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GH C-terminal Fragment 16 Amino Acids

HGH Fragment
176–191

HGH Frag · GH Fragment · C-terminal Lipolytic Fragment

"The fat-burning fragment of growth hormone — isolated from the C-terminus of HGH specifically because it carries the lipolytic signal without the anabolic, IGF-1 raising, or glucose-disrupting effects. Promising in animals. A Phase IIb failure in humans. Still widely used."

Type
16 amino acid synthetic fragment
Structure
C-terminal residues 176–191 of HGH
Status
UK: not illegal to buy or possess · WADA: prohibited · US FDA: Category 2 — restricted compounding (still on list)
Key trial
Phase IIb 2007 · 536 patients · weight-loss endpoint not met
Protocol summary
Community dose
250–500 mcg/day SubQ
Route
SubQ pre-cardio
Cycle
8–12 weeks
Community-reported
250–500 mcg/day SubQ · pre-cardio · 8–12 week cycles
Phase IIb did not meet weight-loss endpoint; community use continues
How we read the evidence
Animal lipolytic data · the modified version (AOD-9604) failed Phase 2b · marketing far ahead of evidence
Animal evidence

The fragment corresponds to the C-terminal 16 amino acids (176–191) of human growth hormone, with the lipolytic domain isolated from the rest of GH's effects. Animal work (rats, pigs, mice) shows lipolysis stimulation and lipogenesis inhibition without the insulin-resistance effects of intact GH. Fragment was reported up to 12.5× more potent than full GH for fat burning in some preclinical models. Doesn't significantly affect IGF-1, glucose tolerance, or insulin sensitivity in animal studies — that's the mechanistic claim.

Community & clinical practice

Community protocols typically use 250–500 mcg SubQ daily, often pre-cardio on empty stomach, in 8–12 week cycles. Sometimes split into morning + pre-bed doses. The marketing claim is selective fat oxidation without GH side effects, but real-world community results are inconsistent — many users report no measurable fat loss, which is consistent with the underlying clinical evidence rather than contradicting it.

Human trial data

Direct human clinical trials of fragment 176-191 are essentially absent. The closely related modified version AOD-9604 went through Phase 2b trials by Metabolic Pharmaceuticals — multiple trials in ~300 obese adults at 1–30 mg oral over 12 weeks. Best result: 2.0–2.8 kg additional fat loss vs placebo at 1 mg/day. Effect size was insufficient for drug registration; Metabolic Pharmaceuticals abandoned the obesity program and pivoted to other indications. A 2013 meta-analysis confirmed adverse events at placebo-level rates but the efficacy signal didn't justify Phase 3.

Regulatory status

Not FDA-approved. WADA-banned for tested athletes. Sourcing via research-peptide vendors. Fragment 176-191 specifically (the unmodified version) has no formal regulatory pathway; AOD-9604 (the modified version with stabilising tyrosine added) was pursued for obesity and abandoned at Phase 2b. Some intra-articular AOD-9604 work for osteoarthritis (under the name LAT-8881) was conducted but preliminary results not promising.

Convergence

HGH Fragment 176-191 has a real lipolytic mechanism in animal models, but the closely related AOD-9604 demonstrated insufficient efficacy in human Phase 2b trials — only 2–3 kg additional fat loss vs placebo, well below what would justify drug registration. Pep IQ is honest about this: the marketing of HGH Fragment as a fat-loss compound runs ahead of the evidence. Far better-evidenced options (GLP-1s, tesamorelin for VAT specifically) deliver bigger and better-characterised effects. Considering HGH Fragment for body composition? The evidence base says you'll likely be disappointed.

Origin & Background

The lipolytic fragment of growth hormone

Human growth hormone has two biologically distinct domains: the N-terminal region responsible for growth promotion, IGF-1 stimulation, and anabolic effects; and the C-terminal region (amino acids 176–191) responsible for fat metabolism. HGH Fragment 176-191 was isolated by researchers who wanted the fat-burning properties of growth hormone without the growth, IGF-1, insulin resistance, and glucose disruption that comes with the full molecule.

The fragment activates β3-adrenergic receptors in adipocytes — stimulating lipolysis and inhibiting new fat storage — without binding the growth hormone receptor at all. This means no IGF-1 elevation, no insulin resistance, no glucose disruption, and no anabolic effects. In theory, a clean, targeted fat-loss signal.

Early Phase II human data was encouraging. A 2004 study showed 1mg/day produced approximately 3kg of fat loss over three months compared to placebo. Then came the Phase IIb trial in 2007: 536 participants, multiple dosing protocols, systematic failure to demonstrate clinical efficacy. The developer (Metabolic Pharmaceuticals) discontinued development. The fragment is now classified by the FDA as a Category 2 compound — meaning it cannot be legally compounded. Despite this, it remains one of the most widely used research peptides in the community.

HGH Frag 176-191 vs AOD-9604: AOD-9604 is a modified version of the same fragment with an additional tyrosine residue at the N-terminus. The two are closely related but technically distinct compounds. Both failed clinical development for the same reasons. AOD-9604 has a separate FDA Category 2 designation. The community often uses the names interchangeably, but they are different molecules.

