Peptide Profile
HGH Fragment 176-191
What Is HGH Fragment 176-191?
HGH Fragment 176-191 — sometimes shortened to HGH frag 176-191 or simply HGH frag — is a 16-amino-acid peptide derived from the tail end of the 191-amino-acid human growth hormone sequence. Its molecular formula is C78H125N19O23S2, with a molecular weight of 1,817.12 g/mol (CAS: 221231-10-3).
Early literature sometimes refers to this fragment as AOD-9401, though this designation is now largely replaced. The fragment was first characterised by Wu and Ng in 1993, who identified the C-terminal domain of GH as the region responsible for its lipid-modulating properties [1]. Subsequent work by the same group confirmed that this short peptide sequence could replicate the fat-reducing effects of full-length growth hormone while remaining structurally distinct from the portions of GH that drive skeletal growth and IGF-1 elevation [2].
It is important to note that HGH Fragment 176-191 is a research compound. It is not approved by the FDA or any regulatory body for therapeutic use. All findings discussed here relate to preclinical and laboratory research contexts.
Compound Profile
Mechanism of Action
The mechanism by which HGH Fragment 176-191 influences fat metabolism appears to operate through two complementary pathways: stimulation of lipolysis (the breakdown of stored triglycerides) and inhibition of lipogenesis (the formation of new fat).
Wu and Ng’s original 1993 study demonstrated that the synthetic C-terminal sequence 177-191 of human growth hormone exhibited antilipogenic activity in adipose tissue preparations [1]. This finding was significant because it suggested that the fat-metabolising properties of growth hormone could be isolated to a small structural domain, independent of the somatogenic (growth-promoting) regions.
Research in Zucker fatty rats and ob/ob mice revealed that the fragment’s lipolytic action involves the beta-3 adrenergic receptor (β3-AR) pathway. Heffernan et al. (2001) showed that AOD9604 — the stabilised version of the fragment — reduced body fat in obese mice but was significantly less effective in beta-3 adrenergic receptor knockout mice, strongly implicating this receptor in the mechanism [5]. The beta-3 adrenergic system is predominantly expressed in adipose tissue and plays a central role in thermogenesis and fat oxidation.
Unlike full-length growth hormone, the fragment does not appear to activate the JAK2-STAT5 signalling pathway in a manner that drives IGF-1 production. This selective activity profile is what distinguishes it from exogenous GH administration, where lipolytic benefits come alongside anabolic, proliferative, and potentially diabetogenic effects.
Fat Loss & Lipolysis Research
The primary research interest in HGH Fragment 176-191 centres on fragment 176-191 fat loss mechanisms, specifically its ability to promote fat oxidation without the side-effect profile of full GH.
Ng et al. (2000) conducted metabolic studies of the synthetic lipolytic domain in both in vitro adipocyte preparations and Zucker fatty rats [2][3]. The fragment stimulated lipolysis in a dose-dependent manner and significantly reduced body weight gain in the obese rat model over a 14-day treatment period. Importantly, the authors noted that the fragment did not alter food intake — suggesting the weight reduction was driven by increased fat oxidation rather than appetite suppression.
Heffernan et al. (2001) extended these findings using chronic treatment protocols in ob/ob mice. Their study demonstrated that both human GH and the modified C-terminal fragment increased fat oxidation and produced weight loss in obese mice [4]. The fragment produced comparable fat reduction to full-length GH but without elevating circulating IGF-1 levels — a critical distinction for researchers interested in isolating the metabolic benefits of GH.
A companion study from the same group examined the role of beta-3 adrenergic receptors, finding that the fat-reducing effects were substantially attenuated in β3-AR knockout mice [5]. This provided direct mechanistic evidence for how the fragment interfaces with adipose tissue signalling, and suggested that its effects are particularly relevant to metabolically active fat depots where β3-AR expression is highest.
While these animal studies are consistent and mechanistically coherent, it is worth noting that no published human trials have investigated HGH Fragment 176-191 in its unmodified form. The fragment 176-191 peptide research base, while promising, remains predominantly preclinical.
