# What intermittent fasting does and what remains uncertain

_By Maurice Lichtenberg · Published 2026-10-31_

Not a scientific paper, but our own look at 24 trials without known links to diet companies or diet books: intermittent fasting helps you lose weight, but barely more than a classic diet. That it makes you live longer is not proven.

## At a glance

- In 11 trials, people who fasted lost 2.1 kg more on average than people eating as usual. But most ran for only about three months, and results varied widely.
- Against a classic diet in which you eat a little less every day, fasting came out only just under 1 kg ahead across 14 trials.
- We left out 27 of 72 trials because we found links to diet companies or diet books.
- Long-term studies capture eating habits other than deliberate 16:8. Whether 16:8 harms or helps does not follow from them.

Intermittent fasting can help you lose weight, but compared with a classic diet the advantage is small. That is the result of our own analysis of 24 trials. There is no proof, however, that fasting makes you live longer.

We ran the numbers ourselves

This article is not a scientific paper. We simply wanted to check the research ourselves, so we collected 72 trials in which chance decided who fasted and who did not. All of them ran for at least eight weeks.

We left out 27 of them because we found a link to the diet industry: a food or diet company gave money or products, or someone on the research team worked for such a company or had already written a diet book. Another 13 did not fit our question, for example because they studied lean, healthy people, or we could not check their numbers in the original. Another 8 tested forms other than 16:8 and 5:2, such as fasting every other day. We left those out to keep the comparison simple.

That left 24 trials, mostly of adults with overweight or type 2 diabetes. These groups were broadly comparable, even though age and support differed. We did not rate the quality of the individual trials.

Fasting helps you lose weight, but barely more than a diet

All trials tested one of two forms: 16:8 and similar, where you eat only within some hours each day, or 5:2, where you eat very little on two days a week. The fasting groups were compared either with people eating as usual or with people on a classic diet, eating a little less every day.

In 11 trials, people who fasted lost 2.1 kg more on average than people eating as usual. With 5:2 it was 3.0 kg, with 16:8 and similar 1.6 kg. That is not a fair comparison of the two forms, though, because they were mostly tested in different trials with different people.

[Chart: Against usual eating, people who fasted lost more. Extra weight loss with fasting compared with people eating as usual, in kg. All combined: −2.1 kg (−3.1 to −1.2); 16:8 & co.: −1.6 kg (−2.6 to −0.5); 5:2: −3.0 kg (−4.8 to −1.1). Source: Longevity Cities analysis, not a scientific paper; study list on request]

Most trials ran for only about three months, and their results varied widely. Smaller trials tended to find bigger effects than large ones, and the only trial that ran for six months came to just half a kilo, which may also be chance. None of these trials shows whether the weight stays off for years.

Against a classic diet, the advantage shrinks. In 14 trials, fasting came out only 0.8 kg ahead on average. For 16:8 and similar, that may also be chance.

[Chart: Against a classic diet, only a little more. Extra weight loss with fasting compared with people eating a little less every day, in kg. All combined: −0.8 kg (−1.5 to −0.1); 16:8 & co.: −0.9 kg (−2.3 to +0.6); 5:2: −0.9 kg (−1.5 to −0.2). Source: Longevity Cities analysis, not a scientific paper; study list on request]

A 2026 Cochrane review that included only trials of six months or more comes to a similar result. Against no treatment at all, people who fasted there lost about 3.4 percent more of their weight, and against conventional dietary advice it found no clear advantage. In our analysis, too, the lead over a diet was small at just under one kilo.

Fasting mainly helps you eat less

A one-year Chicago trial of 90 adults with obesity shows why the gap to a diet is so small. One group ate 16:8 for six months and then within a ten-hour window, a second counted calories, and a third ate as usual. Both weight-loss groups reported eating about 400 kcal a day less and lost a similar amount, 3.5 kg with 16:8 and 4.3 kg with calorie counting. That was not a clear difference. This suggests that fasting mainly helps because you eat less. This trial is not in our calculation, because one of the researchers has written books on fasting.

Does eating early in the day help, then? In a trial with delivered meals, everyone got exactly the same calories, and those who ate early did not lose measurably more. With equal calories, the time of day seems to change little.

Long-term studies measure other eating habits

Whether fasting extends life only shows over many years, and for that there are only observational studies. In a US study of 33,052 adults, people who ate within fewer than eight hours a day had a 34 percent higher death rate than people whose eating was spread over 12 to 13 hours. At eight to ten hours, roughly 16:8, the difference was not clear. People who skipped breakfast also died more often in several studies.

It does not follow that 16:8 is harmful. Whether people fasted on purpose was not recorded, and those who ate within only a few hours a day were poorer on average and more often unsure whether they could afford food. The researchers adjusted for this, but that rarely works fully. On top of that, people were asked about their eating times on just two days, yet followed for about eight years.

More cell recycling is not proven yet

Autophagy, the recycling of worn-out cell parts, is often said to switch on after about 16 hours without food. That is not proven. In a trial from Adelaide, this recycling in blood cells was slightly stronger after six months of fasting than in a group that only got nutrition tips, but in the planned analysis the result was not clear-cut. The trial did not measure anything during the fast itself.

What fasting can cost you

People who lose weight almost always lose some muscle too, whatever the method. So the real question is whether fasting costs more muscle than a classic diet. In a large analysis, that was not clearly the case, and in mostly small trials in which everyone trained, there was no extra loss either. Still, make sure you get enough protein and training, because both help you keep muscle while losing weight. Our protein calculator gives a rough idea of how much protein you need.

