Mature human growth hormone, also called somatotropin, contains 191 amino-acid residues.

HGH Fragment 176-191 consists of residues 176 through 191 inclusive. It therefore contains 16 residues rather than 15.

The fragment begins with the phenylalanine located at position 176 and ends with the phenylalanine at position 191.

Two cysteine residues within the sequence form a disulphide bridge. This recreates one of the two disulphide bonds present in full-length human growth hormone.

The fragment is not produced as a separately established circulating human hormone.

It is usually manufactured synthetically for biochemical, metabolism, anti-doping or analytical research.

Human growth hormone is secreted from somatotroph cells within the anterior pituitary.

Its release is regulated by growth-hormone-releasing hormone, somatostatin, ghrelin, sleep, exercise, nutrition, age and metabolic signals.

Full-length growth hormone activates the growth-hormone receptor on tissues including liver, muscle and adipose tissue.

Receptor activation can stimulate production of insulin-like growth factor 1, influence protein metabolism and alter glucose and lipid handling.

Growth hormone can increase lipolysis, but sustained excessive exposure can also impair insulin sensitivity and raise blood glucose.

Its growth and metabolic effects depend on the complete folded protein interacting with two receptor molecules.

A 16-residue fragment cannot be assumed to reproduce receptor binding, endocrine feedback or tissue distribution of the 191-residue protein.

The first phenylalanine corresponds to position 176 of mature human growth hormone.

The leucine that follows corresponds to position 177.

The final phenylalanine corresponds to position 191 and retains an ordinary free C-terminal carboxylic-acid group.

The molecule is not C-terminally amidated.

A product beginning with tyrosine is AOD-9604 rather than native HGH Fragment 176-191.

The cysteines are located at positions seven and fourteen of the isolated fragment.

Within full-length human growth hormone, they correspond to Cys182 and Cys189.

Oxidation of their sulphydryl groups produces an intramolecular disulphide bond and removes two hydrogen atoms.

The oxidised cyclic form differs chemically from a fully reduced linear peptide containing two free thiol groups.

Inappropriate oxidation can also connect two separate peptide molecules and form disulphide-linked dimers or larger aggregates.

Reduction, disulphide scrambling and intermolecular crosslinking can alter biological activity and immunogenicity.

Replacing phenylalanine with tyrosine adds a hydroxyl group to the aromatic side chain.

This changes molecular mass, polarity, chromatographic behaviour, oxidation profile and potentially receptor or membrane interactions.

A one-residue difference may be small numerically while still being important for analytical identity and regulatory classification.

AOD-9401 was an earlier synthetic growth-hormone-derived experimental compound studied in oral animal research.

Publications described lipolytic and antilipogenic effects in obese mice and human adipose-tissue preparations.

The naming history is not always described consistently across summaries, patents and later reviews.

Findings from AOD-9401 should therefore be linked to its exact sequence and formulation rather than grouped automatically under “HGH Fragment 176-191”.

Supplier pages frequently combine AOD-9401, AOD-9604, native hGH 176-191 and hGH 177-191 as though they were one substance.

Lipolysis

Lipolysis is the process through which stored triglycerides are broken down into glycerol and free fatty acids.

Full-length growth hormone can increase lipolytic responsiveness in adipose tissue.

Some animal studies of modified C-terminal fragments reported increased fat oxidation or sensitivity to lipolytic signals.

Lipogenesis

Lipogenesis refers to synthesis and storage of fatty acids and triglycerides.

A 1993 rat study reported that hGH 177-191 reduced incorporation of labelled glucose into lipid within adipose tissue.

The same experiment did not find a significant acute lipolytic effect when glycerol release was measured.

Adrenergic Signalling

Later AOD-9604 mouse studies proposed involvement of beta-adrenergic pathways, particularly beta-3 adrenergic receptors in adipose tissue.

Reduced effect in beta-3-receptor knockout mice supported a relationship in that model.

Mouse beta-3 adrenergic biology does not translate directly to human weight reduction.

The full growth-hormone receptor recognises several surfaces created by the folded 191-residue protein.

A short C-terminal fragment lacks most of these receptor-binding surfaces.

AOD-9604 experiments reported no meaningful competition for the classical growth-hormone receptor and no comparable cellular proliferation response.