Metabolic Effects
Beyond direct lipolysis, research on the HGH fragment has examined its broader metabolic implications, particularly regarding glucose neutrality and insulin sensitivity preservation.
The original work by Wu and Ng (1993) specifically noted the absence of hyperglycaemic effects from the C-terminal fragment — a striking contrast to earlier work by Ma et al. (1982) showing that synthetic peptides from the same region of GH could produce hyperglycaemia depending on the specific sequence used [1]. The 176-191 sequence appeared to isolate the lipolytic function while excluding the glucose-disrupting domains.
Berryman et al. (2011) reviewed the broader relationship between growth hormone and adipose tissue, noting that GH’s effects on fat metabolism involve complex interactions between lipolysis, adipogenesis, insulin sensitivity, and inflammatory signalling [7]. The fragment approach — using only the lipolytic domain — potentially avoids the insulin-desensitising effects that make full GH problematic for metabolic health applications.
Sackmann-Sala et al. (2014) provided additional context by demonstrating that GH receptor activity has depot-specific effects on white adipose tissue, with different fat depots responding differently to GH signalling [8]. This research suggests that fragment-based approaches might selectively target metabolically harmful visceral fat while having less impact on subcutaneous depots, though this hypothesis remains untested for the fragment specifically.
For researchers interested in peptides with more established metabolic profiles, tesamorelin (FDA-approved for lipodystrophy) and the GLP-1 receptor agonist class offer more robust evidence bases for metabolic health applications.
Side Effects & Safety Profile
The safety data on HGH Fragment 176-191 is limited, reflecting its status as a research-stage compound without clinical trials in its unmodified form.
In published animal studies, the fragment was generally well-tolerated. Heffernan et al. (2001) reported no significant adverse effects during chronic administration in mice, and the absence of IGF-1 elevation and glucose disruption was interpreted as a favourable safety signal compared with full GH [4][5].
Theoretical HGH fragment side effects considerations include:
- Injection site reactions: As a subcutaneously administered peptide, local irritation is a common consideration for all injectable research peptides.
- Hypoglycaemia: While the fragment does not appear to raise blood glucose, its lipolytic activity could theoretically interact with other compounds that affect glucose metabolism.
- Limited long-term data: No studies have evaluated chronic exposure beyond the timeframes used in published animal research (typically 14-30 days).
- Peptide degradation products: The short half-life of the unmodified fragment means rapid degradation occurs, and the biological activity of breakdown products has not been characterised.
The safety profile of the stabilised analogue AOD-9604 is better characterised, with Phase II clinical trial data and GRAS evaluation providing a broader dataset. Wilding’s 2004 review noted that AOD-9604 showed a safety profile comparable to placebo in human trials [6].
Pharmacokinetics
HGH Fragment 176-191 is characterised by very rapid pharmacokinetics, which presents both opportunities and challenges for research applications.
The estimated half-life of the unmodified fragment is approximately 15-20 minutes — considerably shorter than full-length GH (approximately 3-4 hours) and substantially shorter than the stabilised AOD-9604 analogue. This rapid clearance means the fragment is quickly degraded by peptidases in plasma and tissue.
Key pharmacokinetic considerations include:
- Administration route: Subcutaneous injection is the standard research administration route. Oral bioavailability is expected to be negligible for the unmodified fragment due to gastrointestinal proteolytic degradation.
- Stability: The lack of protective modifications (unlike AOD-9604’s tyrosine cap) makes the native fragment vulnerable to rapid enzymatic breakdown. This was one of the primary motivations for developing AOD-9604 as a more practical research tool.
- Distribution: As a small peptide, Fragment 176-191 is expected to distribute primarily to peripheral tissues with significant adipose tissue penetration, consistent with its peripheral signalling mechanism.
The rapid clearance profile means that any research applications would require careful consideration of timing and frequency of administration. This pharmacokinetic limitation is one reason the stabilised AOD-9604 form has been preferred in most formal research settings.
FAQ
What is HGH Fragment 176-191?
