Questions, Answered From the Record

What is tirzepatide?

Tirzepatide (Mounjaro, Zepbound) is a linear 39-amino-acid synthetic peptide based on the native sequence of glucose-dependent insulinotropic polypeptide, carrying a C20 fatty-diacid arm that binds albumin and gives it a long enough residence time for once-weekly administration. It was the first approved dual incretin agonist, acting at both the GIP receptor and the GLP-1 receptor. Approval came first for type 2 diabetes in May 2022, then for chronic weight management, and subsequently for moderate-to-severe obstructive sleep apnoea in adults with obesity; it is not approved for type 1 diabetes ^[2].

How does tirzepatide work?

One peptide engages two incretin receptors. Together, GIP-receptor and GLP-1-receptor activation enhance glucose-dependent insulin secretion, suppress glucagon, slow gastric emptying, and reduce appetite and food intake. The pharmacology is deliberately unbalanced: signalling assays show greater engagement of the GIP receptor than the GLP-1 receptor, with GLP-1-receptor signalling biased toward cyclic AMP generation over beta-arrestin recruitment and weaker receptor internalisation than native GLP-1 produces ^[6]. In the discovery work the molecule improved glucose-dependent insulin secretion and glucose tolerance in mice and, on chronic administration, lowered body weight and food intake more than a selective GLP-1 receptor agonist did ^[7]. Note what that mechanism does not include: nothing in it acts on skeletal muscle.

What is tirzepatide used for?

In approved use, type 2 diabetes, chronic weight management in adults with obesity or with overweight plus a weight-related condition, and moderate-to-severe obstructive sleep apnoea in adults with obesity ^[2]. In the trial literature, it has been studied against placebo in 2,539 adults with obesity over 72 weeks ^[4], head-to-head against semaglutide in 751 adults with obesity over the same period ^[1], and against semaglutide for glycaemic control in 1,879 adults with type 2 diabetes over 40 weeks ^[5]. Whether any of that applies to a particular person is a prescribing question, and this review does not answer prescribing questions.

What is semaglutide?

Semaglutide (Ozempic, Wegovy) is a 31-amino-acid acylated analogue of human glucagon-like peptide-1, sharing roughly 94% sequence homology with the native hormone. An alpha-aminoisobutyric acid substitution at position 8 blocks the enzyme that would otherwise degrade it rapidly, and a C18 fatty di-acid side chain binds albumin, extending circulation sufficiently for the once-weekly trial schedules. An oral tablet co-formulated with an absorption enhancer also exists ^[11].

What is semaglutide used for?

Approved uses span type 2 diabetes, chronic weight management, reduction of major adverse cardiovascular events in adults with established cardiovascular disease and overweight or obesity, and, since 2025, metabolic dysfunction-associated steatohepatitis. The supporting trials are unusually large: 17,604 participants for the cardiovascular endpoint, with a hazard ratio of 0.80 and a 95% confidence interval of 0.72 to 0.90 ^[9], and 3,533 participants with type 2 diabetes and chronic kidney disease for kidney endpoints, with a hazard ratio of 0.76 and a 95% confidence interval of 0.66 to 0.88 ^[8].

How does semaglutide work for weight loss?

Centrally. The published mechanism has it reaching hypothalamic and brainstem appetite circuits — the arcuate nucleus and the area postrema in particular — activating anorexigenic POMC and CART neurons and inhibiting orexigenic NPY and AgRP neurons, so food intake falls and food preference shifts. The same mechanism slows gastric emptying, which contributes to fullness and to most of the gastrointestinal side effects that dominate its tolerability profile ^[11]. The published description adds a detail that matters for body composition: intake falls without energy expenditure falling. In STEP 1, mean body-weight change was -14.9% at week 68 against -2.4% on placebo ^[10]. That is a weight figure, not a fat figure.

What is tesamorelin?

Tesamorelin is a synthetic 44-amino-acid analogue of human growth hormone-releasing hormone, modified at the N-terminus with a trans-3-hexenoic acid group that resists enzymatic cleavage and extends plasma stability. It was approved in the United States in 2010 to reduce excess abdominal fat in HIV-infected patients with antiretroviral-related lipodystrophy, and it has no approval outside that indication ^[13]. It is prohibited in sport under the World Anti-Doping Agency list as a growth-hormone-axis agent.

