GLP-1 medicines and fasting

Medicines such as semaglutide and tirzepatide have changed obesity treatment. Many people now use them alongside an eating schedule, and many fasters wonder how the two fit together. This guide explains what the trials show, the specific risks of combining them, and how to use a fasting app to support treatment rather than work against it. It's not a substitute for advice from the doctor who prescribes your medicine.

How these medicines work

GLP-1 is a hormone your gut releases after eating. It tells your brain you've had enough, slows how quickly your stomach empties and helps your pancreas release insulin. Semaglutide mimics GLP-1; tirzepatide mimics both GLP-1 and a second gut hormone, GIP. The result is less hunger, smaller meals and, for most people, substantial weight loss.

The muscle question

Any large weight loss includes some lean mass, and these medicines produce very large losses. In a detailed body scan analysis from SURMOUNT-1, about 75% of the weight lost with tirzepatide was fat and 25% was lean mass, the same proportion as in the placebo group ⁴. Across studies, a 2026 review found a median of about 28% of weight lost came from muscle-related measures, with wide variation between people ⁵.

Why combining them needs care

Fasting and GLP-1 medicines both work largely by helping you eat less. Put together, it can become too easy to eat far too little, and especially too little protein. Nausea, a common side effect, can make it worse ^1,2. So on these medicines, the goal of a fasting window changes: it's no longer to push your fasts longer, but to give your day a regular structure with protein-rich meals.

What happens when you stop

The same happened with tirzepatide: in a trial where some people switched to placebo after 36 weeks, they regained 14% of their weight over the next year, while those who continued kept losing ⁸. This doesn't mean the medicines fail. It means obesity behaves like a long-term condition. If you and your doctor decide to stop or lower the dose, that's when habits built with an app, such as a regular eating window, protein at every meal and strength training, matter most.

Sources

1. Wilding JPH, Batterham RL, Calanna S, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. N Engl J Med. 2021;384(11):989-1002. doi.org/10.1056/NEJMoa2032183
2. Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide Once Weekly for the Treatment of Obesity. N Engl J Med. 2022;387(3):205-216. doi.org/10.1056/NEJMoa2206038
3. Lincoff AM, Brown-Frandsen K, Colhoun HM, et al. Semaglutide and Cardiovascular Outcomes in Obesity without Diabetes. N Engl J Med. 2023;389(24):2221-2232. doi.org/10.1056/NEJMoa2307563
4. Look M, Dunn JP, Kushner RF, et al. Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight. Diabetes Obes Metab. 2025;27(5):2720-2729. doi.org/10.1111/dom.16275
5. Batsis JA, Gavras A, Gross DC, et al. Effect of Incretin-Based and Nonpharmacologic Weight Loss on Body Composition: A Systematic Review. Ann Intern Med. 2026;179(7):996-1013. doi.org/10.7326/ANNALS-25-00478
6. Mechanick JI, Butsch WS, Christensen SM, et al. Strategies for minimizing muscle loss during use of incretin-mimetic drugs for treatment of obesity. Obes Rev. 2025;26(1):e13841. doi.org/10.1111/obr.13841
7. Wilding JPH, Batterham RL, Davies M, et al. Weight regain and cardiometabolic effects after withdrawal of semaglutide: The STEP 1 trial extension. Diabetes Obes Metab. 2022;24(8):1553-1564. doi.org/10.1111/dom.14725
8. Aronne LJ, Sattar N, Horn DB, et al. Continued Treatment With Tirzepatide for Maintenance of Weight Reduction in Adults With Obesity: The SURMOUNT-4 Randomized Clinical Trial. JAMA. 2024;331(1):38-48. doi.org/10.1001/jama.2023.24945