people eating three meals a day
4- and 6-hour windows 2	58 adults with obesity, 8 weeks	Eating only within 4 or 6 hours	About 3% of their weight, compared with a control group
Liu, NEJM 3	139 adults with obesity, 12 months	8 a.m. to 4 p.m. plus a calorie target	8.0 kg, against 6.3 kg with the calorie target alone
Alternate-day fasting 4	100 adults with obesity, 12 months	Eating very little every other day	6.0% of their weight, against 5.3% for daily calorie cutting

When researchers pool the trials, the picture is the same. An umbrella review of 130 randomized trials found real benefits for weight and body fat, but most of the evidence was low quality and the trials were short, typically three months ⁶. A network meta-analysis of 99 trials with 6,582 adults found that every fasting plan and daily calorie cutting led to weight loss compared with eating freely; only alternate-day fasting did a little better than calorie cutting, by about 1.3 kg ⁵.

What to expect, week by week

• The first week or two: the scale may drop quickly. Some of that is water, because when insulin falls your kidneys let go of more sodium and water ^7,8.
• The first two or three months: in the 8-week trial, people on a 4- or 6-hour window lost about 3% of their weight, because they ate around 550 fewer calories a day without counting ².
• Six months to a year: weight loss slows down. In the year-long alternate-day fasting trial, people had lost 6.8% of their weight at six months and 6.0% at a year, after a phase meant for keeping it off ⁴.

Why weight loss slows down

As you lose weight, your body needs less energy to run, so the gap between what you eat and what you burn shrinks. Researchers at the US National Institutes of Health modelled this and found that the body's response to eating less is slow, with half of the eventual change taking about a year ⁹. People with more body fat to start with lose more for the same change in eating. A slowdown is the expected shape of the curve, not a sign that fasting has stopped working. The weight loss plateau guide explains what helps.

Why results vary so much

Averages hide a wide range. Your starting weight, what you eat in your window, how much protein you get, how you sleep, and whether the plan fits your life all matter. Fasting works mainly by helping you eat less ³, so a window filled with large meals can close the gap. And day-to-day weight jumps around: across daily weigh-ins from 1,421 adults, weight rose about 0.35% over the weekend and fell again during the week ¹⁰. Compare weekly averages, not single days.

MYTH

Every kilo you lose on a fast is fat.

FACT

Most of it is, but some lean mass can go too. In the TREAT trial, the 16:8 group lost a little more lean mass than people eating three meals a day ¹. Enough protein and strength training help you keep it ^11,12.

Tracking your own results

Fasting Tracker reads your weight from Apple Health and draws a trend line through the daily ups and downs. Set a weight goal and it shows your progress and the date you could reach it at your current pace. The weight goal calculator gives you a first idea.

Sources

1. Lowe DA, Wu N, Rohdin-Bibby L, et al. Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial. JAMA Intern Med. 2020;180(11):1491-1499. doi.org/10.1001/jamainternmed.2020.4153
2. Cienfuegos S, Gabel K, Kalam F, et al. Effects of 4- and 6-h Time-Restricted Feeding on Weight and Cardiometabolic Health: A Randomized Controlled Trial in Adults with Obesity. Cell Metab. 2020;32(3):366-378.e3. doi.org/10.1016/j.cmet.2020.06.018
3. Liu D, Huang Y, Huang C, et al. Calorie Restriction with or without Time-Restricted Eating in Weight Loss. N Engl J Med. 2022;386(16):1495-1504. doi.org/10.1056/NEJMoa2114833
4. 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
5. 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
6. Patikorn C, Roubal K, Veettil SK, et al. Intermittent Fasting and Obesity-Related Health Outcomes: An Umbrella Review of Meta-analyses of Randomized Clinical Trials. JAMA Netw Open. 2021;4(12):e2139558. doi.org/10.1001/jamanetworkopen.2021.39558
7. DeFronzo RA, Cooke CR, Andres R, et al. The effect of insulin on renal handling of sodium, potassium, calcium, and phosphate in man. J Clin Invest. 1975;55(4):845-55. doi.org/10.1172/JCI107996
8. Sigler MH. The mechanism of the natriuresis of fasting. J Clin Invest. 1975;55(2):377-87. doi.org/10.1172/JCI107941
9. Hall KD, Sacks G, Chandramohan D, et al. Quantification of the effect of energy imbalance on bodyweight. Lancet. 2011;378(9793):826-37. doi.org/10.1016/S0140-6736(11)60812-X
10. Turicchi J, O'Driscoll R, Horgan G, et al. Weekly, seasonal and holiday body weight fluctuation patterns among individuals engaged in a European multi-centre behavioural weight loss maintenance intervention. PLoS One. 2020;15(4):e0232152. doi.org/10.1371/journal.pone.0232152
11. Moro T, Tinsley G, Bianco A, et al. Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. J Transl Med. 2016;14(1):290. doi.org/10.1186/s12967-016-1044-0
12. Tinsley GM, Moore ML, Graybeal AJ, et al. Time-restricted feeding plus resistance training in active females: a randomized trial. Am J Clin Nutr. 2019;110(3):628-640. doi.org/10.1093/ajcn/nqz126
13. Grajower MM, Horne BD. Clinical Management of Intermittent Fasting in Patients with Diabetes Mellitus. Nutrients. 2019;11(4):873. doi.org/10.3390/nu11040873