Fibroblast growth factor 21
FGF-21
Trial Metabolic hormone
What it is
FGF-21 is a metabolic hormone produced mainly by the liver, released in response to fasting and stress. Unlike most growth factors it acts systemically like a hormone rather than locally. It is of interest in metabolic disease because it improves insulin sensitivity and lipid handling in animal models, and several engineered analogs have entered human trials.
Benefits in plain language
What researchers and reference users usually mean when they talk about this compound. These are study themes and common claims — not a promise that it will do this in you.
- This is a fasting hormone — your liver releases it when you have not eaten, and it pushes the body toward fat burning.
- Drug companies have engineered long-acting versions for fatty liver disease and metabolic conditions.
- Trial results have been mixed: real biomarker improvements, but often modest clinical benefit.
- It is a metabolic compound, not a muscle or repair peptide; body-composition claims online are extrapolation.
If a branded, FDA-approved product exists, that label is the only dose that is actually authorized. Research-chemical vials are a different object.
Mechanism of action
FGF-21 signals through the FGFR1c / β-Klotho complex, activating downstream pathways that enhance insulin sensitivity, increase glucose uptake in adipose tissue, and shift lipid metabolism toward oxidation and away from lipogenesis. Its dependence on β-Klotho restricts action to specific tissues.
Possible side effects / caveats
Trials of analogs have reported injection-site reactions, transient changes in bone-turnover markers (a concern with the FGF19 subfamily), and variable efficacy. Native FGF-21 has a short half-life and is impractical as sold. Any research vial labeled FGF-21 is likely an analog or an uncharacterized preparation.
Protocol ranges
| Source tier | Amount | Frequency / duration | Notes |
|---|---|---|---|
| Trial | mg-level weekly or daily SC, analog-specific | Per trial protocol | Engineered analogs; supervised |
| Cited range | Community figures for research vials | — | No established human protocol for native protein |