What Is HGH Fragment 176–191?

HGH Fragment 176–191, also called somatotropin 176–191 or hGH 176–191, is a synthetic version of the final 16 amino acids of native 191-residue human growth hormone.

Major research themes

• Lipolysis, or release of stored fatty acids
• Lipogenesis, or storage and synthesis of fat
• Adipocyte metabolism
• Beta-adrenergic signaling
• Glucose and insulin responses
• Separation of metabolic from growth-promoting GH domains
• Analytical distinction from AOD-9604

🧬 Structure, Sequence, and Molecular Properties

🧪 Native hGH 176–191 sequence

H-Phe-Leu-Arg-Ile-Val-Gln-Cys-Arg-Ser-Val-Glu-Gly-Ser-Cys-Gly-Phe-OH

FLRIVQCRSVEGSCGF

Disulfide requirement

The two cysteines form an intramolecular disulfide bridge. Reduced, scrambled, intermolecularly linked, or overoxidized cysteine species may have different conformation, stability, and biological activity.

Free-acid termini

The native fragment has a free N terminus and a free C-terminal carboxyl group. Acetylation, amidation, or other terminal modifications create different molecules.

Salt forms

Commercial material may be supplied as acetate or TFA salt. Gross powder weight can include counterions, water, and residual solvents and therefore does not equal net peptide content.

HGH Fragment 176–191 vs AOD-9604

Why the names are confused

Commercial websites often use HGH Frag 176–191, Tyr-hGH177–191, and AOD-9604 as synonyms. This collapses two distinct sequences into one category.

Evidence-transfer problem

Animal and human results obtained with AOD-9604 should not automatically be assigned to native FLRIVQCRSVEGSCGF.

COA implication

MS/MS sequencing must confirm whether residue 1 is phenylalanine or tyrosine. HPLC purity alone cannot resolve an incorrect identity claim.

Relationship to Full-Length Human Growth Hormone

Parent hormone

Human growth hormone is a 191-amino-acid protein with two disulfide bonds and a molecular weight of approximately 22 kDa.

C-terminal domain hypothesis

Early research proposed that GH’s C-terminal region contained metabolic activity separable from its growth-promoting domain.

Fragment rationale

Researchers synthesized C-terminal peptides to test whether lipolytic or hyperglycemic actions could be retained without full-length GH’s broader receptor-mediated growth effects.

Not simply “mini-HGH”

A 16-residue fragment does not preserve the complete three-dimensional structure or receptor-binding surfaces of full-length GH.

Distinct pharmacology

Any biological activity may occur through different binding partners, membrane effects, adrenergic pathways, or indirect mechanisms rather than classic full-length GH receptor agonism.

📅 Research Timeline

• 1970s: Synthetic C-terminal GH fragments, including residues 176–191, were studied for hyperglycemic and insulin-related effects in animals.
• 1980s–1990s: Researchers explored GH domains associated with fat metabolism and separated them conceptually from growth activity.
• Late 1990s: AOD-9604, the Tyr-hGH177–191 analogue, entered obesity and metabolic development.
• 2000: AOD-9604 metabolic studies were reported in obese Zucker rats.
• 2001: AOD-9604 reduced body weight and fat in obese mice and was linked to beta-adrenergic signaling.
• Early 2000s: Human AOD-9604 obesity trials produced mixed early signals.
• 2007: A pivotal 24-week obesity trial involving more than 500 participants failed to demonstrate sufficient weight loss, and drug development was discontinued.
• 2014: A safety and metabolism review summarized AOD-9604 toxicology and clinical exposure.
• 2022: Native hGH Fragment 176–191 was investigated as part of doxorubicin-loaded nanoparticle research in breast-cancer cells.
• 2026: Native hGH 176–191 remained unapproved, poorly studied in humans, and prohibited under anti-doping rules.

🧠 Proposed Mechanism of Action

1. Adipocyte signaling

The fragment has been investigated for direct or indirect effects on fat cells rather than for classical systemic growth activity.

