The short version
- In the 12-week TREAT trial of 116 adults, 16:8 time-restricted eating produced a between-group weight difference of just 0.26 kg versus three structured meals a day (p = 0.63).
- TREAT's only significant between-group secondary finding was greater loss of appendicular lean mass index in the time-restricted group.
- A 12-month trial of 139 adults in the New England Journal of Medicine found -8.0 kg with an 8am-4pm window plus calorie restriction versus -6.3 kg with calorie restriction alone, a non-significant difference of 1.8 kg.
- In a one-year trial of alternate-day fasting, dropout reached 38% versus 29% for daily calorie restriction, weight loss was similar, and LDL cholesterol was 11.5 mg/dL higher in the fasting group at 12 months.
- A 2023 meta-analysis of 24 trials in 1,768 people found intermittent fasting produced weight loss equivalent to continuous calorie restriction, with a mean difference of 0.26 kg (p = 0.37).
The claim that made fasting famous was never “you will eat less.” It was that when you eat changes what your body does with the food. Same calories, different result. That claim has now been tested properly several times, and it is worth knowing how it went.
Twelve weeks: TREAT
In 2020, a UCSF group published TREAT in JAMA Internal Medicine. 116 adults with a BMI between 27 and 43, randomised for twelve weeks either to eat three structured meals a day or to eat freely between noon and 8pm and take nothing caloric outside it. Classic 16:8.
The time-restricted group lost 0.94 kg. The three-meal group lost 0.68 kg. The difference between the groups was 0.26 kg, with a p-value of 0.63 — statistically indistinguishable from nothing.
There was one significant between-group finding, and it went the wrong way. In the 50 participants who came in for body composition testing, appendicular lean mass index — muscle in the arms and legs — dropped more in the fasting group. Estimated energy intake did not differ between groups, which is the tell: skipping breakfast did not reliably make people eat less over the day.
Twelve months: the NEJM trial
The obvious objection is that twelve weeks is short. So: 139 adults with obesity in Guangzhou, randomised for a full year to calorie restriction alone or the same calorie restriction inside an 8am-to-4pm window. Everyone was prescribed 1,500-1,800 kcal a day for men and 1,200-1,500 for women. 84.9% finished.
Weight loss at twelve months: -8.0 kg with the eating window, -6.3 kg without. Net difference -1.8 kg, confidence interval -4.0 to 0.4, p = 0.11. Waist, BMI, body fat, lean mass, blood pressure and metabolic risk factors all followed the primary outcome. The authors' conclusion: time-restricted eating was not more beneficial than daily calorie restriction.
Note what both groups did lose. Eight kilos and six kilos in a year is a real result. The calorie restriction was doing that work in both arms.
Alternate-day fasting, over a year
The 2017 trial in JAMA Internal Medicine is the one that should have cooled the whole conversation. 100 obese adults, one year, three arms: alternate-day fasting at 25% of energy needs on fast days and 125% on feast days, daily calorie restriction at 75%, or no intervention.
Weight loss was essentially identical between the two active arms at six months (-6.8% in both) and at twelve (-6.0% versus -5.3%). What differed was everything around it. Dropout was highest in the fasting group at 38%, against 29% for calorie restriction. Participants overate on fast days and under-ate on feast days — the protocol quietly converted itself into ordinary calorie restriction. And by month 12, LDL cholesterol was 11.5 mg/dL higher in the fasting group than in the calorie restriction group.
Harder to stick to, no better result, and one lipid marker moving the wrong way.
So is any of it useful? Yes.
A 2023 meta-analysis pooled 24 randomised trials and 1,768 people across alternate-day fasting, 5:2 and time-restricted eating. Against conventional continuous energy restriction, the difference was 0.26 kg, confidence interval -0.31 to 0.84, p = 0.37. Compliance was generally above 80% in trials shorter than three months.
