Intermittent fasting for weight loss: what the evidence shows

Yes. Every fasting pattern that has been measured has led to weight loss compared with eating freely. Most of the people in those trials already had a health condition. ¹

Across trials, people usually lost between 0.8% and 13% of their starting weight.² As with any weight loss diet, fasting works by helping keep the calories you take in below the calories you burn.³ How much you lose will still depend on how much less you eat overall.

16:8, 5:2 and alternate day fasting explained

What are the main types of fasting patterns? There are 3 main fasting patterns people use when trying to lose weight. They are:

• 16:8, or time restricted eating. You eat inside an eight hour window, say midday to 8pm, and take nothing with calories in it for the other 16 hours.⁴ You can also do this from 9am to 5pm, or whatever pattern best fits your lifestyle.
• 5:2, or whole day fasting. This is when you fast all day for 2 days a week. On your fasting days you should still eat about 400 calories if you are a woman, or 600 calories for men. Your fasting days should not be next to each other. So have at least one eating day between them. You then eat as usual on the other five.⁵
• Alternate day fasting. You alternate a fasting day with a normal eating day. In most trials the fasting day still included a small amount of food.⁸

Does the fasting window itself do anything?

Not really much on its own. When a 16:8 window was the only change people made, they lost just under 1kg over 12 weeks. That wasn't really any better than eating across the day.⁴

The larger results come from fasting plus eating less. Fasting often makes this easier to do. People who kept to an 8am to 4pm window and counted calories lost around 8kg in a year. Calorie counting on its own resulted in an average loss of 6.3kg.⁶

So eating less overall is what shifts the weight. A fasting window is one way of helping make that happen.

Yes, but only slightly. Alternate day fasting beat the 16:8 pattern by about 1.7kg and whole day fasting by about 1kg.¹

If you are considering fasting, pick the pattern that best fits your day. For example, a midday to 8pm window suits someone who skips breakfast anyway. But if you love your breakfast, and enjoy an early dinner, 8am-4pm might be best for you. Meanwhile, two low calorie days a week suit someone who would rather not change every day.

Each method works about as well as the others. Alternate day fasting is the one exception, and it resulted in higher weight loss than daily calorie cutting, by about 1.3kg.

Fasting does not necessarily give a significantly better weight loss result than ordinary dietary advice.⁷ If you are able to adopt both healthier eating habits AND reduce your calories, this can be your best long term route.

Some people might struggle to do both, and so smaller, more gradual changes in your habits can be easier to make. This can ultimately be more beneficial, as they are easier to stick to.

Everyone is different, and some people will also find "treat" days an effective way to motivate themselves to complete their fasting days.

Yes, fasting continues to work long term.

However, most of the weight comes off in the first one to six months. People can sometimes hit a plateau after that.⁸

Possibly a little. Fasting has been linked to a drop in fat-free mass. This is the weight of everything in your body that is not fat.⁸

Increasing your protein intake can help prevent this.

If that concerns you, our advice on how to prevent muscle loss while losing weight covers what helps.

Alternate day fasting lowered total cholesterol and triglycerides, another fat in your blood, compared with a 16:8 window. A 16:8 window slightly raised total cholesterol and LDL, the unhealthy type, compared with whole day fasting. Fasting, daily calorie reduction and eating freely made no difference to HbA1c, which is the longer term measure of blood sugar, or to HDL cholesterol.¹

Fasting does not raise the overall risk of side effects.⁵ Around 1 in 7 people felt tired when fasting, and about 1 in 7 had a headache, while around 1 in 10 were constipated or felt dizzy.⁵

No trial reported a major or severe side effect.⁵ If you feel dizzy, you should have a small snack.⁵

Don't push yourself if you find you are incredibly fatigued or dizzy. You may want to take things a little slower until your body adjusts.

When to stop fasting and get help

Stop fasting if you get symptoms of low blood sugar. This mainly affects people with diabetes who take insulin or some other diabetes medicines, and is rare without diabetes.⁹

If you take diabetes medicines and your blood sugar drops below 4mmol/L, you should break your fast. Treat the low blood sugar, or hypo, with sugary food or drink, then follow it with something starchy.¹⁰

Use NHS 111 online if you need urgent medical help that is not an emergency.¹⁰ See a GP if you do not have diabetes and get symptoms such as shaking, sweating, a fast heartbeat and feeling confused.⁹

Check with your GP, pharmacist or diabetes team before you start if you take insulin or a medicine that can cause low blood sugar. Fasting can be dangerous if you have diabetes.¹⁰

If you are at risk of low blood sugar, do not skip or delay meals.⁹ That is the opposite of what a fasting window asks of you.

Any dietary approach has to be tailored around conditions such as eating disorders, disordered eating and type 1 diabetes. If one of these applies to you, speak to your GP or diabetes team before you try a fasting window.³

Restrictive diets that are nutritionally unbalanced should be avoided, because they don't work long term and can be harmful.³

For children and young people, healthy eating rather than overly restrictive diets is advised. So fasting windows are for adults only.³

Treat a fasting window as one way of eating less overall. Skipping meals can backfire, because you may end up snacking more when you get hungry. Losing weight suddenly through diets is not recommended either.¹²

A realistic rate of loss

You can aim to lose 1 to 2lbs, or 0.5 to 1kg, a week. Aim to be active for around 150 minutes a week, which you can break into shorter sessions. Change one thing at a time rather than everything at once.¹²

In many cases the weight comes back, so decide now what you will do when the fasting stops.³ Our tips for losing weight cover the parts that have nothing to do with the clock.

Habits that make an eating window easier

Swap sugary drinks for water. Aim for your 5 A Day, counting 80g of fruit or vegetables as one portion. Cut down on food high in sugar and fat. Tell someone you trust about your plan, so you have support on a bad day.¹²

You may be able to refer yourself to an NHS weight management service without seeing a GP. Ask the reception staff at your GP surgery what is available near you. NHS Better Health also runs a free weight loss plan app.¹²

• Fasting helps because it makes you eat less. Pick the pattern you can keep to.
• Expect 0.5 to 1kg a week, and judge your progress over months rather than days.
• Check with your GP, pharmacist or diabetes team before you start if you take insulin or a medicine that can cause low blood sugar.