2. Lipolysis

Preclinical claims center on increased breakdown of stored triglycerides into glycerol and free fatty acids.

3. Lipogenesis inhibition

Experimental literature also describes reduced conversion or storage of nutrients into triglyceride.

4. Adrenergic sensitization

AOD-9604 studies suggested enhanced sensitivity to beta-adrenergic lipolytic signaling, particularly beta-3 pathways in rodent adipose tissue.

5. Unresolved direct target

No universally accepted high-affinity receptor or direct molecular target has been validated for native HGH Frag 176–191.

Lipolysis and Lipogenesis Research

Fat-cell mobilization

Early GH-fragment work suggested that selected C-terminal sequences could influence fatty-acid mobilization.

Animal weight-loss studies

The best-known body-fat reductions were reported with AOD-9604 rather than definitively authenticated native HGH Frag 176–191.

Human translation failure

Although an early AOD-9604 study reported a modest placebo-adjusted weight-loss signal, the larger 24-week trial failed its primary endpoint.

No established localized fat loss

There is no reliable evidence that injecting the fragment near a body area selectively reduces fat in that location.

No proof of clinically meaningful monotherapy

Current evidence does not establish native HGH Frag 176–191 as an effective obesity treatment.

Beta-Adrenergic and Adipocyte Signaling

Beta-3 adrenergic pathway

Rodent AOD-9604 studies linked metabolic effects to beta-3 adrenergic receptor signaling, a pathway involved in adipose lipolysis and thermogenesis.

Species differences

Human adipose tissue differs from rodent tissue in beta-3 receptor expression and physiological importance.

Catecholamine sensitivity

AOD-9604 appeared to increase lipolytic responsiveness in obese mice, but whether native HGH Frag produces the same effect in humans is unknown.

Not a validated receptor agonist

The fragment is not established as a direct selective beta-3 adrenergic receptor agonist.

Glucose and Insulin Research

Early hyperglycemia findings

A 1978 study found that several synthetic GH C-terminal fragments, including hGH 176–191, produced a short-lived increase in blood glucose and a more sustained increase in plasma insulin in rats.

Conflict with modern marketing

Claims that HGH Frag has no glucose or insulin effects are too absolute given these early findings and the limited human evidence.

AOD-9604 safety studies

Later AOD-9604 development generally did not show the same diabetogenic profile as full-length GH, but this cannot prove metabolic neutrality for native fragment products.

Human uncertainty

Effects may vary with dose, route, fasting, diabetes status, insulin sensitivity, concomitant drugs, and product identity.

Growth-Hormone Receptor, IGF-1, and Anabolic Effects

Incomplete receptor-binding structure

HGH Frag lacks most residues required for full-length GH’s normal receptor dimerization and signaling.

IGF-1 claims

AOD-9604 clinical development did not demonstrate the typical IGF-1 elevation associated with full-length GH. Direct human evidence for native HGH Frag remains absent.

Muscle growth

No controlled human evidence establishes HGH Frag 176–191 as anabolic or muscle building.

Linear growth

The fragment is not an approved treatment for childhood growth failure.

Receptor assay needed

A credible mechanistic study should directly test GH receptor binding, receptor phosphorylation, and IGF-1 output rather than infer absence of activity from sequence length alone.

Animal and Preclinical Evidence

Native fragment studies

Early animal studies evaluated glucose and insulin responses to native C-terminal fragments.

AOD-9604 obesity studies

Obese rodent models showed reduced body weight, body fat, and increased lipolytic sensitivity after AOD-9604.

Cell studies

Research has examined adipocytes, metabolic enzymes, adrenergic response, and more recently drug-delivery and cancer-cell systems.

Translation limits

Rodent fat metabolism, receptor expression, dose scaling, and route of administration differ substantially from human obesity biology.

