Intermittent Fasting: Does It Really Work? A Look Across 99 Trials

Few diet ideas have been as popular, or as hotly debated, as intermittent fasting. Fans call it simple and powerful. Critics say it is just another way to eat less. A large 2025 review in The BMJ, which pooled the results of 99 randomized trials, gives us the clearest picture so far, and it is friendlier news than you might expect.

What intermittent fasting means

It is not one diet but a family of eating patterns that focus on when you eat, not only what you eat. The main types studied are:

• Alternate-day fasting: eating very little (or nothing) every other day and eating normally on the days in between.
• Whole-day fasting (such as 5:2): a couple of low-calorie days each week and normal eating on the others.
• Time-restricted eating: eating only within a set window each day, for example eight hours, and fasting for the rest.

The 2025 review in detail

The BMJ analysis looked at 99 randomized trials with 6,582 adults. It compared intermittent fasting strategies with two things: continuous calorie restriction (the traditional “eat a bit less every day” approach) and eating as you normally would.

• Compared with eating normally, all of the approaches, including intermittent fasting and continuous calorie restriction, helped people lose weight.
• Compared with each other, alternate-day fasting came out slightly ahead of continuous calorie restriction, by about 1.3 kg (roughly 3 pounds) on average.
• That gap is small. It did not reach the 2 kg difference the researchers set as clinically meaningful for people with obesity.
• Some health markers improved with alternate-day fasting, including waist size and some blood fat and inflammation measures.

The authors also noted that long-term safety and how well people stick with these patterns still need more study.

So does it work?

The honest answer is: about as well as other diets, for people who can stick with it. A long-running debate about whether fasting has a special “metabolic magic” has not been settled in its favor by the evidence we have. What it does offer is a different structure, and that can be a gift. Some people find “I eat between noon and eight” easier than counting calories all day. Others find long gaps between meals miserable, and that is perfectly fine too.

Who should be careful

Intermittent fasting is not right for everyone. Please talk with a doctor first if any of these apply to you:

• You are pregnant, breastfeeding, or trying to conceive.
• You have a history of an eating disorder, or fasting makes you obsessive about food.
• You take medication for diabetes or blood pressure, because eating less can change how much you need.
• You are under 18 or an older adult at risk of losing muscle.

Fasting windows can also make it harder to get enough protein and fiber. If you give it a try, our explainer on how much protein you actually need shows what a day’s target looks like, and the keto guide covers another popular way of eating and its trade-offs.

A friendly way to decide

1. Ask what you can keep up. The best pattern is the one you can follow for months, not days.
2. Look at the whole day. A short eating window full of ultra-processed foods is still a short eating window full of ultra-processed foods.
3. Notice how you feel. Headaches, dizziness, irritability and obsessing about food are signals to stop and talk to a professional.
4. Keep up your strength training. Losing weight too fast can cost muscle.

The big picture

Intermittent fasting works about as well as other ways of eating less, no better and no worse by a meaningful margin. If it suits your life, it is a reasonable tool. If it does not, you are not missing a secret, and you have plenty of other good options.

Sources

1. BMJ Group. Intermittent fasting comparable to traditional diets for weight loss (summary of the 2025 BMJ systematic review and network meta-analysis of 99 randomized trials): bmjgroup.com
2. Intermittent fasting versus continuous calorie restriction: which is better for weight loss? PMC9099935

This article describes research and is not medical advice. Talk with your doctor before starting any fasting pattern. See our Medical Disclaimer.

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