Intermittent fasting has a reputation as a miracle. "Eat in an 8-hour window and you lose weight without counting calories." Sounds nice. In June 2025 the BMJ published one of the largest analyses on this topic so far. It pooled 99 randomized trials and 6,582 people. The result is sober: fasting works, and there is no magic in it.
Short answer: Intermittent fasting works about as well as a classic calorie deficit, not clearly better. The analysis of 99 randomized trials and 6,582 people, published in the BMJ in June 2025, found small to no differences between fasting and a classic deficit. Over the longer run (about 6 months or more) all approaches worked about the same. In my coaching, the winner is usually the one a person can stick to.
What the big analysis found
The researchers compared three common forms of fasting with a classic calorie deficit (simply eating less every day) and with eating without limits. The three forms were eating in a time window (for example 16:8), alternating normal days with very low-calorie or no-food days, and whole-day fasts.
- Fasting and a classic deficit both work. Both lead to weight loss compared with eating without limits.
- Fasting is not clearly better than a classic daily deficit. The differences between intermittent fasting and a classic deficit were small to none.
- One form, alternating a "hungry" day with a normal day, did slightly better than a classic daily deficit (about 1.3 kg extra). That was mostly in short studies up to half a year. In the longer studies (only 17 of them), this difference was not seen.
- Over the longer run (about 6 months or more) all approaches worked about the same. In my coaching, the winner is usually the one a person can stick to.
Why fasting works at all
Fasting has no secret fat-burning route. It works for one simple reason: when you shrink the window in which you eat, many people simply eat less over the day. You skip breakfast, you do not snack in the evening in front of the TV. You are in a deficit without counting it. The mechanism is energy balance maths. Fasting is one of many ways to get into a deficit.
That is good news, because you can choose what suits you. Some people do well eating twice a day, others five times. Neither is more "correct". What decides is what you can stick to and how much you eat in total.
Who it fits, and who it does not
Try it if you do not like breakfast, you snack mostly in the evening, or you like a clear rule: "now we do not eat". A narrower window may lower your intake naturally.
Do not force it if hunger ruins your focus and mood, you train early in the morning, or you tend to overeat in the window so that the deficit disappears anyway. And watch out: if you stack pizza, cola and dessert into the eating window, no fasting will save you. What and how much you eat still matters most.
Fasting is not for everyone. If you are pregnant or breastfeeding, have or have had an eating disorder, or take medication (especially for diabetes), talk to your doctor first.
What to take from it
Intermittent fasting is a legitimate tool, working about as well as a classic deficit, not clearly better or worse. If it helps you eat less and keep it up, great. If it only stresses you out, drop it and work on your total intake another way. The time of day you eat matters little. What counts is how much you eat in a day, and whether you stick with it long enough for it to show.
Frequently asked questions
Why does intermittent fasting work?
When you shrink your eating window, many people simply eat less over the day and end up in a deficit without counting. Fasting has no secret fat-burning route. The mechanism is energy balance maths.
Which form of intermittent fasting did best?
Alternating a "hungry" day with a normal day did slightly better than a classic daily deficit, about 1.3 kg extra, mostly in short studies up to half a year. In the longer studies (only 17 of them), this difference was not seen.
Who might do well with intermittent fasting?
Try it if you do not like breakfast, you snack mostly in the evening, or you like a clear rule like "now we do not eat". For some people, a narrower window lowers intake on its own. It is not for everyone: if you are pregnant or breastfeeding, have or have had an eating disorder, or take medication (especially for diabetes), talk to your doctor first. With any health problem, talk to your doctor too.
When should I not push myself into it?
When hunger ruins your focus and mood, you train early in the morning, or you tend to overeat in the window so that the deficit disappears anyway. If you stack pizza, cola and dessert into the window, no fasting will save you. If you are pregnant or breastfeeding, have or have had an eating disorder, or take medication (especially for diabetes), talk to your doctor first.
Is it better to eat twice a day or five times?
Neither is more "correct". What counts is what you can stick to and how much you eat in total.
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Apply for coaching Get the free Fat Loss KitSources, so you can check:
1) Semnani-Azad Z, et al. Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trials. BMJ. 2025;389:e082007 (99 trials, 6,582 adults). DOI: 10.1136/bmj-2024-082007 | PMID: 40533200
Martin Barna
Online nutrition and fitness coach. Since 2013 I have coached 600+ people through fat loss.
Apply for coachingThis article is for information only. It is not medical advice. If you have a health condition, talk to your doctor.