Intermittent fasting
Quick answer. Intermittent fasting means eating only within certain hours or on certain days. In trials it helps people lose weight but, compared with ordinary calorie cutting, it does not do better. Nobody has tested it in a trial of people on GLP-1 medicines, and people with diabetes can get low blood sugar when they fast.
What it is
Intermittent fasting covers several patterns. limits food to a daily window, such as 8 hours. Alternate-day fasting swaps normal days with very low-calorie days. The 5:2 pattern uses two low-calorie days a week.
What trials show
A 2026 Cochrane review pooled 22 randomized trials with 1,995 adults. Compared with regular dietary advice, intermittent fasting made little to no difference to weight loss, a gap of about 0.3 percentage points of body weight, with low certainty. Against no intervention, fasting groups lost about 3.4 percentage points more of their starting weight, from six studies. All studies were 12 months or shorter.
A 2022 New England Journal of Medicine trial put 139 adults with obesity on calorie restriction, either with eating only between 8 a.m. and 4 p.m. or with no time limit. After 12 months the time-restricted group lost 8.0 kg and the other group lost 6.3 kg. That 1.8 kg gap was not statistically significant, and adverse events were similar.
A 2026 pooled analysis of 28 trials with 1,833 adults found fasting reduced weight and BMI at every age. It also found a loss of fat-free mass in younger and older groups and a rise in LDL cholesterol in some, and it rated the certainty of the evidence low to very low.
As with low-carb eating, the Atlas did not find a randomized trial of fasting in people taking GLP-1 medicines.
What experts say about fasting with a GLP-1 medicine
The 2025 advisory notes that many people on these medicines end up fasting by accident, because they are simply not hungry. It encourages regular meal times either way.
It warns that long fasts without enough protein or variety can lead to nutrient shortfalls, loss of fat-free mass and a lower resting energy burn. It says strength training, adequate protein and calories, and a variety of minimally processed foods can offset that.
For people with diabetes it flags a different risk. Fasting can cause low blood sugar in people who take insulin or other glucose-lowering medicines, so a plan has to come from the clinician managing the diabetes. The Wegovy and Zepbound labels also say low blood sugar risk rises when the medicine is combined with insulin or a sulfonylurea.
What people report
On forums and social media, people on GLP-1 medicines often describe a natural eating window, because food is less appealing for much of the day. Some stick to patterns such as 16 hours fasting with an 8-hour window, and some describe one main meal a day. People who like it commonly say it simplifies decisions.
The common complaints are trouble reaching protein targets in a short window, lightheadedness, constipation and feeling cold or weak. Some people say they struggled to eat enough once the medicine dose went up.
None of this was measured. It comes from people who chose to share, and the Atlas cannot say how many benefit or struggle. It is included because many people use these patterns, and the evidence above does not suggest fasting beats other ways of cutting calories.
Who needs to be careful
- Anyone with diabetes who takes insulin or a sulfonylurea.
- Anyone with a history of disordered eating.
- Older adults and anyone concerned about muscle, where long fasts can make it harder to reach protein targets.
Questions to bring to a clinician or dietitian
- Is a shorter eating window safe with my medicines?
- How do I reach my protein target inside a window?
- Which signs mean I am not eating enough and should call you?
- Should we check my cholesterol if I fast regularly?
What is still unknown
- Whether fasting adds anything to a GLP-1 medicine is untested.
- Trials of fasting mostly last 12 months or less.
- Effects on muscle in people who also take these medicines are not known.
- Which pattern is best, if any, is not settled.
Related pages
Sources
- Intermittent fasting for adults with overweight or obesity Garegnani LI, et al. Cochrane Database Syst Rev. 2026;2(2):CD015610. PMID 41692034. Funding: No dedicated funding. Journal article
- Calorie restriction with or without time-restricted eating in weight loss Liu D, et al. N Engl J Med. 2022;386(16):1495-1504. PMID 35443107. Funding: National Key Research and Development Project and others. Journal article
- Age-specific analysis of the effects of intermittent fasting on body composition and cardiometabolic markers: a systematic review and meta-analysis Xing K, et al. Nutrients. 2026;18(11). PMID 42280443. Funding: Not stated in the abstract. Journal article
- Nutritional priorities to support GLP-1 therapy for obesity: a joint advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association, and the Obesity Society Mozaffarian D, et al. Obes Pillars. 2025;15:100181. PMID 40673264. Funding: Professional societies; several authors report industry relationships. Journal article
- Wegovy (semaglutide) injection and tablets: prescribing information Novo Nordisk, on DailyMed (U.S. National Library of Medicine). Label version 6/2026. One label covers both the injection and the tablets. Official FDA label on DailyMed
- Zepbound (tirzepatide) injection: prescribing information Eli Lilly and Company, on DailyMed (U.S. National Library of Medicine). Label version 8/2026. Official FDA label on DailyMed