This supported the concept that the modified fragment might influence metabolism independently of classical growth-hormone-receptor activation.

Direct receptor-characterisation evidence for the exact phenylalanine-starting hGH 176-191 sequence is much more limited.

It is therefore safer to state that classical receptor activation has not been established than to claim that the native fragment definitively cannot interact with any growth-related pathway.

Researchers tested several overlapping peptides corresponding to the C-terminal region of human growth hormone in rats.

The tested fragments included hGH 172-191, 176-191, 177-191, 178-191, 179-191 and 180-191.

Four of the longer fragments produced a short-lived increase in blood glucose and a more sustained increase in plasma insulin.

Shorter fragments beginning at positions 179 and 180 were inactive in the tested systems.

Follow-up research examined liver glycogen metabolism, pyruvate dehydrogenase activity and adipose-tissue effects.

These studies were performed decades ago using animal models and do not establish a modern therapeutic profile.

Full-length growth hormone can oppose some actions of insulin and raise glucose during sustained exposure.

Early rat research found hyperglycaemic and insulin-related effects from several C-terminal hGH peptides, including the 176-191 fragment.

Later AOD analogue studies were designed partly to identify lipid-related activity without the adverse glucose profile associated with full growth hormone.

AOD-9604 animal studies generally did not reproduce substantial deterioration in glucose control under the conditions tested.

These later findings concern a modified molecule and cannot establish that every commercial HGH Fragment 176-191 product is metabolically neutral.

No adequate controlled human glucose-clamp or long-term diabetes study of exact native HGH Fragment 176-191 was identified.

The 1993 rat study examined the 15-residue hGH 177-191 sequence rather than full native 176-191.

It reported reduced conversion of labelled glucose into lipid within rat adipose tissue.

This was interpreted as an antilipogenic effect.

The researchers did not observe a significant lipolytic effect when glycerol release from epididymal fat pads was measured.

This distinction is important because marketing frequently states that the fragment directly “burns stored fat”.

An antilipogenic effect in isolated or removed rat tissue does not demonstrate substantial human fat loss.

AOD-9401

Oral research in genetically obese mice reported reduced weight gain and changes in lipid metabolism.

Human adipose-tissue preparations were also used to explore lipolytic activity outside the body.

AOD-9604

Studies in obese Zucker rats and obese mouse models reported reductions in weight gain or body fat under selected experimental conditions.

AOD-9604 increased fat oxidation and enhanced lipolytic responsiveness in some mouse studies.

Experiments involving beta-3 adrenergic receptor knockout mice indicated that this receptor pathway contributed to the observed effect.

Translation Limitations

Genetically obese rodents differ from typical human obesity in appetite regulation, energy expenditure, adrenergic signalling and disease duration.

Animal exposure, formulation and route may also differ from research products marketed online.

A compound can reduce weight gain in young growing rodents while failing to produce clinically relevant weight loss in adults.

No adequate published randomised obesity trial of the exact phenylalanine-starting native fragment was identified in the sources reviewed.

No controlled human trial established clinically meaningful reductions in body weight, waist circumference or total body fat from the exact native sequence.

No human study established improved strength, muscle mass, exercise recovery or athletic performance.

No adequate pharmacokinetic study defined absorption, circulating half-life, tissue distribution or metabolism after commercial routes of administration.

No long-term immunogenicity or cancer-safety programme was identified.

Human claims usually rely on AOD-9604 data, full growth-hormone physiology, animal fragment research or unverified personal reports.

Historical AOD-9604 development included intravenous studies, oral pilot studies and phase-two obesity trials.

Some smaller or earlier studies produced encouraging weight-related signals.

FDA’s review found that most identified AOD-9604 studies did not demonstrate benefit compared with placebo.

The largest programme enrolled approximately 536 participants with obesity.

The study did not find a significant difference in weight loss after twelve weeks, which was the primary endpoint.

Development for obesity was subsequently terminated.

FDA also noted that available summaries lacked sufficient methodological and safety detail.

FDA did not identify human information supporting proposed subcutaneous or transdermal use of AOD-9604.

Researchers investigated chitosan nanoparticles containing doxorubicin and HGH Fragment 176-191 against MCF-7 breast-cancer cells.