Identity limits

Many secondary summaries fail to specify whether the tested compound was native FLRIVQCRSVEGSCGF or AOD-9604 YLRIVQCRSVEGSCGF.

Human Evidence

Native HGH Frag 176–191

No robust controlled human intervention trial of authenticated native FLRIVQCRSVEGSCGF has been established.

AOD-9604 evidence

Human studies evaluated AOD-9604, a related tyrosine-substituted analogue. Early studies suggested possible short-term weight change, but a pivotal 24-week trial of approximately 536 participants failed to show statistically significant weight loss for the primary endpoint.

Safety data are not efficacy data

AOD-9604 toxicology and clinical exposure reports may inform questions but do not establish effective obesity treatment or prove safety of native HGH Frag injections.

No approved human protocol

There is no FDA-approved dose, route, duration, monitoring plan, or manufacturing specification for HGH Frag 176–191.

Cancer and Cellular Research

Nanoparticle study

A 2022 study incorporated hGH Fragment 176–191 into chitosan nanoparticles containing doxorubicin and reported increased toxicity against MCF-7 breast-cancer cells.

Interpretive limit

This was an in-vitro drug-delivery experiment, not evidence that HGH Frag itself treats cancer.

Growth-factor context

Because the fragment derives from a growth hormone, direct receptor, proliferation, and survival effects should be evaluated rather than assumed absent.

No oncology indication

HGH Frag 176–191 is not an approved cancer drug, chemotherapy enhancer, or preventive therapy.

Major Evidence Limitations

• Native HGH Frag and AOD-9604 are routinely confused
• Direct human evidence for native FLRIVQCRSVEGSCGF is essentially absent
• Most obesity evidence belongs to AOD-9604
• AOD-9604’s pivotal obesity trial failed
• No validated direct receptor has been established for native fragment action
• Mechanistic claims rely heavily on rodent adipose biology
• Early glucose and insulin findings complicate “metabolically neutral” claims
• No established human pharmacokinetic profile for native fragment
• No approved dose, route, duration, or monitoring framework
• No established long-term cardiovascular, glucose, immune, or cancer safety
• Commercial products may contain AOD-9604 while labeled as HGH Frag, or vice versa
• No FDA-reviewed finished-product specification exists

FDA and Anti-Doping Status

FDA approval

HGH Fragment 176–191 is not FDA approved for any therapeutic indication.

Unapproved drug status

Products marketed for treating obesity, fat loss, or disease without an approved application may be considered unapproved new drugs.

AOD-9604 distinction

AOD-9604’s historical food-ingredient or GRAS-related discussions do not equal FDA approval as an injectable drug and do not transfer to native HGH Fragment 176–191.

WADA prohibition

The World Anti-Doping Agency prohibits growth hormone, its analogues, and its fragments. WADA materials explicitly identify AOD-9604 and hGH 176–191 as prohibited examples.

Research-use labeling

A research-use label does not establish human safety, approved clinical use, or sports compliance.

Potential Side Effects and Safety Considerations

No established native-fragment safety profile

The absence of controlled human exposure data prevents reliable estimates of adverse-event frequency or severity.

Potential metabolic effects

• Transient glucose elevation
• Insulin changes
• Possible effects on fatty-acid mobilization
• Unknown interactions with diabetes drugs
• Unknown effects during fasting or intense exercise

Injection and immune risks

• Injection-site pain, redness, swelling, or infection
• Hypersensitivity
• Anti-peptide antibodies
• Endotoxin or microbial contamination
• Particulates

Disulfide and impurity risks

Reduced peptide, scrambled disulfides, intermolecular dimers, cysteine overoxidation, sequence errors, and incorrect AOD-versus-native identity may change activity and immunogenicity.

Growth and cancer uncertainty

Although the fragment is not expected to reproduce full-length GH signaling, direct long-term human data addressing proliferation, neoplasia, organ growth, or IGF pathways are insufficient.

Pregnancy and pediatric use

Reproductive, developmental, pediatric, and pregnancy safety have not been established.