Science & Mechanism

β3-adrenergic activation — without GH receptor binding

Mechanism of Action

1
No GH receptor binding: Unlike full HGH, Fragment 176-191 does not bind the growth hormone receptor. This eliminates IGF-1 stimulation, insulin resistance, glucose disruption, and anabolic effects. The fragment is entirely focused on adipose tissue metabolism.
2
β3-adrenergic receptor activation: Acts on β3-adrenergic receptors (ADRB3) in white and brown adipose tissue. ADRB3 activation increases intracellular cAMP, activates protein kinase A (PKA), and initiates lipolytic signalling cascades — breaking down stored triglycerides into free fatty acids.
3
Lipogenesis inhibition: Simultaneously inhibits the uptake of dietary fats into adipose cells — reducing new fat storage while increasing breakdown of existing fat stores.
4
Mitochondrial effects (2023 in vitro): A 2023 investigation using primary human adipocytes found the fragment increased uncoupling protein expression and mitochondrial oxygen consumption — suggesting fat oxidation capacity enhancement beyond simple lipolysis. This is the most recent mechanistic evidence.
5
Obesity-selective effect: Animal studies showed the fragment reduced fat in obese animals but had minimal effect in lean animals — suggesting the ADRB3 mechanism is more active under conditions of excess adiposity. This selectivity is mechanistically interesting but did not translate to human clinical efficacy.

The clinical failure requires honest explanation. The Phase IIb trial (536 participants, multiple dose arms) found no statistically significant advantage over placebo across any dosing protocol. The disconnect between animal success and human failure likely reflects the complexity of human energy balance — isolated lipolytic stimulation is insufficient to overcome compensatory mechanisms including appetite upregulation, metabolic adaptation, and behavioural factors. GH itself produces meaningful fat loss in humans through a more complex hormonal cascade; its isolated C-terminal fragment, operating through only one mechanism, appears insufficient.

Benefits & Evidence

What the data shows

🔥
Fat loss (animal models)
Consistent in obese rodent models — reduces fat mass, increases lipolysis, inhibits lipogenesis without affecting blood glucose or IGF-1. The mechanism is sound and well-characterised in preclinical settings.
● Moderate — strong animal data
👤
Fat loss (limited human data)
2004 early trial: ~3kg fat loss at 1mg/day over 3 months vs placebo. Phase IIb 2007 (n=536): failed to demonstrate clinical efficacy across multiple doses. The one positive signal did not replicate at scale.
● Limited — one positive, one failed Phase IIb
⚡
No IGF-1, no glucose disruption
The clean safety profile — no IGF-1 elevation, no insulin resistance, no glucose disruption — is genuinely distinctive. This makes it more tolerable than full GH for people concerned about those effects, even if the efficacy evidence is weak.
● Strong — well-established mechanistic characteristic
🦴
Cartilage regeneration (emerging)
A study combining HGH Frag 176-191 with hyaluronic acid showed greater cartilage formation in rabbit osteoporosis models than hyaluronic acid alone. An unexpected finding warranting further investigation.
● Emerging — single animal study
Things to know

Risks & considerations

⚠️
Better safety profile than full GH — but not approved and cannot be compounded. The absence of IGF-1 elevation and glucose effects makes this a cleaner compound than GH or most GH secretagogues. The main practical concern is FDA Category 2 status making legal access difficult and unregulated market quality uncertain.
Mild
Injection site reactions — transient redness and swelling. Common with SubQ administration.
Mild
Transient blood glucose effects — some animal data suggests mild transient insulin-like effects. Not confirmed to be clinically relevant in humans at standard doses.
Serious
FDA Category 2 — cannot be compounded: Unlike some other peptides, HGH Fragment 176-191 is FDA Category 2, meaning it cannot be legally prepared by compounding pharmacies in the US. It is only available through unregulated research chemical markets — meaning no quality control, no purity verification, and no dose accuracy assurance.
Unknown
Long-term effects unknown — clinical development was discontinued. No long-term safety data in humans exists. The animal safety profile is reassuring but limited.

⚠ Key Warnings

FDA Category 2 status means this compound cannot be legally compounded in the US. All available material is from unregulated sources — purity and dosing accuracy cannot be verified.
The Phase IIb trial failure is significant. Clinical development was discontinued for efficacy reasons, not safety. The community use continues despite the lack of demonstrated human efficacy at scale.
WADA prohibited at all times for competitive athletes.
Do not confuse with AOD-9604 — related but distinct compound with its own separate regulatory status.
Fast Facts
Structure16 AA · C-terminal HGH fragment
FDA statusCategory 2 · cannot be compounded
Key trialPhase IIb 2007 · n=536 · failed
vs AOD-9604Related but distinct · tyrosine modified
No IGF-1Does not bind GH receptor
RouteSubQ 500mcg–2mg/day · fasted
WADAProhibited at all times
Related Peptides
AOD-9604Modified version · same mechanism
SemaglutideProven fat loss alternative
Ipamorelin / CJCGH stack combination
MOTS-cMitochondrial fat oxidation overlap