The experimental system assessed whether the peptide-containing nanoparticle preparation enhanced cellular uptake or toxicity of doxorubicin.

The work was performed in cultured cancer cells rather than patients.

The fragment was incorporated into a complex drug-delivery system and was not tested as an authorised stand-alone cancer therapy.

Increased toxicity to one laboratory cell line does not establish tumour shrinkage, survival benefit, safe systemic delivery or selectivity for cancer over healthy tissue.

Full-length growth hormone can raise hepatic and tissue IGF-1 through classical growth-hormone-receptor signalling.

IGF-1 contributes to growth, protein metabolism and tissue remodelling.

AOD-9604 experiments did not demonstrate the same receptor binding or proliferative activity as full growth hormone.

This observation is frequently extended to native HGH Fragment 176-191 without direct confirmation.

No controlled human trial established that the native fragment increases IGF-1, lean mass, muscle protein synthesis or strength.

There is also no adequate human evidence proving that it can never influence IGF-related or growth-related signalling.

• Direct controlled human research on the exact native fragment is extremely limited.
• No adequate human obesity trial of FLRIVQCRSVEGSCGF was identified.
• Most human data concern AOD-9604.
• AOD-9604 begins with tyrosine rather than phenylalanine.
• AOD-9401 is another separately named experimental compound.
• hGH 177-191 is one residue shorter than hGH 176-191.
• Animal findings cannot establish human weight loss.
• Rodent beta-3 adrenergic biology differs from human biology.
• Reduced weight gain in young mice is not equivalent to adult human weight loss.
• Historical studies used different exposure routes and formulations.
• Early native-fragment research reported glucose and insulin effects.
• The exact metabolic receptor remains uncertain.
• Classical growth-hormone-receptor inactivity is better characterised for AOD-9604.
• No reliable human pharmacokinetic profile was identified for native HGH Frag.
• No established circulating half-life was identified.
• No long-term body-composition study was identified.
• No cardiovascular-outcome study was identified.
• No diabetes-safety programme was identified.
• No long-term cancer-safety programme was identified.
• No reproductive-safety programme was identified.
• No paediatric clinical indication was identified.
• No muscle-strength trial was identified.
• No exercise-performance trial was identified.
• No established recovery or injury-treatment trial was identified.
• The breast-cancer study was performed in cultured cells.
• The cancer-cell study used a multi-component nanoparticle system.
• AOD-9604 failed the primary endpoint in its larger obesity study.
• AOD-9604 safety information cannot prove native-fragment safety.
• Native sequence does not prove correct disulphide formation.
• Reduced peptide may differ from the oxidised native-like form.
• Intermolecular disulphide bonds can produce dimers and aggregates.
• A high HPLC area percentage does not confirm sequence or folding.
• Research-market vials may contain AOD-9604 instead of native fragment.
• Research products may lack endotoxin and sterile controls.

No authorised prescribing information defines contraindications, interactions, monitoring or adverse-event frequencies for native HGH Fragment 176-191.

Direct human exposure data for the exact native sequence are sparse.

AOD-9604 studies provide limited indirect information but concern a chemically distinct peptide.

FDA’s AOD review identified insufficient long-term information and significant uncertainty concerning physicochemical characterisation, impurities, aggregates and immunogenicity.

Reported adverse events in historical AOD studies included gastrointestinal symptoms and chest tightness, although available summaries did not always permit reliable assessment of causation.

FDA also noted equivocal findings in some genotoxicity testing and liver findings in non-human primate research involving AOD-9604.

These observations cannot be assigned directly to native HGH Fragment 176-191, but they prevent claims that the entire fragment class is proven harmless.

Blood Glucose

Historical native-fragment studies reported short-lived increases in glucose in rats.

People with diabetes, prediabetes or impaired glucose tolerance could respond differently from healthy animal models.

Insulin

Sustained insulin increases were reported after selected C-terminal fragments in early rat studies.

Long-term human effects on insulin sensitivity have not been established for exact hGH 176-191.

IGF-1

No adequate evidence establishes a clinically significant IGF-1 increase from the native fragment.

It is equally inappropriate to promise that every research product is completely incapable of influencing growth-related signalling.

Lipid Metabolism

Changes in lipolysis or lipogenesis may alter circulating free fatty acids, liver metabolism and insulin action.

