Thermogenesis

Overview

Thermogenesis means heat production. In metabolism research it usually stands in for energy expenditure: how many calories the body burns, as opposed to how many it takes in. Peptide marketing leans on it with claims like "boosts metabolism" and "activates brown fat". This page explains what the term covers, how large the effects are in adults, and what human studies show for the peptides sold on that claim. Much of the strongest-sounding evidence comes from mice, and we flag each place where a rodent finding hasn't been shown in people.

Pages in this section

• Brown fat activation covers claims that specific peptides switch on brown or beige fat.
• Metabolic rate enhancement looks at claims about raising resting metabolic rate.

For the wider category, start at the fat-loss and metabolic peptides hub.

What thermogenesis covers

Resting energy expenditure, the energy needed to keep the body running at rest, typically accounts for 60 to 70 percent of daily energy use, according to the National Academies' 2023 Dietary Reference Intakes for Energy. The same report puts the energy cost of digesting food at about 10 percent, and physical activity makes up most of the rest.

In the narrower sense, thermogenesis means making heat on top of that baseline. Shivering does it through muscle contraction. Non-shivering thermogenesis is the quieter version, where tissue burns fuel to make heat without movement. In rodents and newborn babies, much of that heat comes from brown fat.

Brown and beige fat

Brown fat cells are packed with mitochondria that carry uncoupling protein 1 (UCP1). Normally, mitochondria capture the energy from burning fuel as ATP. UCP1 lets that energy leak away as heat instead.

Adults were long thought to have no meaningful brown fat. A 2009 NEJM study of PET-CT scans from 1,972 patients changed that. It found active depots in the neck and upper chest in 7.5% of women and 3.1% of men, and tissue from that region was positive for UCP1. Brown fat showed up less often in older people and in people taking beta-blockers.

A 2012 Cell paper described "beige" cells that sit inside white fat. At rest they have very little UCP1, but they can switch it on strongly when stimulated. The paper also reported that adult human brown fat looks more like mouse beige fat than classical brown fat. Much of the "browning" literature behind peptide claims describes white fat turning beige in mice.

How big these effects are in adults

A 2013 study in the Journal of Nuclear Medicine measured oxygen use directly in cold-activated brown fat. Adults with detectable brown fat had about 60 g of active tissue, and mild cold raised its energy use to roughly 6 kcal per day on average. Even with generous assumptions, the authors put the ceiling at about 15-25 kcal per day. A 2016 study using the same method found that cold added about 4 kcal per day of brown fat energy use, around 1% of the rise in whole-body energy expenditure. Nearby muscle accounted for far more.

Drugs can push harder. In a 2015 Cell Metabolism study, a single 200 mg oral dose of mirabegron (a bladder drug, not a peptide) raised brown fat activity in 12 healthy men and lifted resting metabolic rate by about 203 kcal per day. Heart rate rose by 14 beats per minute and systolic blood pressure by 11 mmHg. That dose is four times the approved 50 mg bladder dose, and it was given once. A real rise in energy expenditure is possible in adults, but here it came with measurable cardiovascular effects.

Brown fat matters far more for heat balance in rodents than in adults. A peptide that raises heat output in a mouse may do little for a person's daily calorie burn.

What human studies show for peptides sold on this claim

GLP-1 drugs: appetite, not extra heat

Semaglutide is a prescription GLP-1 receptor agonist that is sometimes described as a metabolism booster. Its mechanism studies point the other way. In a 2017 crossover trial in 30 adults with obesity, semaglutide (escalated to 1.0 mg weekly for 12 weeks) cut energy intake across the day's test meals by 24%, while resting metabolic rate adjusted for lean mass did not differ from placebo. A 2021 trial of semaglutide 2.4 mg weekly in 72 adults found 35% lower intake at a test lunch and less hunger. Weight loss on these drugs comes mainly from eating less, which is why they're covered under appetite regulation.

Growth hormone and GH secretagogues

Growth hormone does act on fat directly. A 2009 Endocrine Reviews summary of human studies calls a marked rise in lipolysis GH's most prominent metabolic effect. The trade-off is that GH works against insulin.

Lipolysis is not the same as fat loss. Releasing fatty acids only shrinks fat mass if they're burned rather than stored again, and for the research secretagogues that outcome hasn't been tested in a published trial. FDA's 2024 compounding reviews evaluated ipamorelin for growth hormone deficiency and postoperative ileus, and CJC-1295 for growth hormone deficiency only. Neither was reviewed for fat loss. A Phase 2 CJC-1295 trial in 192 people with HIV-related abdominal fat gain was halted in 2006 after a participant died, and we found no published results from it.

Tesamorelin

Tesamorelin is the exception, with a narrow label. It is a prescription GHRH analog (Egrifta) approved by FDA to reduce excess abdominal fat in adults with HIV-associated lipodystrophy, and the current label says it is not for weight loss management. In the two 26-week trials on the label, 2 mg daily cut visceral adipose tissue by 18% and 14% on average, against +2% and -2% with placebo. That's a real effect on one fat depot in one population. It doesn't show a rise in energy expenditure. As of October 2026, the newer Egrifta WR formulation is labeled at 1.28 mg once daily. Our tesamorelin profile covers the label and user reports.

MOTS-c

MOTS-c is the peptide most directly tied to heat production, and that evidence comes from mice. In the 2015 Cell Metabolism paper that described it, daily injections (0.5 mg/kg in mice) prevented weight gain on a high-fat diet without changing food intake, and treated mice produced more body heat. The authors linked this to AMPK activation in muscle.

We found no peer-reviewed study giving native MOTS-c to people and measuring energy expenditure or fat loss. The only clinical-stage analog, CB4211, ran a 4-week Phase 1b in 20 adults with obesity and fatty liver, at 25 mg by subcutaneous injection daily. It lowered liver enzymes versus placebo, but liver fat fell in both groups. In 2023 the developer, CohBar, said the trial formulation wasn't suitable for further development and then moved to dissolve the company. WADA's 2026 Prohibited List names MOTS-c under S4.4.1. Our MOTS-c profile has more detail.

AOD-9604

AOD-9604 is a modified growth hormone fragment developed as a fat-loss drug. Mouse studies cited by FDA reported more fat oxidation, but the human program failed. FDA's December 2024 compounding briefing notes that all human data come from oral or intravenous dosing. In the largest trial, which enrolled 536 adults with obesity (502 randomized), participants took oral doses from 0.25 to 1 mg daily with diet and exercise. Weight loss did not differ significantly from placebo at the 12-week primary endpoint, and in February 2007 the developer ended development for obesity. FDA found no human data for the subcutaneous route most users describe.

What users report

Community reports fit the trial picture better than the marketing. In forum threads about AOD-9604, most users describe little or no change. MOTS-c reports are more mixed: some users describe better endurance and others notice nothing, and injection-site reactions come up often. These are self-selected anecdotes about gray-market products of uncertain quality, so they can't settle whether a peptide raises energy expenditure.

What we still don't know

• Whether any research peptide raises 24-hour energy expenditure in adults. None of the human studies cited here measured that for a research peptide.
• Whether MOTS-c's heat effect in mice carries over to people. As of October 2026, there is no published human trial of the native peptide.
• Whether expanding brown or beige fat over months could matter, given the 15-25 kcal per day ceiling estimated for acute activation.