Intermittent fasting means alternating periods of normal eating with periods of no, or almost no, calorie intake. The 16/8 method, the 5:2 diet, alternate-day fasting: the randomised trials published since 2011 make it possible to separate what has been demonstrated in humans from what has mainly been observed in animals, and to identify the situations in which it is better to abstain.
Key points
- Over 12 months, the large trials show weight loss comparable to that of conventional calorie restriction: about -8.0kg versus -6.3kg in the Liu trial (2022), a non-significant difference.
- The 2026 Cochrane review (22 trials, 1,995 participants) concludes that there is little or no difference in weight loss compared with usual dietary advice.
- Eating early in the day may be more favourable: in a small controlled trial, a 6-hour window ending before 15:00 improved insulin sensitivity, at stable weight, compared with a 12-hour window.
- Fasting should be ruled out during pregnancy and breastfeeding and in people with eating disorders; it is not advised for people with diabetes, and anyone on medication should seek medical advice.
What exactly is intermittent fasting?
Intermittent fasting is not a diet in the strict sense: it sets when to eat rather than what to eat. There are three main families, each of which has been tested in clinical trials.
- Time-restricted eating (TRE): all meals fit into a daily window of 4 to 10 hours. The 16/8 method, with 16 hours without food and an 8-hour eating window, is the best-known version.
- Alternate-day fasting: a day of very low intake (often around 25% of energy needs) alternates with a day of unrestricted eating.
- The 5:2 diet: two days a week of very low intake, five days of usual eating.
| Method | Principle | Reference trial | Main result |
|---|---|---|---|
| 16/8 (12:00–20:00) | 8-hour window, no calorie counting | TREAT, 116 adults, 12 weeks | -0.94kg, no difference from three regular meals |
| 8:00–16:00 + calorie restriction | Daytime window and limited calories | Liu, 139 adults, 12 months | -8.0kg versus -6.3kg, non-significant difference |
| 4 or 6 hours (afternoon) | Very short window | Cienfuegos, adults with obesity, 8 weeks | About -3% of body weight, intake reduced by about 550kcal per day |
| Alternate-day | 25% of energy needs one day, 125% the next | Trepanowski, 100 adults, 12 months | -6.0% versus -5.3% (relative to control), more frequent dropouts (38%) |
| 5:2 | 2 days a week at about 2,710kJ (650kcal) | Harvie, 107 women, 6 months | -6.4kg versus -5.6kg, no significant difference |
What happens in the body during intermittent fasting?
After the last meal, the body first draws on glycogen, the liver's glucose store. It then mobilises fat, part of which is converted into ketone bodies, a fuel for the brain and muscles. The New England Journal of Medicine review (2019) places this “metabolic switch” at the heart of the intermittent fasting hypothesis: regularly switching from one mode to the other is thought to stimulate adaptive cellular responses.
This switch involves the mitochondria, the power plants of the cells. However, these mechanisms have mainly been described in animals; in humans, the measurements concern weight, blood glucose, insulin and blood lipids.
Does intermittent fasting lead to more weight loss than a conventional diet?
It does lead to weight loss, but no more than well-followed calorie restriction, mainly through a spontaneous reduction in intake. In the Lin trial (2023), the 16/8 group, without calorie counting, reduced its intake by about 425kcal per day and lost 4.61kg more than the control group over 12 months, with no significant difference from the calorie restriction group.
The longest trials point in the same direction. With an eating window from 8:00 to 16:00 combined with calorie restriction, Liu and colleagues found no significant advantage for weight, body fat or metabolic risk factors compared with restriction alone. Alternate-day fasting did no better than daily restriction after one year, and its participants dropped out more often.
The BMJ network meta-analysis (2025), which pools 99 trials and 6,582 adults, finds that all fasting strategies and continuous restriction produce weight loss compared with unrestricted eating. Only alternate-day fasting does slightly better than continuous restriction (-1.29kg), but this advantage disappears in trials lasting 24 weeks or more. The 2026 Cochrane review concludes, with low certainty, that there is little or no difference in weight loss compared with usual dietary advice.