These outcomes were not assessed adequately in long-term human studies.

Sixteen-residue peptides can contain deletion sequences, epimers, oxidation products and incomplete synthesis products.

The two cysteine residues can remain reduced, form the intended intramolecular bond or connect separate peptide molecules.

Disulphide-linked dimers and higher aggregates may increase immunogenic potential.

Changes in pH, temperature, concentration, oxygen exposure and metal contamination can alter oxidation and aggregation.

Research-market certificates often report only reverse-phase HPLC area purity and mass.

Those measurements alone do not establish aggregate content, biological potency, endotoxin or sterile quality.

Human pregnancy studies have not established embryo-fetal safety.

Transfer into breast milk and effects on a nursing infant are unknown.

Children and adolescents should not be exposed to an unauthorised growth-hormone-derived research product.

People with diabetes, pituitary disorders, active malignancy, liver disease, kidney disease or unexplained endocrine symptoms may have additional risks.

A lack of full growth-hormone activity does not establish safety in conditions sensitive to metabolic, growth or immune signalling.

• AOD-9604 substituted for native HGH Fragment 176-191
• Native fragment substituted for AOD-9604
• Fifteen-residue hGH 177-191 material
• Incorrect first residue
• Incorrect final phenylalanine
• Missing amino-acid residues
• Incorrect amino-acid order
• D-amino-acid or epimerised impurities
• Reduced cysteine residues
• Incorrect intramolecular disulphide bond
• Intermolecular disulphide-linked dimers
• Higher-molecular-weight aggregates
• Truncated peptide
• Deletion-sequence impurities
• Incomplete coupling products
• Phenylalanine oxidation products
• Glutamine deamidation
• Incorrect C-terminal amidation
• Incorrect net peptide quantity
• Residual trifluoroacetate
• Undeclared acetate or other counterions
• Residual coupling reagents
• Residual scavengers
• Residual organic solvents
• Elemental contamination
• Incorrect pH
• Unverified bacterial-endotoxin control
• Unverified sterile quality
• Visible or subvisible particles
• Loss of material during warm transport
• Certificate unrelated to the supplied batch
• Raw-material certificate presented as finished-vial evidence

What Meaningful HGH Fragment Documentation Should Include

• Identity as native hGH Fragment 176-191
• Complete sequence FLRIVQCRSVEGSCGF
• Confirmation of N-terminal phenylalanine
• Exclusion or quantification of Tyr-starting AOD-9604
• Confirmation of the free C-terminal acid
• Observed intact molecular mass
• Tandem-MS sequence evidence
• Confirmation of L-amino-acid stereochemistry
• Cys7-to-Cys14 disulphide map
• Reduced-thiol result
• Disulphide-linked dimer result
• Aggregate and particle analysis
• Net peptide-content assay
• Chromatographic purity
• Named and unknown impurity results
• Deletion-sequence profile
• Truncation profile
• Deamidation assessment
• Epimerisation assessment where relevant
• Counterion identification
• Trifluoroacetate or acetate quantity
• Water-content result
• Residual-solvent results
• Residual synthesis-reagent results
• Biological-potency result where relevant
• Bacterial-endotoxin result
• Sterility result for a finished injectable medicine
• Visible and subvisible particle results
• Batch-specific stability evidence
• Container-closure compatibility
• Testing-laboratory identity
• Analytical methods and acceptance criteria
• A statement identifying tests not performed

Why “99% HPLC” Is Not Enough

HPLC area purity does not prove that the main peak begins with phenylalanine rather than tyrosine.

It does not establish complete sequence, amino-acid stereochemistry or the intended intramolecular disulphide bond.

It may not reveal disulphide-linked dimers, larger aggregates, endotoxin, microorganisms or subvisible particles.

It does not establish biological potency, human weight-loss efficacy or pharmaceutical equivalence.

Medicinal-Product Classification

MHRA guidance states that a product may be medicinal where it is presented as treating or preventing disease.

A product may also be medicinal where it is intended to restore, correct or modify physiological functions through pharmacological, immunological or metabolic action.

Claims involving fat metabolism, obesity treatment, glucose control, hormone replacement or physiological body-composition change are strongly medicinal in character.

“Research Use Only” Wording

A research disclaimer does not determine legal status by itself.

