01 // Definition
What Is AOD-9604? The hGH Fragment 176-191 Peptide, Explained
The C-terminal piece of growth hormone, rebuilt as a standalone fat-metabolism fragment.
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What is AOD-9604? It is a small lab-made peptide (a short chain of amino acids) copied from the tail end of human growth hormone — the stretch known as residues 176-191, with one extra amino acid (tyrosine) added at the front [3]. Scientists noticed decades ago that this tail is the part of growth hormone that handles fat, so they built just that piece [4]. The idea was to get the fat effect without the rest of growth hormone — and crucially, without raising IGF-1 (a growth signal) [1][3].
In the lab it does its job: in fat cells it blocks the enzyme that builds new fat and nudges cells toward burning fat [4][1]. In animals it lowered body fat [1][2]. But in people it did not produce real weight loss in the main trial, and development for obesity stopped around 2007 [12][14]. It is investigational, not a supplement, and banned in sport [15].
hGH fragment 176-191: the molecule
The name hGH fragment 176-191 tells you exactly what AOD-9604 is: a fragment corresponding to the 176-191 region of human growth hormone (hGH) [3]. The synthetic version is a 16-amino-acid hexadecapeptide (a peptide of sixteen amino acids) modelled on hGH residues 177-191 with an N-terminal tyrosine substituted in place of the native phenylalanine [3].
Its reported sequence is Tyr-Leu-Arg-Ile-Val-Gln-Cys-Arg-Ser-Val-Glu-Gly-Ser-Cys-Gly-Phe (YLRIVQCRSVEGSCGF). Its molecular formula is C78H123N23O23S2 and its molecular weight is 1815.1 Da. The CAS number is 221231-10-3; the FDA UNII is 7UP768IP4M. Two cysteine residues form an intramolecular disulfide bridge that recreates the cystine loop of the parent hormone [3]. It is not endogenous on its own — it is made synthetically from a region of growth hormone the body produces naturally.
What the peptide AOD9604 does in fat cells
Two mechanisms are documented. First, AOD-9604 inhibits acetyl-CoA carboxylase (ACC) — a rate-limiting enzyme in fat synthesis — by interacting with the fat-cell membrane and releasing a second messenger that increases the enzyme's phosphorylation, reducing de novo lipogenesis (the making of new fat) [4]. This antilipogenic action — suppressing the formation of new fat — is the earliest and best-characterised property of the hGH C-terminal fragment [3].
Second, AOD-9604 up-regulates beta-3 adrenergic receptor (beta3-AR) expression in white adipose tissue — the receptor that, when active, drives lipolysis (the breakdown of stored fat) and energy expenditure [1]. In obese mice, chronic dosing restored suppressed beta3-AR levels toward those of lean mice and increased fat oxidation [1]. In beta3-AR knockout mice the long-term weight/fat effect was abolished, while acute increases in energy expenditure persisted — evidence that the chronic effect depends on functional beta3-AR signalling [1].
What it is not (the conflation guard)
AOD-9604 is gh-receptor-sparing: it does not bind the growth-hormone receptor (GHR), the docking port intact growth hormone uses, so it does not raise IGF-1 [1][3]. That single fact separates it from growth hormone itself, which acts through GHR and elevates IGF-1.
It is not a growth-hormone secretagogue. CJC-1295, ipamorelin, and sermorelin work by prompting the body to release more of its own growth hormone; AOD-9604 does not — it is a fragment that acts directly on fat metabolism [3]. It is not a steroid: there is no androgenic mechanism here. And it is not a dietary supplement and not an approved drug, despite the "fat-loss peptide" framing it is often sold under [14]. See the cited AOD-9604 side effects and safety record for the rest of the context.