HGH Fragment 176-191

In the early 1990s, Frank Ng's group at Monash established that the fat-related actions of growth hormone could be reproduced by a short synthetic peptide of its C-terminal sequence, Leu-Arg-Ile-Val-Gln-Cys-Arg-Val-Ser-Glu-Gly-Ser-Cys-Gly-Phe, residues 177-191. It was antilipogenic in fat tissue to the same degree as the intact hormone, with no significant lipolytic effect in the same experiment [2]; it reduced basal and insulin-stimulated glucose uptake in rat fat cells, more strongly than growth hormone itself at the same concentration [5]; and given chronically to obese mice it reduced cumulative body weight gain, adipose mass and adipose lipogenesis [1].

That is essentially the whole case for this peptide, and it is the same body of work that underpins AOD-9604, which is this fragment with a tyrosine added at the N terminus [6]. The later and better-documented experiments were not done on the plain fragment: the 30-day oral study in ob/ob mice and the 20-day study in Zucker rats used a peptide the authors called AOD-9401 [3][7], and the beta-3 adrenergic receptor knockout work used AOD9604 [8]. Findings for all three are routinely quoted for each other.

Why it is interesting in principle. Growth hormone mobilizes fat, but it also raises IGF-1, causes fluid retention and worsens insulin sensitivity. If the fat-mobilizing activity lives in a separable domain, a peptide of that domain should give the first without the rest. In animals it does [2].

Why that is not enough. No human trial of hGH fragment 176-191 has been published, for weight, fat, or anything else. The only human material in this literature is isolated adipose tissue [3]. A 2026 review of the peptides sold for physique purposes places this fragment among those with no human evidence [4]. A related caution sits in the oldest papers: synthetic peptides from this region of growth hormone have a hyperglycemic action of their own in animals [9].

The fragment acts on the fat cell. In the experiment that defined it, it suppressed lipogenesis as strongly as intact growth hormone and produced no measurable lipolysis - glycerol release from rat epididymal fat pads did not change [2]. The "fat-burning peptide" framing came later, from work on the AOD-coded analogs, where the same C-terminal domain stimulated hormone-sensitive lipase, inhibited acetyl-CoA carboxylase and raised lipolytic activity in adipose tissue ex vivo [3][7]. Both descriptions are in the literature; the fragment's best-established action is on fat synthesis rather than fat breakdown.

A third thread is glucose handling. The fragment cut both basal and insulin-stimulated deoxyglucose uptake in adipocytes from obese Zucker rats, and did so more potently than growth hormone at equal concentrations, which the authors took to be part of how it suppresses lipogenesis [5]. Reducing glucose uptake into fat is not the same as improving metabolic health, and the oldest paper in this line is about the hyperglycemic action of peptides from this region of the hormone [9].

No receptor. Both growth hormone and the AOD analog restore the suppressed expression of the beta-3 adrenergic receptor, the main lipolytic receptor on fat cells, in obese mice - the tidiest available explanation. But in beta-3 knockout mice the chronic weight and lipolysis effects disappear, while an acute dose still raises energy expenditure and fat oxidation, so the peptide does not act directly through that receptor [8]. Three decades on, nothing else has been identified either, and the mechanism is a set of observed effects on fat tissue without a target.

The peptide is residues 177-191 of human growth hormone, also numbered 176-191 — 15 amino acids, including the disulfide-closed loop of the parent hormone [2]. The variant with a tyrosine added at the N terminus is AOD9604 [6], and the two names are used interchangeably in the wider literature even though the molecules differ by that residue. Peptides of this size are detectable in doping control [10], and AOD9604 specifically is detectable in urine down to 50 pg/mL [6].

No human safety data exist, because no human study has been published. The animal work reports no toxicity but was not designed to look for it [1][3].

Two things in the preclinical literature are worth carrying forward: peptides from this region of growth hormone raise blood glucose in animals [9], and the fragment reduces basal and insulin-stimulated glucose uptake in fat cells [5]. Neither has been examined in people. Against that, 20 days of the AOD-9401 analog in obese Zucker rats produced neither insulin resistance nor glucose intolerance, where growth hormone itself does [7], and the chronic mouse study that reported reduced weight gain and fat mass reported no toxicity alongside it [1]. No study in this literature measured IGF-1, so the common claim that the fragment leaves IGF-1 alone is untested rather than established.