The MHRA may assess names, website text, before-and-after imagery, testimonials, administration material, body-transformation claims and intended customer use.

A product sold as a laboratory reagent while being promoted for personal injection may still attract medicinal-product enforcement.

Advertising

Regulation 279 of the Human Medicines Regulations restricts advertising medicinal products without the required authorisation, registration or certificate.

This section provides general regulatory education rather than legal advice.

FDA assessed AOD-9604 free base and AOD-9604 acetate for possible inclusion on the US section 503A bulk-drug-substances list.

FDA concluded that the available balance of characterisation, effectiveness and safety information weighed against inclusion.

The agency described deficiencies concerning peptide impurities, aggregates, microbial controls, bacterial endotoxin and product characterisation.

It noted that peptides containing disulphide bonds can degrade through reduction, dimer formation and aggregation.

FDA found insufficient evidence of effectiveness for obesity and highlighted the failed larger 12-week study.

FDA also considered long-term safety information inadequate.

In December 2024, the Pharmacy Compounding Advisory Committee voted twelve to zero against placing AOD-9604 free base and acetate on the list.

FDA’s current safety-risk information continues to identify potential immunogenicity, peptide-impurity and characterisation concerns for compounded AOD-9604 products.

WADA section S2 covers peptide hormones, growth factors, related substances and mimetics.

The growth-hormone section prohibits growth hormone, its analogues and its fragments.

The published examples include both AOD-9604 and hGH 176-191.

The prohibition applies at all times, including during training and outside competition.

A “Research Use Only” label does not alter an athlete’s anti-doping responsibility.

Athletes are subject to strict liability for prohibited substances detected in their samples.

Anti-doping laboratories can investigate parent peptides and characteristic metabolites using mass-spectrometric methods.

HGH Fragment 176-191 appears in Peptides UK catalogues as lyophilised powder, research vials and blends with growth-hormone secretagogues.

Common claims include targeted fat loss, increased lipolysis, reduced lipogenesis, preservation of muscle, improved recovery and avoidance of every growth-hormone adverse effect.

Some product pages use native-fragment and AOD-9604 names interchangeably.

Others display the native FLR sequence while citing human studies conducted with Tyr-starting AOD-9604.

Some certificates identify only “HGH Frag” without a complete amino-acid sequence.

Blended products create further uncertainty because each active ingredient, quantity and degradation profile must be measured independently.

No clinical finding authenticates an online vial without batch-specific chemical and pharmaceutical testing.

• Is the product native HGH Fragment 176-191 or AOD-9604?
• Does the sequence begin with phenylalanine or tyrosine?
• Is the complete 16-residue sequence stated?
• Is hGH 177-191 distinguished from hGH 176-191?
• Is AOD-9401 distinguished from AOD-9604?
• Is the C-terminal free acid confirmed?
• Is the Cys7-to-Cys14 disulphide bond mapped?
• Was intact molecular mass measured?
• Was tandem-MS sequence evidence provided?
• Were reduced peptide and disulphide dimers quantified?
• Were aggregates and particles measured?
• Was net peptide quantity measured?
• Was the counterion identified?
• Were microbial limits tested?
• Was bacterial endotoxin tested?
• Was finished-product sterility demonstrated?
• Does the certificate match the supplied batch?
• Was the study conducted with the exact sequence?
• Was the research performed in cells, rats, mice or humans?
• Was the outcome lipogenesis, lipolysis or body weight?
• Was actual human fat mass measured?
• Was the study randomised and placebo controlled?
• How many participants completed the study?
• Was the primary endpoint met?
• Is AOD-9604 evidence being attributed to native HGH Frag?
• Is full-growth-hormone evidence being attributed to the fragment?
• Are early glucose and insulin findings acknowledged?
• Is animal beta-3-receptor evidence being presented as proven human action?
• Is a cell-culture cancer study being described as human treatment?
• Is absence of evidence being presented as proof of safety?
• Is an authorised somatropin product being used to imply fragment approval?
• Is WADA’s express prohibition acknowledged?
• Is the source selling the product it describes?

The reviewers and contributors are identified to provide clear authorship and accountability. Their inclusion does not represent endorsement of any research product, supplier, personal use, treatment claim or commercial statement discussed in this article.

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