🧪 Laboratory Testing Methods

📄 How to Interpret an HGH Frag 176–191 COA

1. Confirm the exact native sequence: FLRIVQCRSVEGSCGF.
2. Confirm phenylalanine—not tyrosine—at residue 1.
3. Verify formula C₇₈H₁₂₃N₂₃O₂₂S₂.
4. Verify molecular weight near 1,799.1 Da.
5. Confirm the Cys7–Cys14 intramolecular disulfide bond.
6. Measure reduced, scrambled, and intermolecular-disulfide species.
7. Use MS/MS sequence mapping rather than intact mass alone.
8. Confirm all amino acids have the intended L stereochemistry.
9. Confirm free N- and C-terminal groups.
10. Measure deletion peptides, epimers, cysteine oxidation, and dimers.
11. State free base versus acetate or TFA salt.
12. Report net peptide content after correcting for counterions and water.
13. Use adipocyte lipolysis and lipogenesis assays when metabolic potency is claimed.
14. Include direct GH-receptor and IGF-1 pathway testing rather than assuming no activity.
15. For finished injectables, require sterility, endotoxin, particles, pH, osmolality, fill accuracy, container closure, and post-reconstitution stability.
16. Do not use AOD-9604 reference standards or clinical data as though they prove native-fragment identity or efficacy.
17. A COA does not establish FDA approval, human safety, or clinical effectiveness.

📊 Comparison Tables

HGH Frag 176–191 vs AOD-9604

HGH Frag vs Full-Length HGH

HGH Frag vs GLP-1-Based Obesity Medicines

Basic Product Claim vs Research-Qualified Material

🖼️ Original Diagram Specifications

1. Sequence identity: Native FLRIVQCRSVEGSCGF versus AOD YLRIVQCRSVEGSCGF with residue 1 highlighted.
2. Parent-protein map: Full 191-aa hGH with residues 176–191 highlighted at the C terminus.
3. Disulfide structure: Cys7–Cys14 loop with free N and C termini.
4. Proposed metabolic pathway: Adipocyte, catecholamine signaling, lipolysis, fatty-acid release, and lipogenesis.
5. Evidence timeline: Early native-fragment animal studies, AOD rodent work, human AOD trial failure, and current status.
6. Evidence pyramid: Chemistry and animal data, limited native-fragment cellular work, absent human native-fragment trials.
7. COA workflow: Phe-versus-Tyr identity, sequence, disulfide mapping, net content, potency, sterility, and stability.

❓ Frequently Asked Questions

Is HGH Fragment 176–191 a peptide?

Yes. It is a cyclic 16-amino-acid fragment of human growth hormone.

What is the exact native sequence?

FLRIVQCRSVEGSCGF.

What is its molecular formula?

C₇₈H₁₂₃N₂₃O₂₂S₂.

What is its molecular weight?

Approximately 1,799.1 Da.

Does it contain a disulfide bond?

Yes. Cys7 and Cys14 form an intramolecular disulfide bond.

Is HGH Frag the same as AOD-9604?

No. Native HGH Frag begins with phenylalanine; AOD-9604 begins with tyrosine.

Why are the names often used interchangeably?

Both represent closely related C-terminal GH sequences, but commercial and secondary sources often fail to preserve the residue-1 distinction.

Is HGH Frag FDA approved?

No.

Is it approved for weight loss?

No.

Does it burn fat?

Preclinical studies support a metabolic hypothesis, mainly using AOD-9604, but clinically meaningful human fat loss from native HGH Frag has not been established.

Did AOD-9604 work in human obesity trials?

Early results were mixed, and a pivotal 24-week trial failed to show sufficient statistically significant weight loss.

Does HGH Frag raise IGF-1?

Direct human evidence for the native fragment is absent. It should not be assumed completely inactive without receptor and IGF-pathway testing.

Can it affect glucose?

Early animal research found transient hyperglycemia and sustained insulin elevation with hGH 176–191.

