HGH Fragment 176-191: growth hormone's tail and its tangled name

The tail end of a famous hormone

Human growth hormone is a single chain of 191 amino acids ^[1]. From 1978 to 2000, papers co-written by biochemist Frank Ng tested synthetic pieces of its far end to see which of the hormone's many jobs a piece could do on its own ^[2,5].

The answer they eventually championed was fat metabolism. But the story did not start there.

The earliest study we could find that tested synthetic hGH 176–191 was not looking for fat loss. In rats, it found a brief rise in blood sugar ^[2].

Today "Fragment 176-191" is discussed almost entirely as a fat-metabolism peptide. Most of the evidence behind that reputation was gathered on two close relatives, so knowing exactly which molecule is which is the key to reading the literature.

Origins: chopping up growth hormone

In 1978, Ng and Bornstein tested six synthetic tail fragments in normal rats: 172–191, 176–191, 177–191, 178–191, 179–191 and 180–191. Four of them, including 176–191, caused a short-lived rise in blood glucose and a longer rise in insulin; the two shortest did nothing ^[2]. Peptides containing residues 178–191 also reduced the animals' sensitivity to insulin ^[2].

A 1982 follow-up, using chemically modified versions of 177–191, located this insulin-opposing core within residues 178–190 ^[3].

The fat story began in 1993. Synthetic 177–191 blocked fat synthesis in rat fat tissue as effectively as the intact hormone, although it did not increase fat breakdown ^[4]. By 2000, under the code name AOD9401, the same 15-residue peptide stimulated the fat-releasing enzyme hormone-sensitive lipase, slowed weight gain in obese Zucker rats over 20 days, and shrank their fat cells from about 110 to 80 µm across, without causing insulin resistance ^[5]. Alongside it, the developers made a version with an extra tyrosine at the front: AOD-9604 ^[7,10].

One tail, three names

Tyrosine is phenylalanine with one extra hydroxyl group. So the natural fragment (about 1799 Da) and AOD-9604 (about 1815 Da) differ by a single oxygen atom. Both carry the same disulfide bond between their two cysteines, which in the full hormone links Cys182 and Cys189 ^[1].

The confusion is in print. A 2022 cancer-cell study modelled "hGH fragment 176-191" using the tyrosine sequence ^[12], and a 2026 review in Frontiers in Endocrinology describes AOD9604 as "hGH 176-191" ^[13]. Even the developers' 2000 structure paper used the label "AOD" for the 15-residue 177–191 peptide ^[6].

The practical upshot: the sequence and measured mass on a certificate of analysis tell you which molecule is in the vial. Our guide to reading a COA shows where to look, and our AOD-9604 article covers the tyrosine version's full history.

How it is thought to work: one blade from a Swiss Army knife

Growth hormone is like a Swiss Army knife, with separate blades for growth, blood sugar and fat. The developers' idea was that the tail might be the fat blade, still working after it is snapped off. They argued that growth hormone partly behaves as a "pro-hormone" whose fragments have actions of their own ^[8].

The supporting data come from the relatives. AOD-9604 did not compete with growth hormone for its receptor and did not drive receptor-dependent cell growth ^[8]. In obese mice it raised expression of the β3-adrenergic receptor, a fat-cell receptor that triggers fat release, and its long-term weight effect disappeared in mice lacking that receptor ^[9]. The 177–191 peptide also inhibited acetyl-CoA carboxylase, the enzyme that commits carbon to fat synthesis ^[5]. NMR work showed the 177–191 ring folds into turns that partly resemble the same region inside the intact hormone ^[6].

None of these mechanism studies used the natural 176–191 sequence, and the FDA's 2024 review concluded that AOD-9604's molecular target remains unknown ^[11].

What the research shows

The natural 176–191 sequence (rats)

The only direct study is the 1978 rat experiment above: a short-lived rise in blood glucose and a longer rise in insulin ^[2]. We found no published study of its effect on body fat.

hGH 177–191 (rats)

It blocked fat synthesis in rat fat tissue ^[4] and, in obese Zucker rats, slowed weight gain and shrank fat cells without causing insulin resistance ^[5].

AOD-9604 (rodents, then people)

• Obese Zucker rats: given orally for 19 days, weight gain was 15.8 g versus 35.6 g in controls, with insulin sensitivity preserved ^[7].
• Obese mice: less weight gain and more fat oxidation, without the rise in blood sugar seen with growth hormone ^[8].
• People: six trials in about 900 adults showed a placebo-like safety profile and no rise in IGF-1 ^[10]. The decisive 24-week OPTIONS trial (536 enrolled) found no significant weight loss versus placebo, and obesity development ended in February 2007 ^[11].

A recent "176-191" paper

In 2022, researchers loaded chitosan nanoparticles with the chemotherapy drug doxorubicin plus "hGH fragment 176-191", using the tyrosine sequence for their computer modelling. In a dish of MCF-7 breast-cancer cells, the dual-loaded particles suppressed cell growth more than doxorubicin particles alone ^[12]. It says nothing about fat metabolism.

What we don't know yet

• The natural fragment's effect on fat. Untested in published work.
• Whether phenylalanine versus tyrosine matters. We found no head-to-head comparison.
• Blood sugar. Early studies found that tail fragments raised blood glucose and opposed insulin in rats ^[2,3]; later AOD-9604 work found no such effect in rodents or people ^[7,8,10]. The peptides and methods differed, so the question is open for the natural sequence.
• Human data. Only AOD-9604 has been tested in people, and it failed as a weight-loss drug ^[11].
• Mechanism and safety. No confirmed target ^[11], and no human safety data at all for the natural fragment.

Our guide to levels of evidence explains how to weigh animal and human data; evidence borrowed from a related molecule sits lower still.

Regulatory status

• Approval: not approved as a medicine by the FDA, the EMA or any other regulator, and no human trial is registered under this name on ClinicalTrials.gov (our check, October 2026).
• US compounding: the FDA's 2024 compounding evaluation covered AOD-9604 ^[11]; we found no separate FDA review of the natural fragment.
• Sport: WADA added "growth hormone fragments" to its list for 2018, naming AOD-9604 and hGH 176-191 as examples ^[14]. Both remain listed under S2.2.3 of the 2026 list and are prohibited at all times ^[15].

In the lab: handling and storage

The HGH Fragment 176-191 research peptide is supplied as a lyophilised powder. Long-term storage is usually at −20 °C, with 2–8 °C acceptable for short periods, protected from light and moisture. Its ring depends on the disulfide bond, so reducing agents will open it and change the molecule. Once reconstituted, solutions are usually kept cold and split into single-use aliquots to avoid repeated freeze–thaw cycles. Mass spectrometry separates the two common versions at a glance: about 1799 Da for the natural sequence, 1815 Da for the tyrosine form.

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