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Does intermittent fasting work for weight loss?
Yes, if it helps you eat fewer calories than you burn, and for many people it does. But in trials, intermittent fasting loses about the same weight as ordinary daily calorie cutting; it isn't a metabolic shortcut. For a woman of 70 kg (154 lb) burning 1,890 calories a day, skipping a 400-calorie breakfast is a weekly deficit of 2,800, about 1.6 kg of fat a month. If she eats that breakfast back at lunch, the deficit is zero.
Does intermittent fasting work? The short answer
Intermittent fasting (IF) means eating only in certain windows or on certain days. It works the same way every diet works: by putting you in a calorie deficit. Fewer meals make it easier for some people to eat less, and harder for others. In studies, people who fast lose weight; people on a regular calorie-controlled diet lose about the same. The best plan is the one you can keep.
Why does intermittent fasting work?
Mostly because of calories, not because of a special fat-burning switch. Take our example woman: she burns 1,890 calories a day. If a shorter eating window means she skips a 400-calorie breakfast and eats the rest as usual:
- weekly deficit: 400 × 7 = 2,800 calories
- fat lost: 2,800 ÷ 7,700 = 0.36 kg a week, about 1.6 kg a month (how much weight you can lose in a month)
- but if she eats an extra 400 calories at lunch and dinner, the deficit is 0.
That's the whole mechanism. See your own number in the box above, and the plan behind it: how many calories to eat to lose weight.
What do the trials show?
| Study | What was tested | Result |
|---|---|---|
| Trepanowski 2017, JAMA Intern Med | Alternate-day fasting vs daily calorie restriction vs no diet, 1 year, about 100 adults with obesity | Similar weight loss at 1 year (6.0% vs 5.3%); 38% dropped out of fasting vs 29% of daily restriction |
| Lowe 2020 (TREAT), JAMA Intern Med | 16:8 time-restricted eating vs regular meal timing, 12 weeks, no calorie targets | Small, non-significant difference in weight loss; some lean mass lost |
| Liu 2022, NEJM | 8-hour eating window plus calorie restriction vs calorie restriction alone, 12 months, about 140 adults | No significant difference in weight loss between the groups |
The pattern: fasting is a valid way to eat less, not a better one. Some people find a window easier and some find it harder: in the 2017 trial more people dropped out of alternate-day fasting than of daily restriction. Weight trends and food logs tell you what's working for you (weight-loss plateau).
What types of intermittent fasting are there?
| Type | How it works | The deficit |
|---|---|---|
| 16:8 | Eat in 8 hours, fast 16 (e.g. noon to 8 pm) | Only if you eat less in the window; typical 200–400 kcal/day cut |
| 14:10 | Eat in 10 hours, fast 14 | A gentler version, smaller deficit |
| 5:2 | Eat normally 5 days, about 25% of needs on 2 days | Eat about 450 kcal on fast days; up to 2,830 kcal/week less |
| Alternate-day | Fast (or about 25%) every other day | Up to 4,950 kcal/week if normal days are at maintenance |
| OMAD | One meal a day | Hard to eat enough protein; the deficit depends on the meal |
How to start 16:8
- Pick an eating window that fits your life, such as noon to 8 pm. Don't copy an influencer's.
- Start by pushing breakfast later by an hour a day.
- Drink water, black coffee or tea during the fast.
- Eat proper meals in the window with protein and fibre, not just treats.
- Set calories with the box above. A window is a tool, not a licence.
- After 2–3 weeks check your weight trend, as with any diet.
Is intermittent fasting good for women?
For most healthy women it's safe and works the same way as for men. Be careful if you are pregnant or breastfeeding, trying to conceive, underweight, or have had an eating disorder: fasting isn't advised. Evidence that fasting changes periods or hormones is thin and mixed; if your periods become irregular or stop, or you feel weak or cold, loosen the fast and see a doctor. Women who feel worse in the second half of the cycle may find a longer window easier then. The cycle itself moves your weight; see weight gain on your period.
Will you lose muscle when fasting?
Any weight loss can cost some muscle. Protect it with enough protein across your window, such as 1.6 g per kg (how much protein to lose weight) and strength training two or three times a week. In the TREAT trial lean mass fell in the fasting group, so protein and lifting matter more with a short window.
Who should not do intermittent fasting?
- People with type 1 diabetes, or on insulin or other glucose-lowering drugs (risk of low blood sugar): only with a doctor.
- Anyone pregnant or breastfeeding.
- Under 18s.
- Anyone with a history of an eating disorder, or who binges after fasting.
- People on medicines that must be taken with food, and older adults who are frail or underweight.
If in doubt, ask your doctor first.
Intermittent fasting or counting calories?
IF sets a rule about when you eat; counting calories sets a rule about how much. If time rules make you eat less without effort, use IF. If you snack your way through the window, counting is more reliable (how to count calories). You can combine them, as in the NEJM trial, which found no extra benefit but no harm either. The 2022 result is what to remember: pick the one you'll stay with.
Sources
- 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.
- 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.
- 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.