Fragment 176-191

Fragment 176-191 (also called hGH Fragment 176-191, tyr-hGH 177-191, or sometimes loosely referred to as AOD-9604) is a synthetic 16-amino acid peptide derived from the C-terminal region of human growth hormone. Specifically, it represents amino acids 176-191 of the hGH molecule - the region scientists identified as responsible for hGH's fat-burning (lipolytic) activity.

The critical insight behind Fragment 176-191 was that human growth hormone has distinct functional domains. The N-terminal region is responsible for IGF-1-mediated growth promotion and anti-insulin activity. The C-terminal fragment - amino acids 176-191 - was hypothesised to carry the lipolytic properties independently. By isolating this fragment, researchers could potentially capture the fat-burning benefits of hGH without the unwanted effects on glucose metabolism or tissue growth.

The first amino acid of the native fragment (tyrosine at position 176) is retained or substituted depending on the formulation, giving rise to the alternative name "tyr-hGH 177-191." This terminal tyrosine addition may enhance receptor binding affinity and stability.

Mechanistically, Fragment 176-191 appears to target beta-3 adrenergic receptors in adipose tissue, stimulating lipolysis through a cAMP-mediated pathway. Unlike full hGH, it does not appear to activate the insulin receptor or the IGF-1 receptor, explaining its metabolic selectivity. In rodent studies, this translated to weight loss preferentially from fat stores rather than lean mass.

Research in obese animal models demonstrated that daily administration of Fragment 176-191 significantly reduced body weight and total fat mass, with observable effects on abdominal and visceral adiposity. The peptide also showed preliminary evidence for improvements in bone mineral density in some preclinical models - an effect potentially mediated through indirect pathways rather than direct IGF-1 receptor activation.

Fragment 176-191 is frequently used in research comparing selective fat-loss strategies to broader GH axis manipulations. Its relatively short half-life (approximately 30 minutes) and fasted-state dosing protocols have made it a popular subject in metabolic research.