Intermittent fasting: what it is, types and if it works

What intermittent fasting is and the main types

Intermittent fasting does not tell you what to eat, but when. You alternate periods of not eating, or eating very little, with periods of eating normally. These are the three most studied formats (Varady et al., 2022):

Types of intermittent fasting
Type	How it works	Example
16:8 (time-restricted eating)	You eat everything within an 8-hour window and fast for the other 16	You eat from 12 p.m. to 8 p.m.
5:2	Two days a week you eat about 25% of what you burn; the other five, as normal	If you burn 2,000 kcal, about 500 kcal on Mondays and Thursdays
Alternate-day fasting	A fast day (0 to 500 kcal) followed by a normal day, always alternating	Every other day

While fasting you only have drinks with no calories, such as water, black coffee or unsweetened tea. In the TREAT trial, for example, the instruction was to eat from 12 p.m. to 8 p.m. and have nothing with calories the rest of the day (Lowe et al., 2020).

Does it work for weight loss?

Yes, but not because of some special effect of the clock. It works because, with fewer hours or fewer days to eat, most people eat less. What drives fat loss is still the calorie deficit.

When studies compare fasting with a diet that cuts calories every day, the result is almost the same:

5:2 vs. a daily diet. In 107 women with overweight followed for 6 months, the 5:2 group lost 6.4 kg (14 lb) and the daily diet group 5.6 kg (12 lb), a difference that was not significant (Harvie et al., 2011).
Alternate-day fasting vs. a daily diet. After a year, both groups lost about 6% of their body weight, but 38% of the alternate-day fasters dropped out compared with 29% of the daily dieters (Trepanowski et al., 2017).
Time-restricted eating with the same calories. In 139 people with obesity eating the same calories for 12 months, eating only from 8 a.m. to 4 p.m. added a 1.8 kg (4 lb) difference that was not significant (Liu et al., 2022).
16:8 with nothing else changed. Skipping breakfast and eating from 12 p.m. to 8 p.m., with no other change, led to less than 1 kg (2 lb) of weight loss in 12 weeks, about the same as eating three meals a day (Lowe et al., 2020).

A meta-analysis of 11 trials reached the same conclusion: fasting and daily dieting lead to similar weight loss (Cioffi et al., 2018). The International Society of Sports Nutrition does not find an advantage for body composition either (Aragon et al., 2017). The best option is the one you can stick with.

Do you lose muscle with intermittent fasting?

Any weight loss diet costs some lean mass, and fasting is no different from daily dieting: about 75% of the weight lost is fat and 25% is lean mass (Varady et al., 2022). An umbrella review of meta-analyses also linked fasting with a drop in lean mass (Patikorn et al., 2021).

What changes the outcome is what you do outside the fast. In the TREAT trial, people who fasted without training lost more lean mass in their arms and legs than the control group (Lowe et al., 2020). By contrast, in men who lifted and ate all their calories within 8 hours, fat went down while lean mass and strength held steady over 8 weeks (Moro et al., 2016).

Lift weights. Strength training is the signal that tells your body to hold on to muscle while it loses fat. See the fat loss routines.
Eat enough protein within your window. 1.6 to 2.2 g per kilo of body weight per day covers most people who train (Morton et al., 2018). With 8 hours to eat, split it across 2 or 3 meals. Work out yours with the protein calculator.
On fast days, put protein first. If you do 5:2 or alternate-day fasting, the advice is to eat at least 50 g of lean protein that day to control hunger and limit lean mass loss (Varady et al., 2022).

How to start intermittent fasting in 4 steps

Pick a format that fits your day. 16:8 is usually the simplest: you finish dinner at a set time and push back your first meal the next day. If you would rather eat normally most of the week, try 5:2.
Work out your calories. Fasting does not replace the deficit. Find out how much you burn with the calorie calculator and aim to eat less than that within your window.
Give it one or two weeks to adjust. That is how long the body usually takes to get used to it. Headaches are common at first and tend to ease when you drink more water (Varady et al., 2022).
Check your progress every two weeks. Look at your weekly average weight. If it is not going down, the problem is not the schedule but the calories: eat a little less within your window.

Example with 5:2: if you burn 2,000 kcal a day and eat 500 kcal on your two fast days, you save 1,500 kcal on each, 3,000 kcal a week. That is about 430 kcal a day on average and close to 0.4 kg (0.9 lb) of fat a week if you eat what you burn on the other five days, because a kilo of fat is worth about 7,700 kcal (Hall, 2008).

Side effects and precautions

In a meta-analysis of 15 trials with 1,365 adults with overweight or obesity, intermittent fasting did not cause more fatigue, headaches or dropouts than the control groups (Zhong et al., 2024). Still, a few things are worth keeping in mind:

Hunger in the first few days. It is normal while you adjust. Starting with a slightly longer eating window and shortening it little by little can make it easier.
Overeating when it is time to eat. In the alternate-day trial, sticking to the plan was hard: fasters ate more than prescribed on their fast days (Trepanowski et al., 2017). Plan your meals so you do not make up for it.
If you take medication. Medications for blood pressure, cholesterol or blood sugar may need adjusting as you lose weight, so talk to your doctor before you start (Varady et al., 2022).
Pregnancy, breastfeeding and eating disorders. The evidence on fasting comes from trials in adults (Patikorn et al., 2021). If you are pregnant, breastfeeding, under 18 or have a history of eating disorders, check with a health professional before trying it.

Questions

How many hours do you need to fast to lose weight?

There is no magic number. What makes you lose weight is your overall calorie deficit; the schedule only helps you eat less. One of the most common formats in studies is 16:8, with 8 hours to eat and 16 hours of fasting (Varady et al., 2022).

Does coffee break a fast?

Black coffee with no sugar or milk has almost no calories, so in practice it does not break your fast. In the trials, the rule was to have nothing with calories outside the eating window (Lowe et al., 2020).

Is intermittent fasting better than counting calories?

For weight loss, no: the trials come out about even (Cioffi et al., 2018). It is only better if you find it easier to stick to. If it makes you very hungry or leads you to overeat in your window, a diet spread across the day works just as well.

Can you build muscle while doing intermittent fasting?

You can keep it if you lift and eat enough protein (Moro et al., 2016). Building muscle takes eating a bit more than you burn (Aragon et al., 2017), and that is harder to reach in a short window. See how to build muscle.

Sources

This guide is for information only and does not replace advice from a health professional.

Intermittent fasting: what it is, types and whether it works

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