Intermittent fasting helps some people lose weight, but trials show it works about as well as ordinary calorie reduction, not better. A Cochrane review published in February 2026 found little to no difference against standard dietary advice. Choose it only if it suits you, and avoid it if you are pregnant, have diabetes or have had an eating disorder.
What intermittent fasting actually means
Intermittent fasting is an umbrella term for eating patterns that alternate periods of eating with periods of little or no food. It is not one diet. The trials group it into a handful of distinct approaches, and they do not all behave the same way.
- Time-restricted eating (TRE) keeps all food inside a daily window. The popular 16:8 pattern means a 16-hour fast and an 8-hour eating window, often noon to 8 pm. Early time-restricted eating moves the window to the morning and early afternoon.
- Alternate-day fasting swaps normal eating days with fasting or very low-energy days.
- The 5:2 diet means eating normally on five days and cutting right back on two non-consecutive days. Diabetes UK describes the low days as about 500 to 600 kcal (Diabetes UK).
- Whole-day fasting means one or two full days a week with no food at all.
The comparison that matters most is not fasting against doing nothing. It is fasting against continuous energy restriction, the ordinary approach of eating somewhat less every day. That is the question most people are really asking: is fasting a better way to cut calories, or just a different one?
| Pattern | What a typical week looks like | What the trials compared it with |
|---|---|---|
| 16:8 time-restricted eating | Eat inside an 8-hour window each day | Usual meal timing, or the same calories spread across the day |
| Early time-restricted eating | Eat inside a 6 to 8-hour window ending mid-afternoon | A 12-hour eating window at the same calorie intake |
| Alternate-day fasting | Fast or eat very little every other day | Daily calorie restriction |
| 5:2 | Five normal days, two low days of about 500 to 600 kcal | Daily calorie restriction or usual care |
| Whole-day fasting | One or two days a week with no food | Daily calorie restriction |
The headline evidence: Cochrane and the BMJ
Two large evidence syntheses published in 2025 and 2026 now set the tone for this topic, and they broadly agree.
Cochrane review, February 2026
Cochrane reviewed 22 randomised trials with 1,995 adults living with overweight or obesity, followed for up to 12 months. Compared with standard dietary advice, intermittent fasting "may result in little to no difference" in percentage weight loss. The pooled difference was 0.33 percentage points, and the certainty of the evidence was rated low. Cochrane's own summary says fasting "did not appear to have a clinically meaningful effect on weight loss", and notes that "the evidence base remains limited, with only 22 trials, many with small sample sizes and inconsistent reporting" (Cochrane).
BMJ network meta-analysis, June 2025
A network meta-analysis in the BMJ pooled 99 randomised trials with 6,582 adults and compared each fasting style with each other and with continuous energy restriction (Semnani-Azad and colleagues, BMJ 2025). Every strategy, fasting or not, beat eating freely. Alternate-day fasting produced a little more weight loss than continuous energy restriction, a mean difference of 1.29 kg with moderate certainty, which the authors described as trivial. The extra effect appeared only in trials shorter than 24 weeks. Time-restricted eating and whole-day fasting "generally did not result in additional benefits" over daily calorie restriction.
Put the two together and the message is consistent. Intermittent fasting is a legitimate way to eat less. It is not a metabolic shortcut that beats eating less in other ways.
What the landmark trials found
The headline reviews are built on individual trials, and three of them are worth knowing because they are cited so often.
TREAT: 16:8 without calorie advice
The TREAT trial, published in JAMA Internal Medicine in 2020, randomised 141 adults with overweight or obesity to 16:8 eating (noon to 8 pm) or to three structured meals a day, for 12 weeks (Lowe and colleagues, JAMA Internal Medicine). The 16:8 group lost 0.94 kg and the control group 0.68 kg. The difference of 0.26 kg was not statistically significant, and the authors concluded that time-restricted eating "is not more effective in weight loss than eating throughout the day."
The more striking finding was body composition. In participants measured in person, the 16:8 group lost significantly more appendicular lean mass, the muscle in the arms and legs, than the control group. That finding is why protein intake and resistance exercise come up so often in discussions of fasting.
Time-restricted eating plus calorie restriction, 12 months
A 2022 trial in the New England Journal of Medicine went further. It randomised 139 adults with obesity in China to calorie restriction with an 8 am to 4 pm eating window, or to the same calorie restriction with no time limit, for 12 months (Liu and colleagues, NEJM 2022). Weight loss was 8.0 kg with the window and 6.3 kg without it. The 1.8 kg difference was not significant, and the window added no clear benefit for body fat or metabolic risk factors. When calories were matched, timing did not change much.
Early time-restricted eating and insulin
The case for timing rests mostly on a small, tightly controlled study. Sutton and colleagues fed eight men with prediabetes a weight-maintaining diet within a 6-hour window ending by 3 pm, and within a 12-hour window, for five weeks each (Cell Metabolism 2018). On the early, short window, fasting insulin fell, insulin sensitivity improved, and systolic and diastolic blood pressure fell by 11 and 10 mm Hg. These changes happened with no significant difference in weight.