How does tesamorelin work?

It binds the growth hormone-releasing hormone receptor on pituitary somatotrophs and stimulates the synthesis and pulsatile release of the body's own growth hormone, which drives hepatic insulin-like growth factor-1 production; together those promote lipolysis preferentially in visceral fat. Because it amplifies an endogenous pulsatile rhythm rather than supplying growth hormone at a flat concentration, its metabolic profile differs from recombinant growth hormone. Measured directly in 13 healthy men over two weeks, it raised mean overnight growth hormone by 0.5 micrograms per litre and insulin-like growth factor-1 by 181 micrograms per litre, with no significant change in fasting glucose or insulin-stimulated glucose uptake ^[15].

What does the MOTS-c peptide do?

Inside the body, MOTS-c is a 16-amino-acid peptide encoded in the mitochondrial 12S ribosomal RNA gene that acts as a signal about cellular energy state. It inhibits the folate cycle and de novo purine biosynthesis, raising AICAR and activating AMP-activated protein kinase, with improved glucose handling and insulin sensitivity described mainly in skeletal muscle ^[19]. Under metabolic stress it translocates into the nucleus and regulates nuclear gene expression in an AMPK-dependent way, including antioxidant-response genes through NRF2 — the first demonstrated retrograde signalling by a mitochondrially encoded peptide ^[21]. A 2024 study identified casein kinase 2 as a direct binding target, with muscle activation and fat suppression proposed as the route to muscle glucose uptake and atrophy prevention ^[17]. What it does when administered to a person is not established, because no human efficacy trial has been completed.

What are the negative side effects of MOTS-c?

There is no adverse-effect profile to report, and that absence is the answer. No completed human efficacy or safety trial exists for MOTS-c, so there is no adverse-event table, no incidence figure and no characterised safety profile in people. The documented risks are structural instead of pharmacological: the peptide is not approved for human use anywhere and is supplied only as a laboratory research chemical, so identity, purity and sterility vary by supplier and are not held to pharmaceutical standards; there is no published, measured human half-life, bioavailability or dose-response; and anti-doping authorities treat it as prohibited in elite sport. The strongest human dataset in this reference list measures circulating endogenous peptide as a risk biomarker in 94 haemodialysis patients ^[18] — it administered nothing, and therefore reports no adverse effects of administration. Any source describing MOTS-c as well tolerated in humans is describing evidence that has not been generated.

How much of the weight lost with these compounds is lean tissue?

Not one of the trials numbered on this site answers that, which is the finding this desk is built around. SURMOUNT-5, SURMOUNT-1, STEP 1 and SURPASS-2 reported body weight, waist circumference and glycaemic markers ^{[1][4][10][5]}; SELECT and FLOW reported clinical events ^[9][8]. None of them partitions fat mass from lean mass.

The compound record does state, qualitatively, that a body-composition substudy within the STEP programme found the weight lost with semaglutide comprised both fat mass and a meaningful proportion of lean mass, and it lists lean-mass loss during rapid weight reduction as an active area of investigation for the dual agonist. The supporting publications sit outside this site's composed reference list, so this digest does not reproduce their numerical estimates.

The only lean-mass measurement in this reference list belongs to tesamorelin, where pooled randomised data reported lean body mass rising by 1.42 kg in adults with HIV-associated lipodystrophy ^[12] — a different mechanism, a different population and a far smaller scale of fat loss.

Does this site supply, compound or private-label any peptide?

No. Despite what the domain name suggests, this is a literature review and nothing else. It manufactures nothing, compounds nothing, stocks nothing and ships nothing. It names no vendor, quotes no price, compares no source, links to no shop and publishes no purchasing guidance of any kind. It also publishes no protocol, schedule or quantity for a person to follow; where a dose appears on these pages it is a description of what a cited study administered under clinical supervision. The compounds reviewed here range from approved prescription medicines to a research chemical that is not approved for human use anywhere, and in every case the decision about whether and how a person uses one belongs to a qualified prescriber.