Read that as the good news it is. Intermittent fasting is equivalent to counting calories. If the clock is easier for you than arithmetic — and for a lot of people it plainly is — then you have found a rule you can follow, and following a rule is most of the job.
Timing may matter slightly. A 2022 trial put 90 adults with obesity on energy restriction with either an early 7am-to-3pm window or a window of twelve hours or more. The early group lost 2.3 kg more over fourteen weeks, though body fat difference did not reach significance. The authors put the effect as equivalent to cutting about 214 kcal a day — which is exactly the point. It behaved like a smaller calorie intake, because it was one.
What we'd actually tell you
Choose fasting because the structure suits your life, not because you have been told it unlocks something. It does not appear to. Then protect against the two documented downsides.
- Protein. TREAT's one significant between-group result was extra lean mass loss. A compressed window makes it easy to fall short. Protein's satiety effect also happens to be your best appetite tool inside a short eating window, so this is one intervention that pays twice.
- Resistance training. Same reason. Weight lost as muscle is not the weight you were trying to lose.
- An exit condition. If you are three months in, miserable, and the scale has not moved beyond what ordinary calorie restriction would have given you, the protocol is not failing you — it was never going to beat calorie restriction, because in four separate trials it did not.
And be sceptical of anything sold as a fasting aid. Nothing on the weight-loss shelf changes the arithmetic above, and a capsule that promises to extend a fasted state is selling you a mechanism nobody has shown to matter.
Good questions
Does intermittent fasting work better than just eating less?
No. A meta-analysis of 24 randomised trials in 1,768 people found a difference of 0.26 kg between intermittent fasting and ordinary calorie restriction, which is nothing. That is not a criticism of fasting. It means fasting is a legitimate alternative way to eat less, and you should pick whichever rule you can actually follow.
Is 16:8 enough to lose weight?
Only if it makes you eat less, and in the biggest trial it did not. TREAT found no difference in estimated energy intake between the 16:8 group and the three-meals group, and a between-group weight difference of 0.26 kg over twelve weeks. Some people naturally eat less in a shorter window. Plenty of people just eat the same food faster.
Will fasting make me lose muscle?
It can, and the best trial we have flagged exactly that. TREAT's one significant between-group secondary outcome was greater loss of appendicular lean mass in the fasting arm. The fix is not to abandon the window but to hit your protein inside it and keep lifting. Weight lost as muscle is the wrong weight.
Is alternate-day fasting better than daily calorie restriction?
It was not in the one-year trial designed to answer that. Weight loss was similar at six and twelve months, dropout was higher in the fasting arm at 38%, participants drifted off protocol in both directions, and LDL cholesterol ended up 11.5 mg/dL higher than in the calorie restriction group. Harder, no better, one marker worse.
Does it matter what time my eating window is?
Possibly a little. A 14-week trial comparing an early 7am to 3pm window against twelve hours or more found 2.3 kg more weight loss with the early window, though the body fat difference was not significant. The authors described the effect as equivalent to eating about 214 fewer calories a day, which suggests the timing worked by reducing intake.
Do I need a supplement to support fasting?
No, and we would be sceptical of anything sold that way. Nothing in the supplement aisle changes the calorie arithmetic that these trials keep landing on. Electrolytes if you are training hard fasted and feel it, water, and enough protein in your window. That is the entire list, and none of it needs a subscription.
Sources
- Lowe DA, Wu N, Rohdin-Bibby L, 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. View study
- Liu D, Huang Y, Huang C, et al. Calorie Restriction with or without Time-Restricted Eating in Weight Loss. N Engl J Med, 2022. View study
- Trepanowski JF, Kroeger CM, Barnosky A, 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. View study
- Elortegui Pascual P, Rolands MR, Eldridge AL, et al. A meta-analysis comparing the effectiveness of alternate day fasting, the 5:2 diet, and time-restricted eating for weight loss. Obesity (Silver Spring), 2023. View study
- Jamshed H, Steger FL, Bryan DR, 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. View study
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