The best method is therefore the one you can stick to. Our article on L-carnitine examines another widely publicised weight-loss approach.
What are the effects on blood glucose, insulin and the heart?
This is the most nuanced area. In a small controlled trial in men with prediabetes, fed so as to keep their weight stable, a 6-hour window with dinner before 15:00 for 5 weeks improved insulin sensitivity, the response of the pancreatic beta cells, blood pressure and markers of oxidative stress. Some effects may therefore not depend solely on weight loss, but the sample is very small.
In the Harvie 5:2 trial, fasting insulin and insulin resistance fell slightly more than with continuous restriction, for equivalent weight loss. Conversely, in the Trepanowski trial, alternate-day fasting did no better than daily restriction on blood glucose, insulin or blood pressure, and LDL cholesterol was higher at 12 months in the fasting group. The BMJ meta-analysis finds no difference between strategies for glycated haemoglobin (HbA1c).
The timing of the window may matter, although none of the trials cited directly compares a morning window with a late window of the same length. In the Jamshed trial (2022), 90 adults with obesity followed for 14 weeks lost 2.3kg more by eating between 7:00 and 15:00 than with a window of at least 12 hours, with equal dietary support. See our guide to insulin resistance and our article on fatty liver disease, where weight loss remains the best-documented lever.
Does intermittent fasting trigger autophagy?
Autophagy is the process by which the cell breaks down and recycles its worn-out components. A 2018 review concludes that fasting and calorie restriction are among the most powerful non-genetic stimulators of autophagy, but a large share of the observations it brings together come from animal and cell models. Autophagy is difficult to measure in humans, and no trial has established how many hours of fasting would be needed in humans to obtain a useful effect.
Likewise, the gains in lifespan observed in animals under calorie restriction have no demonstrated equivalent in humans. Fasting is also among the levers discussed in connection with NAD+, a molecule central to energy metabolism; our article how to increase your NAD+ details what is established and what is not.
16/8, 5:2 or alternate-day fasting: which intermittent fasting method should you choose?
No method stands out on the basis of its results. A few pointers:
- The 16/8 method is the easiest to fit into daily life. A short window placed early in the day did better than a window of at least 12 hours in the Sutton and Jamshed trials.
- The 5:2 diet suits those who prefer two demanding days to a daily rule.
- Alternate-day fasting is the most demanding. It showed a slight short-term advantage in the BMJ meta-analysis, but the dropout rate of 38% in the Trepanowski trial is a reminder that it is hard to sustain.
- Very short windows: 4 hours did no better than 6 hours in the Cienfuegos trial and, like eating one meal a day, they make it harder to meet protein and micronutrient needs.
Starting with a 12-hour window, then gradually narrowing it, allows you to test your tolerance. Meal quality matters as much as timing: protein at every meal, vegetables, fruit, wholegrains. Sleep and stress also affect hunger; our article on cortisol explains why its daily rhythm is worth preserving.
Who should not practise intermittent fasting?
The French public health information service (Service public d'information en santé) points out that circadian-type fasting is intended only for healthy adults. It mentions in particular:
- pregnant or breastfeeding women, who should never practise this type of fasting;
- people with anorexia or bulimia, who should never practise it either;
- people with diabetes, for whom skipping a meal impairs blood glucose control.
The risk is real: in a small 12-week trial in 41 people with type 2 diabetes on glucose-lowering treatment, two fasting days a week doubled the rate of hypoglycaemia (RR 2.05), despite a reduction in medication supervised by the care team. Anyone taking medication, particularly for diabetes or blood pressure, should talk to their doctor before starting.
Frequently asked questions
Can you drink coffee while intermittent fasting?
Yes, provided it contains no sugar or milk: black coffee, tea and herbal infusions provide virtually no energy, unlike coffee with milk or fruit juice.
How much weight can you lose with 16/8 fasting?
Results vary widely. In the trials, 16/8 with no other instruction led to a loss of about 1kg in 12 weeks (TREAT) and about 4.6kg more than the control group over 12 months (Lin, 2023). With 4- to 6-hour windows, the loss reached about 3% of body weight in 8 weeks.