HGH Fragment 176-191 is a synthetic peptide comprising amino acids 176 through 191 of human growth hormone. It represents the C-terminal region of GH that has been identified in research as responsible for the hormone’s lipolytic (fat-metabolising) activity, without the growth-promoting or glucose-disrupting effects of the full hormone. It is a research compound and is not approved for therapeutic use.
What are the benefits of HGH Fragment 176-191 in research?
Published research — primarily in animal models — suggests HGH fragment benefits include stimulation of lipolysis, inhibition of lipogenesis (new fat formation), and fat oxidation increases comparable to full GH. The key research advantage is selectivity: the fragment appears to promote fat metabolism without elevating IGF-1 levels or disrupting glucose homeostasis. These findings remain preclinical and have not been confirmed in human trials of the unmodified fragment.
How does Fragment 176-191 differ from AOD-9604?
Fragment 176-191 is the parent molecule — the native GH sequence. AOD-9604 is a stabilised modification that adds a tyrosine residue to improve metabolic stability. AOD-9604 has progressed through Phase II clinical trials and received GRAS status; Fragment 176-191 has not. For detailed AOD-9604 information, see our AOD-9604 guide.
Does HGH Fragment 176-191 affect IGF-1 levels?
Published animal studies consistently report that the fragment does not elevate circulating IGF-1 levels, unlike full-length growth hormone. This is one of its distinguishing features in research settings, as IGF-1 elevation is associated with both benefits (muscle growth) and risks (cell proliferation) of GH therapy.
What is the half-life of HGH Fragment 176-191?
The estimated half-life of the unmodified HGH frag 176-191 is approximately 15-20 minutes — significantly shorter than full GH (~3-4 hours). This rapid degradation was one of the motivations for developing the stabilised AOD-9604 analogue.
Does the HGH fragment affect blood glucose?
In published preclinical studies, Fragment 176-191 did not produce the hyperglycaemic effects associated with full-length growth hormone. This glucose neutrality is considered one of the fragment’s most significant advantages over intact GH in research contexts, as the diabetogenic effects of GH limit its therapeutic utility.
What are the side effects of HGH Fragment 176-191?
The safety data for HGH Fragment 176-191 is limited to animal studies, where it was generally well-tolerated. No human clinical trial data exists for the unmodified fragment. Theoretical considerations include injection site reactions and unknown effects from long-term exposure. The stabilised analogue AOD-9604 has more extensive safety data from Phase II trials.
Is HGH Fragment 176-191 approved for human use?
No. HGH Fragment 176-191 is not approved by the FDA or any regulatory body for human therapeutic use. It is classified as a research compound. Even the stabilised analogue AOD-9604, which progressed further through clinical development, did not achieve drug approval — though it received GRAS status as a food ingredient.
References
- Wu Z, Ng FM. Antilipogenic action of synthetic C-terminal sequence 177-191 of human growth hormone. Biochem Mol Biol Int. 1993;30(1):187-196. PMID: 8358331
- 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. J Mol Endocrinol. 2000;25(3):287-298. PMID: 11116208
- Ng FM, Sun J, Sharma L, Libinaka R, Jiang WJ, Gianello R. Metabolic studies of a synthetic lipolytic domain (AOD9604) of human growth hormone. Horm Res. 2000;53(6):274-278. PMID: 11146367
- 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. Int J Obes Relat Metab Disord. 2001;25(10):1442-1449. PMID: 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
- Wilding J. AOD-9604 Metabolic. Curr Opin Investig Drugs. 2004;5(4):436-440. PMID: 15134286
- Berryman DE, List EO, Sackmann-Sala L, et al. Growth hormone and adipose tissue: beyond the adipocyte. Growth Horm IGF Res. 2011;21(3):113-123. PMID: 21470887
- Sackmann-Sala L, Berryman DE, Lubbers ER, et al. Age-related and depot-specific changes in white adipose tissue of growth hormone receptor-null mice. J Gerontol A Biol Sci Med Sci. 2014;69(1):34-43. PMID: 23873966