Topic
Weight & Metabolism
Quick answers
- What is it?
- This hub covers the research behind GLP-1 and GIP weight-loss medications like semaglutide and tirzepatide, plus related weight-and-metabolism compounds such as retatrutide, 5-Amino-1MQ, and MOTS-c — what the studies actually measured about weight loss, appetite, muscle, bone, heart health, and what happens after stopping.
- Why are people interested in it?
- Millions of people are now taking or considering these drugs for weight loss, and the headline numbers from clinical trials often don't match what people experience once they're off a trial's structured monitoring — so readers want to know what the real-world evidence says about results, side effects, muscle loss, and what happens if they stop.
- What has actually been studied?
- The evidence spans large, multi-year, industry-run randomized trials of FDA-approved drugs (semaglutide, tirzepatide, liraglutide, dulaglutide) in tens of thousands of participants; a smaller Phase 2 trial and an unpublished Phase 3 program for the investigational triple-agonist retatrutide; real-world insurance-claims and cohort studies; and, at the other extreme, compounds like 5-Amino-1MQ and MOTS-c that have never been tested in a single human trial and are studied only in mice.
- How strong is the evidence?
- Strength varies enormously across this hub: semaglutide and tirzepatide rest on some of the largest drug trials ever run, including a 17,604-person cardiovascular outcomes trial, but many specific questions (bone density in older adults, resistance training's effect on muscle, protein intake during GLP-1 therapy) are answered only by post hoc subgroup analyses, small pilot trials, or evidence borrowed from ordinary dieting research rather than dedicated trials. Compounds like 5-Amino-1MQ and MOTS-c have zero human trials of any kind.
- What don't we know yet?
- Across these sources, researchers don't yet know what happens beyond roughly 2-4 years on or off these drugs, whether tapering the dose changes outcomes after stopping, why some people lose much less weight than others, whether resistance training specifically (versus general exercise) preserves muscle during GLP-1-driven weight loss, and how these drugs behave in older, frailer, or more diverse populations than those enrolled in the pivotal trials.
- Where can I read the original research?
- Explore the individual articles in this hub for the study-by-study numbers on real-world weight loss, muscle and bone effects, side effects, and stopping treatment, and check the peptide profiles for what has (and hasn't) been tested in humans for each specific compound.
Most of the peptide conversation online starts here, with GLP-1 medicines like semaglutide and tirzepatide. This is also where the strongest human evidence lives: large, multi-year, placebo-controlled trials. The research covers appetite, blood sugar, how much of lost weight is muscle, what happens after stopping, and what the trials actually reported about side effects.
Peptides in the Evidence Index
Research on this topic
Video briefs
Frequently asked
Does semaglutide or tirzepatide work as well outside of a clinical trial as it did in the trials?
Generally not as well. The pivotal trials reported 14.9% average weight loss for semaglutide (STEP 1) and up to 20.9% for tirzepatide (SURMOUNT-1), but a 2026 review comparing 127 trials against 26 real-world studies found real-world semaglutide users with diabetes lost only 4.7-10.5% of body weight over 6-12 months. The review traced most of the gap to two things: only 13% of real-world semaglutide users ever reached the 2.4 mg trial dose, and 20-50% of people discontinue treatment within a year. Among people who stayed on treatment and reached the target dose, real-world results (14.1% for semaglutide, 16.5% for tirzepatide in one 'persister' analysis) came much closer to trial-level results.
Why does weight come back after stopping a GLP-1 drug?
In the STEP 1 extension, 327 participants who had lost a mean 17.3% of body weight on semaglutide regained 11.6 of those 17.3 percentage points within a year of stopping the drug. In SURMOUNT-4, people switched from tirzepatide to placebo regained 14.0% of body weight over 52 weeks, while those who continued the drug lost an additional 5.5%. Researchers describe this as evidence that obesity behaves like a chronic condition — the drugs suppress appetite while present in the body, and appetite-related signaling returns once the drug clears, rather than the drug having reset the underlying biology.
How much of the weight lost on a GLP-1 is muscle, and does protein or resistance training help?
A 2026 systematic review of 35 randomized trials found the median share of total weight loss from muscle-related measures was about 28%, ranging from roughly 16% to 40% across studies — and comparator groups losing weight through lifestyle alone showed similar losses in nearly half of studies, so this isn't unique to the drugs. Separately, a 2020 meta-analysis of general dieting studies found protein intake above the RDA preserved an extra 0.36 kg of lean mass during dieting, rising to 0.77 kg when combined with resistance training. However, none of this protein-and-exercise evidence has actually been tested in a completed trial of people losing weight on semaglutide or tirzepatide specifically — the first dedicated trial (LEAN-PREP, 232 participants) is still enrolling.
What side effects actually showed up in the semaglutide and tirzepatide trials?
Gastrointestinal events — nausea, vomiting, diarrhea, constipation — were consistently the most common adverse events, typically mild-to-moderate and concentrated during dose escalation. They drove treatment discontinuation in 4.5% of the semaglutide group in STEP 1 and 4.3-7.1% (dose-dependent) of the tirzepatide group in SURMOUNT-1, versus under 3% on placebo in both trials. In the longer, older-population SELECT cardiovascular trial, discontinuation for adverse events reached 16.6%. Serious adverse events, including pancreatitis and gallstones, were consistently reported as uncommon across every trial and meta-analysis reviewed.
Is tirzepatide actually more effective than semaglutide?
In the one published head-to-head trial, SURMOUNT-5 (751 adults with obesity, no diabetes), tirzepatide at its maximum tolerated dose produced significantly greater weight loss than semaglutide at its maximum tolerated dose over 72 weeks (-20.2% vs -13.7%) and a larger drop in waist circumference (-18.4 cm vs -13.0 cm). That's genuine head-to-head evidence, but it's a single trial in one population that excluded people with type 2 diabetes, and it didn't test whether the larger weight loss translates into greater cardiovascular or long-term health benefit — there is no head-to-head cardiovascular trial between the two drugs.
Is 'food noise' — constant, intrusive thoughts about food — a real, measurable thing?
It's a real phenomenon that researchers are still building tools to measure, since it started as a term coined by patients and clinicians rather than a formal clinical construct. A 550-person retrospective survey using a new 5-item Food Noise Questionnaire found scores roughly halved after starting semaglutide, and a smaller controlled study found the same drop after 6 months alongside a measurable shift in sweet-taste detection. But not every study agrees: a small real-world cohort using a different validated craving scale found cravings were essentially unchanged overall after 24 weeks, though people who started with the highest cravings did see the largest reductions.
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Start here for plain-English explanations of clinical trials, evidence levels, and how to read a study.