HGH Fragment 176-191 is a short, sixteen-amino-acid cutting from the tail end of the growth hormone molecule. The idea behind it is simple: growth hormone burns fat, but along the way it upsets glucose metabolism and drives tissue growth — so if only the piece responsible for fat could be cut out, what remained would be "clean fat-burning" without the rest. The same peptide appears in the literature under the name AOD9604, and in Australia it went through a full human trial programme that ended in 2007 without demonstrating efficacy. The animal data are consistent but come from a single institute; the human data say a fair amount about safety and nothing good about effect. Below we untangle the names, the mechanism and what from that programme was actually published.
What fragment 176-191 is
Growth hormone has 191 amino acids. In the early 1990s Frank Ng's group at Monash University in Melbourne was studying its terminal segment on the assumption that the hormone's different actions reside in different parts of the molecule. A synthetic segment 177-191 (15 amino acids) inhibited fat storage in fat cells just as the whole hormone did. The company Metabolic Pharmaceuticals gave it the code AOD9401, and the version with a single extra tyrosine at the start of the chain the code AOD9604. That sixteen-amino-acid peptide is therefore sometimes called "hGH 176-191". Anti-doping analysts describe it exactly so: "the C-terminal fragment of human growth hormone from amino acids 177-191 with an additional tyrosine residue at the N-terminus". In other words, "Fragment 176-191" and AOD9604 are the same molecule in the literature; the difference between them and AOD9401 is one amino acid.
How it works — without the growth hormone receptor
The most important finding about this peptide sounds paradoxical: it does not act through the growth hormone receptor. In Heffernan's 2001 study on cells engineered to carry the human GH receptor, the fragment did not compete with the hormone for binding and did not stimulate cell division — unlike the whole hormone. At the same time, in obese mice both growth hormone and the fragment reduced weight gain, increased fat oxidation and raised blood glycerol (a marker of fat breakdown), but only the whole hormone caused hyperglycaemia and reduced insulin secretion. The authors took this as confirmation that growth hormone is a "pro-hormone" whose fragments act through their own, non-classical routes.
Which route is still not settled. A second paper from the same group pointed to beta3-adrenergic receptors, the main "lipolysis switch" in fat cells: in obese mice both growth hormone and the fragment restored the depressed expression of these receptors to the level seen in lean mice, and in mice lacking the beta3 receptor chronic treatment neither reduced body weight nor increased lipolysis. In an acute experiment, however, the fragment still raised energy expenditure and fat oxidation in mice without the receptor, so the authors wrote cautiously that the lipolytic action is "not mediated directly" through the beta3 receptor, although both compounds increase its expression. The mechanism therefore remains a hypothesis from a single laboratory.
What was studied — cells and animals
The animal literature agrees on one thing: in obese rodents the fragment reduces weight gain. The bare 177-191 segment, without the tyrosine, was given chronically to obese mice as early as 1994 — it reduced cumulative weight gain and fat-tissue mass by inhibiting fat storage in that tissue. The same group tested the AOD9401 version in obese Zucker rats: after 20 days weight gain was lower and the average fat-cell diameter had fallen from 110 to 80 micrometres; in isolated tissue the peptide stimulated hormone-sensitive lipase and inhibited the fat-storing enzyme (acetyl-CoA carboxylase). Unlike whole growth hormone, it caused neither insulin resistance nor glucose intolerance.
An earlier paper from 1993 matters, though, because it shows the limit of this story. The researchers gave rats the synthetic 177-191 segment and measured glycerol release from fat tissue — that is, actual fat breakdown. The antilipogenic action (blocking storage) matched that of the whole hormone, but "no significant lipolytic effect" was found. The popular label "lipolytic peptide" is therefore a simplification: in those experiments the fragment mainly hindered fat accumulation, and only later work with the tyrosine version described increased fat burning and glycerol. The only data on human tissue also come from this institute: in isolated fat tissue from humans and rodents AOD9401 increased lipolysis and decreased lipogenesis in vitro, and given orally to obese mice it reduced weight gain from day 16 onwards without changing food intake.
Human data — six trials, only half published
Metabolic Pharmaceuticals took the tyrosine version (AOD9604) into clinical trials in Australia. The only peer-reviewed publication from that programme is the 2013 paper by Stier, Vos and Kenley — it appeared in a journal outside the PubMed index, and two of the three authors were tied to the company (the medical director of the trials and a shareholder in the parent company). It describes six randomised, double-blind, placebo-controlled trials with a combined total of about 900 adults: from single intravenous infusions in 15 healthy men, through a week of oral dosing in 36 obese men, to two phase IIb trials — a 12-week study in 300 people and a 24-week study in 502 obese adults (534 enrolled) at 16 centres.
The paper, however, is purely a safety report. The peptide did not change IGF-1 levels, did not worsen the oral glucose tolerance test, did not provoke antibodies, and its adverse-event rate was "indistinguishable from placebo". About efficacy — whether people lost weight — the paper says nothing, even though both phase IIb trials were designed to measure exactly that. The answer is supplied by a 2026 review: in the 24-week trial in 502 people the peptide "failed to demonstrate a significant benefit on the primary weight-loss endpoint", and the conclusions about lack of efficacy "are based primarily on sponsor-reported phase 2 programme outcomes rather than peer-reviewed randomized efficacy trials". The source is a 2007 stock-exchange announcement by the company terminating the programme because the results did not support "commercial viability". ClinicalTrials.gov holds no trial of this peptide — the programme predated mandatory registration.
