Does fasting work for weight loss?
Yes, about as well as cutting calories. It helps because people tend to eat less in fewer hours. One fasting plan did edge ahead in trials. But its lead was small.
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Does intermittent fasting work for weight loss?
Yes, fasting helps people lose weight. But in trials it rarely beats a normal calorie cut.
A 2026 Cochrane review pooled 22 trials of adults with overweight or obesity. Next to usual diet advice, fasting may make little to no difference to weight lost.
Why does it work at all? Diabetes UK says people tend to eat fewer calories when they only eat in set hours.
Think of calories as money you spend each week. Fasting shuts the shop for some hours. Spend the same at the till later, and you lose nothing.
Spend less overall, and you lose weight.
| Trial | Fasting group | Other group |
|---|---|---|
| Eat 12pm to 8pm, no calorie cut, 12 weeks | Lost 0.94 kg | Lost 0.68 kg on 3 meals a day |
| Eat 8am to 4pm plus a calorie cut, 12 months | Lost 8.0 kg | Lost 6.3 kg on the calorie cut alone. The gap may be chance. |
| Alternate-day fasting, 1 year | Lost 6.0% more than a group given no diet | Lost 5.3% more than a group given no diet, on a daily calorie cut |
Sources: Lowe 2020; Liu 2022; Trepanowski 2017
NICE, which writes NHS guidance, makes no recommendation on fasting yet. Any weight-loss diet, it says, must keep what you eat below what you burn.
So pick the plan you can keep up. The next section compares them.
What is the best intermittent fasting for weight loss?
| Plan | How it works | What trials found |
|---|---|---|
| 16:8 | Eat in 8 hours, such as 9am to 5pm. Then just water, tea, coffee or sugar-free squash. | No better than 3 meals a day without a calorie cut. |
| 5:2 | Eat normally 5 days. On 2 days, not in a row, have about 500 to 600 calories. | No better than a daily calorie cut. |
| Alternate-day | Every other day, a fast day of about a quarter of your usual calories. | About 1.3 kg more than a daily calorie cut, in shorter trials. |
Sources: Diabetes UK; NHS London; BMJ 2025; Trepanowski 2017
The plan that edged ahead is alternate-day fasting. A 2025 BMJ review of 99 trials found it lost about 1.3 kg more than a daily calorie cut.
That lead only showed in trials under 24 weeks. In longer trials, each plan beat eating freely, but none beat the others.
It’s also hard to keep up. In a year-long trial, 38% quit alternate-day fasting, against 29% on a daily cut. Many ate more than planned on fast days.
Pick the plan that fits your week. Skip breakfast easily? Try 16:8.
Rather eat normally most days? Try 5:2.
What is the best time to fast to lose weight?
Eating earlier may help a little, but the proof is one small trial.
In it, 90 adults with obesity, mostly women, all cut calories for 14 weeks. Those who ate only from 7am to 3pm lost 2.3 kg more than those who ate over 12 hours or more.
That’s like eating about 214 fewer calories a day. But in the main results, they didn’t clearly lose more body fat.
A later window did no better. With no calorie cut, eating from 12pm to 8pm lost about the same as 3 meals a day.
If an early window suits your days, try it. If it doesn’t, choose hours you can keep. To hold on to muscle as you lose weight, see keeping muscle while you lose weight.
Can fasting help you lose weight fast?
Not faster than other diets. The NHS says to aim to lose 0.5 to 1 kg a week, or 1 to 2 lb.
Even the biggest loss in the trials above, 8.0 kg, took 12 months.
Expect hunger at first. Dietitians Australia says hunger is the main side effect, and you may feel weak. Diabetes UK says cravings can ease over time for some people.
Side effects that keep upsetting your daily life? Stop and get advice from a health professional.
Seen the headline linking short eating windows to heart deaths? The British Heart Foundation (BHF) says that study was observational and not yet checked by other scientists (peer-reviewed). It showed a link, not a cause.
Want to add more? Add movement. See walking to lose weight.
How do I start a fasting plan for weight loss?
Six steps for your first fasting week
- Start small: cut out night-time snacks first.
- Pick one plan: 16:8, 5:2 or alternate-day.
- In fasting hours, drink plenty of water, tea, coffee or sugar-free squash.
- In eating hours, fill up on wholegrains, fruit and veg, beans and pulses.
- On 5:2, spread your 2 fast days out. Never do them in a row.
- Weigh yourself once a week. Aim to lose 0.5 to 1 kg a week.
Sources: Dietitians Australia; Diabetes UK; NHS London; NHS
Fasting is not for everyone. Check this first.
| Who | What to do |
|---|---|
| Pregnant or breastfeeding | Don’t fast. |
| Teenagers | Fasting isn’t suitable for teenagers. |
| Underweight, or had an eating disorder | Only fast with a health professional’s support. |
| Take insulin or other diabetes medicines | Talk to your diabetes team first. Fasting can cause a hypo, when blood sugar drops too low. |
| Having cancer treatment | Speak to your medical team before trying fasting. |
Sources: Dietitians Australia; Diabetes UK; NHS London
Never stop or change a prescribed medicine without your prescriber.
For meals in your eating hours, see the weight loss meal plan and protein for weight loss. To compare fasting with low-carb and low-fat, see the best diet to lose weight.
When to speak to a GP
Speak to a GP or your care team if:
- You take insulin or other diabetes medicines and want to fast. Talk to your diabetes care team before you start.
- You eat very little food, make yourself sick or take laxatives after eating, exercise too much, have very strict routines around food, or spend a lot of time worrying about your weight and shape. The NHS says to see a GP as soon as you can.
- You feel cold, tired or dizzy, have pains, tingling or numbness in your arms and legs, feel your heart racing, faint or feel faint, have bloating, constipation or diarrhoea, or your periods stop. The NHS lists these as signs of an eating disorder.
- Side effects of fasting keep upsetting your daily life.
Worried about your eating? You can also call the Beat helpline on 0808 801 0677.