Take care if you use insulin or diabetes tablets such as glimepiride, because when meals drop out, your blood sugar can fall too low. A review on fasting with diabetes therefore advises fasting only with medical supervision. Pregnant and breastfeeding women, people who are underweight, teenagers and anyone who has or had an eating disorder should not fast without medical advice. If you are constantly hungry, start binge eating or sleep worse, spread your eating over more hours or stop.

On a personal note: I have eaten 16:8 since I was 17, for the least scientific reason there is. Skipping breakfast was easier than making it. Even so, today I would advise teenagers against it. After everything we have gathered, intermittent fasting is mainly a comfortable way to eat a little less. If it works for you and you eat enough, especially protein, it can be a good fit. There is no proof yet that it makes you live longer.

## Bottom line

**What the trials show** Intermittent fasting can help you eat less. People who fasted lost about 2 kg more than people eating as usual, but only just under 1 kg more than people on a diet, mostly in trials of a few months.

**What is still open** That fasting lengthens or shortens life is not proven. The long-term studies capture eating habits other than deliberate 16:8, and the groups differed in many ways.

**What is worth a try** If 16:8 or 5:2 suits you, it can be a good fit. Get enough protein and training to keep your muscles. If you take insulin or diabetes tablets, talk to your doctor first.

## FAQ

### Does intermittent fasting help you lose weight?

Yes, but the advantage is small. In our analysis of 11 trials, people who fasted lost 2.1 kg more on average than people eating as usual. Against a classic diet in which you eat a little less every day, the gap across 14 trials was just under 1 kg. We left out trials with known links to diet companies or diet books.

### Is 16:8 better than counting calories?

A clear advantage is not proven. In a one-year trial of 90 adults, one group ate 16:8 for six months and then within a ten-hour window. It lost 3.5 kg, the calorie-counting group 4.3 kg, and that was not a clear difference. This trial is not in our calculation, because one of the researchers has written books on fasting.

### Is it better to eat earlier in the day?

With the same calories, apparently hardly. In a trial with delivered meals, everyone got exactly the same calories, and those who ate early did not lose measurably more.

### Does autophagy start after 16 hours of fasting?

That is not proven. In a trial from Adelaide, cell recycling in the blood cells of people who fasted was slightly stronger after six months than in people who got nutrition tips, but in the planned analysis the result was not clear-cut. The trial did not measure recycling during the fast at all, so it can neither show nor rule out such a switch.

### Does 16:8 raise the risk of death?

That is not proven. In a US study, people who ate within under eight hours a day had a higher death rate, even after adjusting for income and smoking. At eight to ten hours, roughly 16:8, the difference was not clear. And people were asked about their eating times on just two days.

### Do you lose muscle with intermittent fasting?

Losing weight almost always costs some muscle, whatever the method. That fasting costs more muscle than a classic diet is not proven: a large analysis found no clear difference, and neither did mostly small trials in which everyone trained. Enough protein and training help you keep muscle.

## Sources

- Longevity Cities (2026). Own pooled analysis of 24 randomised trials on 16:8 and 5:2 fasting and body weight, excluding trials with known links to diet companies or diet books (random-effects model, quality not rated). Not a scientific paper; study list and calculation available on request.
- Garegnani, L. I., et al. (2026). Intermittent fasting for adults with overweight or obesity. Cochrane Database of Systematic Reviews, 2026(2), CD015610. https://doi.org/10.1002/14651858.CD015610.pub2
- Lin, S., et al. (2023). Time-restricted eating without calorie counting for weight loss in a racially diverse population: a randomized controlled trial. Annals of Internal Medicine, 176(7), 885-895. https://doi.org/10.7326/M23-0052
- Chen, Y. E., Tsai, H. L., Tu, Y. K., & Chen, L. W. (2026). Effects of timing and eating duration of time restricted eating on metabolic outcomes: systematic review and network meta-analysis. BMJ Medicine, 5(1), e001071. https://doi.org/10.1136/bmjmed-2024-001071
- Maruthur, N. M., et al. (2024). Effect of isocaloric, time-restricted eating on body weight in adults with obesity: a randomized controlled trial. Annals of Internal Medicine, 177(5), 549-558. https://doi.org/10.7326/M23-3132
- Mao, Z., et al. (2025). Association of eating window with mortality among US adults: insights from a nationally representative study. Aging Cell, 24(11), e70230. https://doi.org/10.1111/acel.70230
- Wang, Y., Li, F., et al. (2024). Breakfast skipping and risk of all-cause, cardiovascular and cancer mortality among adults: a systematic review and meta-analysis of prospective cohort studies. Food & Function, 15(11), 5703-5713. https://doi.org/10.1039/d3fo05705d
- Bensalem, J., et al. (2025). Intermittent time-restricted eating may increase autophagic flux in humans: an exploratory analysis. The Journal of Physiology, 603(10), 3019-3032. https://doi.org/10.1113/JP287938
- Dai, Z., Wan, K., et al. (2024). The effect of time-restricted eating combined with exercise on body composition and metabolic health: a systematic review and meta-analysis. Advances in Nutrition, 15(8), 100262. https://doi.org/10.1016/j.advnut.2024.100262
- Grajower, M. M., & Horne, B. D. (2019). Clinical management of intermittent fasting in patients with diabetes mellitus. Nutrients, 11(4), 873. https://doi.org/10.3390/nu11040873

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_Canonical: https://longevity-cities.com/en/articles/intermittent-fasting-what-the-science-says · Part of Longevity Cities · Updated 2026-10-31_