HGH Fragment 176-191

What it is

Human growth hormone consists of 191 amino acids. Via the liver it stimulates the production of IGF-1 and thereby drives growth, but it also has direct effects on fat tissue: it promotes fat breakdown and slows fat storage. Unfortunately, in larger amounts it also brings problems, above all poorer insulin action. A working group around the biochemist F. M. Ng therefore searched for decades for the part of the molecule that alone is responsible for fat metabolism.

They found it at the end of the chain. The section 177-191 inhibited fat synthesis in rat fat tissue about as well as the whole hormone. Two development names emerged from this: AOD9401 for the unmodified piece 177-191 and AOD9604 for the same sequence with an additional tyrosine at the start, which is meant to stabilize the peptide. AOD9604 therefore consists of 16 amino acids.

Why the name is confusing

In natural growth hormone, position 176 is occupied by a phenylalanine, according to the protein database UniProt. The true fragment 176-191 therefore begins with phenylalanine. AOD9604 begins at the same position with tyrosine, because the additional tyrosine was placed there in front of position 177. The two peptides thus differ only in their first building block.

A 2026 review notes that AOD9604 is therefore often also referred to as hGH 176-191. According to this classification, what is sold as HGH Fragment 176-191 is in many cases the AOD9604 sequence. WADA and the German Anti-Doping Act nevertheless list both names separately. For the assessment this means: the human data on AOD9604 apply to a product only if that sequence is really in it. This cannot be verified without lab analysis.

How it is supposed to work

In animals, the fragment acts directly on fat tissue. In rat fat it activated hormone-sensitive lipase, the enzyme that releases stored fat, and inhibited acetyl-CoA carboxylase, a key step in fat synthesis. In mice, fat oxidation rose under AOD9604, and the amount of the beta-3 adrenoceptor in fat increased, a receptor that mediates fat breakdown. The receptor itself, however, is not the target: in mice lacking this receptor, energy expenditure and fat oxidation still rose acutely.

The decisive difference from the whole hormone: in the lab, AOD9604 does not bind to the growth hormone receptor and does not trigger cell division there. That is why IGF-1 does not rise, and that was exactly the selling point. Which receptor mediates the effect instead has not been clarified to this day.

What users report

In the fitness and bodybuilding scene, the fragment is regarded as a fat-loss peptide that does not increase hunger and leaves blood sugar alone. A critical 2026 review explicitly lists it among the peptides advertised for fat metabolism and body composition. User reports are uncontrolled and leave open which molecule was in the vial. They are taken seriously here as a picture of user experience, but they are not proof of efficacy.

What the studies show

Ng and Bornstein 1978 — the true fragment

The only paper found that tested the natural piece 176-191. Normal rats received a single injection of several synthetic end pieces of growth hormone. The peptides 172-191, 176-191, 177-191 and 178-191 triggered a brief rise in blood sugar and a longer-lasting rise in insulin; shorter pieces had no effect. The direction was thus blood-sugar-raising, not fat-burning. Fat metabolism and weight were not measured.

Ng 2000 — fat cells get smaller

Obese Zucker rats received the unmodified piece 177-191 for 20 days. Weight gain was lower, mean fat cell diameter fell from 110 to 80 micrometers, and unlike under growth hormone, no insulin resistance occurred. In a parallel paper with the tyrosine variant, Zucker rats gained 15.8 instead of 35.6 grams over 19 days, that is, more than 50 percent less.

Heffernan 2000 — oral and in human fat

Ob/ob mice received the fragment by mouth for 30 days, 10 animals versus 8 controls. From day 16 they gained significantly less weight, while food intake stayed the same. Energy expenditure and fat oxidation rose acutely. In addition, the peptide promoted lipolysis and inhibited lipogenesis in isolated fat tissue from rodents and humans.

Stier 2013 — six studies in humans

The summary of the clinical development of AOD9604: two studies with a single infusion, four oral studies, including one over 12 weeks with 300 and one over 24 weeks with 502 randomized participants. The adverse event profile was similar to that of placebo; headache was the most common. The paper reports only safety, not efficacy, and comes from the circle of the rights holder.

Where the data stop

For the true fragment with phenylalanine there is exactly one animal study, and it shows a brief rise in blood sugar and insulin. Studies on fat loss or body composition with this sequence are entirely lacking, as are human data. The positive animal data come from the piece 177-191 and from the tyrosine variant. Why the same working group earlier found acute blood-sugar-raising effects and later, with continuous administration, metabolically neutral effects is not explained in the abstracts.

On efficacy in humans, the data stop early. According to the manufacturer, the 24-week study with 534 enrolled participants missed its weight endpoint in 2007, and the program was discontinued. The efficacy results were never published in peer-reviewed form. It was tested as a tablet; injected, the peptide was tested only in single doses for safety. There are no human data on abdominal fat, muscle preservation, performance or joints; the cartilage data come from an experiment with 32 rabbits. There is no entry in the ClinicalTrials.gov study registry.

Status, approval and legal

HGH Fragment 176-191 is not an approved medicine in Germany, the EU or the US and is sold as a research chemical. The WADA Prohibited List 2026 explicitly names AOD-9604 and hGH 176-191 under S2.2.3, prohibited in and out of competition. The German Anti-Doping Act likewise lists both names in its annex among the growth hormone fragments. The US Food and Drug Administration (FDA) had classified AOD-9604 as a compounding substance with possible significant risks; the nomination was withdrawn, but the assessment remains on the page as of April 22, 2026.

Safety

For the AOD9604 sequence, the data are unusually good for a research peptide: around 900 people in controlled studies, up to 24 weeks, side effects at placebo level, no rise in IGF-1, no worsening of blood sugar. After infusions, 5 of 23 volunteers reported mild euphoria, none under placebo. The FDA names as open issues the risk of immune reactions against the peptide, impurities and serious events of unclear cause. For the true fragment with phenylalanine, safety data in humans are lacking, and in animals it briefly raised blood sugar and insulin. For people with diabetes, pregnant and breastfeeding women, adolescents and people with a history of cancer there is no data basis. Gray-market products are often mislabeled.

Sources

• Ng and Bornstein, Am J Physiol 1978 (natural fragment 176-191 in rats)
• Ng et al., J Mol Endocrinol 2000 (AOD9401, Zucker rats)
• Heffernan et al., Am J Physiol Endocrinol Metab 2000 (oral, ob/ob mice, human fat tissue)
• Ng et al., Horm Res 2000 (AOD9604 oral, Zucker rats)
• Heffernan et al., Int J Obes 2001 (no GH receptor binding)
• Stier et al., J Endocrinol Metab 2013 (safety in humans, six studies)
• Cox et al., Drug Test Anal 2015 (structure and detection of AOD9604)
• Dominikowski et al., Front Endocrinol 2026 (review of peptides of the GH–IGF-1 axis)
• WADA, Prohibited List 2026, section S2.2.3
• FDA, bulk drug substances with possible safety risks