The study is interesting because it hints that eating earlier in the day might matter. But eight participants, five weeks and fully supplied meals are a long way from real life, and it does not justify broad claims about fasting "resetting" metabolism.
Most fasting trials are short, small and in adults with overweight or obesity. Few run beyond a year, and very few track hard outcomes such as heart attacks, diabetes diagnoses or deaths. Treat claims about long-term disease prevention as unproven.
Blood sugar, diabetes and the heart
Type 2 diabetes
The same Diabetes UK guidance says fasting "can have a big impact on your blood sugar levels", and that "we need more research before we can say that fasting diets can help you go into remission long-term". The charity stresses that talking to your healthcare team is particularly important if you take insulin or other diabetes medication, because doses may need adjusting.
NHS Scotland's diabetes service is more explicit. It advises considering the risk of hypoglycaemia (low blood glucose) on fasting days, and discussing fasting with your team "especially if you manage your diabetes with insulin or medication that can cause hypoglycaemia" (NHS Scotland, My Diabetes My Way). It adds that "the long-term safety is yet to be determined".
The 2024 cardiovascular signal
In March 2024, a conference abstract presented at an American Heart Association meeting reported that adults who ate within a window of less than 8 hours a day had a "91% higher risk of death due to cardiovascular disease" (American Heart Association newsroom). The headline travelled fast. The detail matters more.
- It was an abstract, not a peer-reviewed paper. The AHA itself says abstracts are "considered preliminary until published in a peer-reviewed scientific journal."
- It was observational. Eating windows were estimated from two 24-hour dietary recalls in a US national survey, then linked to death records over a median of eight years.
- The lead author said directly that "this does not mean that time-restricted eating caused cardiovascular death."
The fair reading is that this abstract proves nothing in either direction. It is a reason not to oversell fasting as heart-protective, and a reason for better long-term trials. It is not a reason for someone eating a balanced diet in an 8-hour window to panic.
Intermittent fasting for women
"Intermittent fasting for women" is one of the most searched variations, usually because of claims that fasting disrupts hormones. The evidence here is thin.
A secondary analysis of an 8-week trial with a 4 to 6-hour eating window looked at 12 premenopausal and 11 postmenopausal women with obesity (Kalam and colleagues, Obesity 2023). DHEA, an adrenal hormone, fell by about 14% and 13% respectively. Testosterone, androstenedione and sex hormone-binding globulin did not change, and oestradiol, oestrone and progesterone, measured in the postmenopausal women, did not change either. It is a small, short study, so it neither confirms nor rules out effects on fertility or menstrual cycles.
What is clear is the advice on pregnancy. An NHS service in London states that "pregnant or breastfeeding women should not attempt intermittent fasting" (My Health London, NHS). If you are trying to conceive, pregnant or breastfeeding, your energy and nutrient needs come first, and any change in eating pattern should be agreed with your midwife, GP or dietitian.
Who should not fast without medical supervision
Several UK sources agree on who should avoid intermittent fasting or only try it with professional support. A British Heart Foundation dietitian lists situations where "no type of intermittent fasting is suitable", including diabetes or low blood sugar, being underweight, a history of eating disorders, pregnancy or breastfeeding, and being a child or teenager (British Heart Foundation). The same article notes that "there is a lack of long-term data into the effects of the 5:2 diet".
Do not start fasting on your own if you:
- take insulin or any other diabetes medicine that can cause hypoglycaemia;
- are pregnant, trying to conceive or breastfeeding;
- have had anorexia, bulimia, binge eating disorder or another eating disorder, or notice that skipping meals leads to bingeing;
- are under 18, underweight or frail, or are losing weight without trying;
- have a long-term condition that is being actively treated, or take medicines that must be taken with food.
Restrictive eating patterns can tip some people into disordered eating. If fasting leaves you preoccupied with food, guilty after eating or caught in a fast-and-binge cycle, stop and talk to your GP or a registered dietitian.
Social media versus the trial evidence
Online, intermittent fasting is often sold with promises the trials do not support: fat loss "without dieting", a reset metabolism, autophagy that clears ageing cells, or hormone balancing. Some of these ideas come from animal research or from mechanisms that make biological sense. None has been shown in good human trials to deliver benefits beyond those that come from eating less.
The trials are actually more encouraging than the hype in one respect: fasting works about as well as other approaches. That makes it a reasonable tool if it fits your life. Some people find a fixed eating window easier than counting calories, especially if they rarely feel hungry in the morning. Others find long gaps lead to overeating later, poor sleep or low energy at work or in training. Neither experience is a failure; it is information about which approach suits you.