Will intermittent fasting cause muscle loss?
It can, as with any weight loss. The TREAT trial recorded a more marked decrease in appendicular lean mass in the 16/8 group. Maintaining sufficient protein intake during the eating window and doing strength training limits this risk.
Should you practise intermittent fasting every day?
Not necessarily. The 16/8 trials applied it every day, the 5:2 trials two days a week. What matters is being able to maintain the rhythm for several months.
Is intermittent fasting suitable after 50?
For a healthy adult, perhaps, but the data are limited: the average age of participants was roughly between 40 and 47 in the TREAT, Trepanowski, Jamshed and Lin trials. After 50, preserving muscle and any medication being taken justify medical advice and particular attention to protein.
Is there a link between intermittent fasting and cancer prevention?
There is no evidence to support this. According to the French public health information service, the French National Cancer Institute (INCa) considers that the data do not allow any conclusion that fasting is of benefit in preventing or treating cancer.
Conclusion
Intermittent fasting is one way among others to reduce your intake, not a superior method. Over 12 months, it produces as much weight loss as conventional calorie restriction, with a few favourable signals on insulin when the eating window is placed early in the day. The claims about autophagy and longevity still rest mainly on animal research. If the rhythm suits you, a 10- to 12-hour window, good-quality meals and medical advice if you are on medication are a reasonable starting point; if it feels like a burden, a balanced diet spread across the day gives equivalent results.
Sources
- de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. N Engl J Med. 2019;381(26):2541-2551. doi:10.1056/NEJMra1905136
- Garegnani L, et al. Intermittent fasting for adults with overweight or obesity. Cochrane Database Syst Rev. 2026;2(2):CD015610. doi:10.1002/14651858.CD015610.pub2
- 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. doi:10.1136/bmj-2024-082007
- Liu D, et al. Calorie Restriction with or without Time-Restricted Eating in Weight Loss. N Engl J Med. 2022;386(16):1495-1504. doi:10.1056/NEJMoa2114833
- Lin S, et al. Time-Restricted Eating Without Calorie Counting for Weight Loss in a Racially Diverse Population: A Randomized Controlled Trial. Ann Intern Med. 2023;176(7):885-895. doi:10.7326/M23-0052
- Lowe DA, et al. Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial. JAMA Intern Med. 2020;180(11):1491-1499. doi:10.1001/jamainternmed.2020.4153
- Trepanowski JF, et al. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial. JAMA Intern Med. 2017;177(7):930-938. doi:10.1001/jamainternmed.2017.0936
- Harvie MN, et al. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. Int J Obes (Lond). 2011;35(5):714-727. doi:10.1038/ijo.2010.171
- Cienfuegos S, et al. Effects of 4- and 6-h Time-Restricted Feeding on Weight and Cardiometabolic Health: A Randomized Controlled Trial in Adults with Obesity. Cell Metab. 2020;32(3):366-378. doi:10.1016/j.cmet.2020.06.018
- Sutton EF, et al. Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Cell Metab. 2018;27(6):1212-1221. doi:10.1016/j.cmet.2018.04.010
- Jamshed H, et al. Effectiveness of Early Time-Restricted Eating for Weight Loss, Fat Loss, and Cardiometabolic Health in Adults With Obesity: A Randomized Clinical Trial. JAMA Intern Med. 2022;182(9):953-962. doi:10.1001/jamainternmed.2022.3050
- Corley BT, et al. Intermittent fasting in Type 2 diabetes mellitus and the risk of hypoglycaemia: a randomized controlled trial. Diabet Med. 2018;35(5):588-594. doi:10.1111/dme.13595
- Bagherniya M, et al. The effect of fasting or calorie restriction on autophagy induction: A review of the literature. Ageing Res Rev. 2018;47:183-197. doi:10.1016/j.arr.2018.08.004
- Service public d'information en santé (French public health information service). Is occasional fasting good for your health? Does it help you lose weight? [in French]. Santé.fr. sante.fr
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