Is it anabolic?

No controlled human evidence establishes muscle-building effects.

Is it prohibited in sport?

Yes. WADA prohibits growth-hormone fragments, including hGH 176–191.

Does 99% HPLC purity prove it is native HGH Frag?

No. Residue-1 identity, full sequence, disulfide mapping, stereochemistry, content, potency, sterility, and stability must also be verified.

Final Thoughts

HGH Fragment 176–191 is the native 16-residue C-terminal segment of human growth hormone, with the sequence FLRIVQCRSVEGSCGF and a Cys7–Cys14 disulfide bond. It was investigated to determine whether selected metabolic actions of GH could be separated from the full protein’s growth-promoting activity.

The most important interpretive issue is the distinction from AOD-9604. AOD-9604 begins with tyrosine and has a molecular mass 16 Da higher than the native phenylalanine-containing fragment. Most modern obesity and lipolysis claims trace back to AOD-9604 studies, not authenticated native HGH Frag 176–191.

Preclinical research suggests possible effects on lipolysis, lipogenesis, adrenergic sensitivity, glucose, and insulin. However, no robust human intervention evidence establishes native HGH Frag as an effective weight-loss treatment. AOD-9604 itself failed to achieve sufficient efficacy in a large pivotal obesity trial.

Analytical authentication requires the exact FLRIVQCRSVEGSCGF sequence, phenylalanine at residue 1, free terminal groups, the Cys7–Cys14 intramolecular disulfide, correct stereochemistry, counterion-corrected content, degradation profiling, metabolic potency assays, and route-specific microbiological quality. HGH Frag 176–191 remains unapproved and prohibited in competitive sport.

📚 References

1. PubChem. Somatotropin (176–191), CID 16131230.
2. PubChem. HGH Fragment 176–191 Acetate, CID 172966176.
3. PubChem. AOD-9604, CID 71300630.
4. Ng FM, et al. Hyperglycemic Action of Synthetic C-Terminal Fragments of Human Growth Hormone. Hormone and Metabolic Research. 1978.
5. Ng FM, et al. Metabolic Studies of a Synthetic Lipolytic Domain of Human Growth Hormone, AOD-9604, in Obese Zucker Rats. Hormone Research. 2000.
6. Heffernan MA, et al. The Effects of Human Growth Hormone and Its Lipolytic Fragment AOD-9604 on Lipolysis and Beta-3-Adrenergic Receptors in Obese Mice. Endocrinology. 2001.
7. Moré MI, et al. Safety and Metabolism of AOD9604, a Novel Nutraceutical Ingredient for Improved Metabolic Health. Journal of Endocrinology and Metabolism. 2014.
8. Misra M. Obesity Pharmacotherapy: Current Perspectives and Future Directions. Current Cardiology Reviews. 2013.
9. Habibullah MM, et al. Human Growth Hormone Fragment 176–191 Peptide Enhances the Toxicity of Doxorubicin-Loaded Chitosan Nanoparticles Against MCF-7 Breast Cancer Cells. Drug Design, Development and Therapy. 2022.
10. World Anti-Doping Agency. Prohibited List: Growth Hormone, Its Analogues and Fragments.
11. World Anti-Doping Agency. 2026 Prohibited List.
12. USADA. Athlete Guidance on the 2026 WADA Prohibited List.
13. U.S. Food and Drug Administration. Certain Bulk Drug Substances for Use in Compounding That May Present Significant Safety Risks.
14. U.S. Food and Drug Administration. Warning Letters Concerning Unapproved Peptide Products.
15. International Council for Harmonisation. ICH Q1A(R2), Q2(R2), Q3A, Q3B, Q3C, and Q6B.
16. United States Pharmacopeia General Chapters <621>, <71>, <85>, and <788>.

Sequence, chemistry, AOD-9604 distinction, evidence, regulatory status, safety, and analytical recommendations reviewed in July 2026.