If you want to check how your current diet compares with healthy eating guidance before changing when you eat, the free healthy eating check takes a couple of minutes. To estimate how many calories you burn in a day, and so what a sensible deficit looks like, use the calorie calculator. If you are unsure whether you need to lose weight at all, start with the BMI calculator and talk the result through with a professional.
How to try 16:8 sensibly
If you have no reason to avoid fasting and want to try it, a few practical points come out of the trials.
- Food quality still matters. A window does not turn a poor diet into a good one. Build meals around vegetables, fruit, whole grains, pulses, fish and lean proteins, with patterns like the Mediterranean diet as a template.
- Protect your muscle. The TREAT result on lean mass is a warning. Spread protein across the meals you do eat and keep up resistance exercise. People taking GLP-1 medicines for weight loss face a similar risk, which our weight management guide covers.
- Start gently. Moving from a 12-hour to a 10-hour window, then 8 hours, is easier than jumping straight to 16:8.
- Keep drinking. Plain water, black coffee and unsweetened tea add little or no energy, so most fasting plans allow them, and they help you stay hydrated.
- Watch the window, not just the clock. If fasting pushes you into a large late-night meal, an earlier window may suit you better.
- Judge it at 8 to 12 weeks. Weigh up whether it has helped you eat less, feel better and stick with it. If not, daily calorie reduction is equally effective.
Try the free protein calculator
How fasting compares with other popular diets
Intermittent fasting governs when you eat. Most other diets govern what you eat, and the two can be combined. Restrictive "what" diets such as the ones in our paleo, keto and carnivore evidence review bring their own trade-offs, and stacking a strict window on top of a strict food list raises the risk of nutrient gaps and of disordered eating. Quick-fix plans, covered in our piece on detox diets, have even less evidence behind them.
For most people, the strongest long-term evidence still points to the boring basics, the same ones behind the dietary guidelines in our explainer on the new food pyramid: an eating pattern built on plants, whole grains and unsaturated fats that you can keep up for years. Our guide to the anti-inflammatory diet looks at how that pattern is being marketed now.
The practical takeaway
Intermittent fasting is neither a miracle nor a danger for most healthy adults. The best evidence available in October 2026, a Cochrane review and a BMJ network meta-analysis of 99 trials, shows it helps people lose weight about as well as eating a little less every day. Alternate-day fasting may have a small short-term edge that fades. Timing may matter for blood sugar, but that evidence comes from very small studies.
So choose the approach you can sustain. If a fixed window helps you, use it, with good food quality and enough protein. If you take diabetes medicines, are pregnant, have a history of eating disorders or have a long-term condition, do not fast without speaking to your doctor first. For a plan built around your health, medicines and routine, you can book a session with a registered dietitian.
Frequently asked questions
How long should you eat at maintenance to reset your metabolism?
There is no trial-backed number of weeks that "resets" metabolism. Energy use does fall somewhat as you lose weight, mainly because a smaller body needs less energy. A period of eating at maintenance can make the next phase of dieting feel easier and help you practise keeping weight stable. A registered dietitian can help you plan the timing around your goals.
How do you curb cravings when fasting?
Plan the fast so that it does not cover your usual hungriest time of day. Drink water, black coffee or unsweetened tea during the fast, and break it with a meal containing protein, fibre and vegetables rather than sugary snacks. Get enough sleep, as tiredness drives cravings. If cravings turn into bingeing at the end of a fast, the pattern is not right for you.
How do you stick to a diet mentally?
Choose a pattern that fits your work, family and social life, because the trials show most approaches give similar results when people keep them up. Set small, specific goals, plan meals ahead and avoid labelling foods as forbidden. Expect off days and simply return to your routine. If food is taking over your thoughts, talk to a registered dietitian or your GP.
Is reverse dieting necessary?
Reverse dieting means raising calories slowly after a diet. There is little trial evidence that it is necessary, though a gradual, planned return to maintenance eating can help some people avoid swinging straight back into overeating. The important part is having a plan for maintaining weight after the loss phase, whether you fasted or not. A dietitian can tailor that plan to you.
Is 16:8 intermittent fasting safe for women?
For healthy, non-pregnant women it appears to be safe in the short term, but research on hormones and fertility is small and brief. A 2023 study of 23 women found little change in sex hormones over eight weeks. Women who are pregnant, breastfeeding, trying to conceive or with a history of eating disorders should not fast without professional advice.
Does intermittent fasting work better than counting calories?
Not according to the current evidence. A 2025 BMJ analysis of 99 trials found time-restricted eating and whole-day fasting gave no clear extra benefit over daily calorie restriction, while alternate-day fasting gave a small extra loss of about 1.3 kg in short trials. A February 2026 Cochrane review found little to no difference against standard dietary advice.
Can I fast if I take diabetes medication?
Only with your healthcare team's agreement. Diabetes UK and NHS Scotland both warn that fasting can change blood glucose sharply, and insulin or medicines that can cause hypoglycaemia may need adjusting on fasting days. Never stop or change your medicines on your own to make a fasting plan work.