Safety, stability and the quality of the material
On the available data the fragment looks well tolerated: in the clinical programme there was not a single serious event attributed to the peptide, IGF-1 did not rise and glucose tolerance did not deteriorate — precisely the effects that limit the use of whole growth hormone were missing. That, however, is a safety assessment of oral tablets and intravenous infusions given under supervision in a trial, not of material bought online.
Stability is a separate problem. Anti-doping analysts developing a urine detection method identified six breakdown products of the peptide after incubation in serum and urine; one of them proved "significantly more stable than the other metabolites or the parent compound", which is why detecting misuse means looking for that breakdown fragment rather than the peptide itself. The same paper recalls that the compound is banned by the World Anti-Doping Agency and had been identified in confiscated vials in the USA. A short, disulfide-closed peptide that breaks down faster than its own metabolites is demanding material, and it is the quality and identity of what is in the vial that constitute the real risk here, not the pharmacology.
The wider context — fragment versus whole hormone
Fragment 176-191 is the mirror image of the idea behind full-length growth hormone and behind the peptides that stimulate its release, such as ipamorelin: instead of supplying the whole molecule or provoking its release, it tries to detach one function from the rest. Biologically the plan worked — the fragment leaves the GH receptor alone, does not raise IGF-1 and does not disturb glucose handling. Clinically it did not — because once the rest was cut away, too little remained for people to lose weight. A separate article on AOD-9604 covers the same molecule's programme from the regulatory side; here we have focused on what the two names share.
Summary
HGH Fragment 176-191 is the sixteen-amino-acid tail of the growth hormone molecule, identical in the literature to AOD9604 (without the extra tyrosine it is AOD9401, "177-191"). In obese rodents at a single institute it limited weight gain and fat storage without binding the GH receptor or disturbing glucose; in the earliest paper, however, it had no lipolytic action, only an antilipogenic one. In humans it went through six trials with about 900 participants: only the safety data were published (and they were good), while the efficacy result — no advantage over placebo in a 24-week trial in 502 people — is known only from the sponsor's announcement, after which the programme was closed in 2007. State of the evidence: weak; the one solid finding is a documented lack of effect in the sole application it was created for.
Sources
- Stier H, Vos E, Kenley D. Safety and Tolerability of the Hexadecapeptide AOD9604 in Humans. Journal of Endocrinology and Metabolism. 2013;3(1-2):7-15. DOI: 10.4021/jem157w. doi.org/10.4021/jem157w
- Dominikowski A, Rękoś Z, Olejarz M, et al. The emerging landscape of performance-enhancing peptides modulating GH-IGF1 axis: bridging the gap between clinical evidence and patient self-administration. Frontiers in Endocrinology. 2026;17:1822475. PMID: 42395176. DOI: 10.3389/fendo.2026.1822475. pubmed.ncbi.nlm.nih.gov/42395176
- Heffernan MA, Thorburn AW, Fam B, et al. Increase of fat oxidation and weight loss in obese mice caused by chronic treatment with human growth hormone or a modified C-terminal fragment. International Journal of Obesity and Related Metabolic Disorders. 2001;25(10):1442-1449. PMID: 11673763. DOI: 10.1038/sj.ijo.0801740. pubmed.ncbi.nlm.nih.gov/11673763
- Heffernan M, Summers RJ, Thorburn A, et al. The effects of human GH and its lipolytic fragment (AOD9604) on lipid metabolism following chronic treatment in obese mice and beta(3)-AR knock-out mice. Endocrinology. 2001;142(12):5182-5189. PMID: 11713213. DOI: 10.1210/endo.142.12.8522. pubmed.ncbi.nlm.nih.gov/11713213
- Ng FM, Jiang WJ, Gianello R, Pitt S, Roupas P. Molecular and cellular actions of a structural domain of human growth hormone (AOD9401) on lipid metabolism in Zucker fatty rats. Journal of Molecular Endocrinology. 2000;25(3):287-298. PMID: 11116208. DOI: 10.1677/jme.0.0250287. pubmed.ncbi.nlm.nih.gov/11116208
- Heffernan MA, Jiang WJ, Thorburn AW, Ng FM. Effects of oral administration of a synthetic fragment of human growth hormone on lipid metabolism. American Journal of Physiology. Endocrinology and Metabolism. 2000;279(3):E501-E507. PMID: 10950816. DOI: 10.1152/ajpendo.2000.279.3.E501. pubmed.ncbi.nlm.nih.gov/10950816
- Natera SH, Jiang WJ, Ng FM. Reduction of cumulative body weight gain and adipose tissue mass in obese mice: response to chronic treatment with synthetic hGH 177-191 peptide. Biochemistry and Molecular Biology International. 1994;33(5):1011-1021. PMID: 7987248. pubmed.ncbi.nlm.nih.gov/7987248
- Wu Z, Ng FM. Antilipogenic action of synthetic C-terminal sequence 177-191 of human growth hormone. Biochemistry and Molecular Biology International. 1993;30(1):187-196. PMID: 8358331. pubmed.ncbi.nlm.nih.gov/8358331
- Cox HD, Smeal SJ, Hughes CM, Cox JE, Eichner D. Detection and in vitro metabolism of AOD9604. Drug Testing and Analysis. 2015;7(1):31-38. PMID: 25208511. DOI: 10.1002/dta.1715. pubmed.ncbi.nlm.nih.gov/25208511
For in-vitro laboratory research only. It is not a human medicine and is not for treatment.