What IGF-1 is
Insulin-like growth factor 1 (IGF-1) is a polypeptide hormone produced primarily by the liver in response to growth hormone (GH) signalling via the JAK-STAT pathway. It mediates most of GH's anabolic effects — stimulating protein synthesis, promoting cell proliferation and differentiation, inhibiting apoptosis, and supporting bone and muscle growth.
IGF-1 has a plasma half-life of 15–20 hours, reflecting integrated GH secretion over the preceding day. This makes it a stable and practical marker for GH axis activity.
Why IGF-1 is the better test for GH status
Growth hormone itself is pulsatile — it is secreted in 6–8 pulses per day, primarily during sleep, with up to 50-fold variation in serum levels throughout the day. A single random GH measurement is essentially meaningless for assessing GH status. By contrast, IGF-1 reflects the integrated GH exposure over 24 hours and is a reliable proxy for GH axis function.
IGF-1 is the appropriate test to order when evaluating growth hormone status — not GH itself.
The longevity paradox
IGF-1 sits at the heart of one of the most fascinating debates in longevity biology:
- Animal models: Reducing IGF-1/GH signalling consistently extends lifespan. Ames dwarf mice with GH receptor knockout live 55% longer. Worms with reduced IGF-1 signalling (daf-2 mutations) live twice as long. In every major model organism, lower insulin/IGF-1 signalling pathway activity is associated with extended lifespan.
- Human observational data: Very high IGF-1 is associated with elevated cancer risk (breast, prostate, colorectal). Very low IGF-1 is associated with sarcopenia, frailty, and increased metabolic disease risk.
- Current consensus: The relationship in humans appears J-shaped — both extremes are unfavourable. Mid-range IGF-1 — preserving muscle and metabolic function without excessive anabolic drive — appears optimal for healthspan and lifespan.
Higher IGF-1 promotes muscle growth, bone density, and metabolic efficiency — important for healthspan. But the insulin/IGF-1 signalling pathway (IIS) drives growth at the expense of longevity pathways (mTOR activation vs autophagy). The balance differs by age: in youth, higher IGF-1 supports development; in mid-life and beyond, moderating it may extend longevity.
IGF-1 reference ranges by age
| Age Group | Lab Reference Range (ng/mL) | Longevity Target | Notes |
|---|---|---|---|
| 20–30 years | 182–780 ng/mL (broad range) | 180–260 ng/mL | High natural IGF-1 in youth; mid-range optimal |
| 30–45 years | 115–307 ng/mL | 150–220 ng/mL | Primary longevity target range for this demographic |
| 45–60 years | 94–252 ng/mL | 130–200 ng/mL | Preserve muscle while not driving cancer-associated IGF-1 elevation |
| 60+ years | 67–205 ng/mL | Upper half of age-range (120–180 ng/mL) | Low IGF-1 in elderly = frailty risk; maintain higher end of age range |
What moves IGF-1
Raises IGF-1:
- Protein intake — the strongest dietary driver; above 1.2g/kg/day supports liver IGF-1 production; high animal protein particularly effective
- Resistance training — stimulates GH pulsatility; chronic training raises basal IGF-1
- Sleep quality — 70% of GH release occurs during slow-wave sleep; poor sleep dramatically reduces IGF-1
- GH-stimulating peptides — sermorelin, ipamorelin, CJC-1295; these are prescription items and not for unsupervised use
Lowers IGF-1:
- Caloric restriction — fasting and caloric restriction consistently lower IGF-1 by reducing hepatic production
- Plant-based diet — lower animal protein reduces IGF-1 production; one proposed mechanism for longevity benefits of plant-based diets
- Insulin resistance — paradoxically, insulin resistance can lower IGF-1 despite high insulin, by reducing hepatic GH receptor sensitivity
How often to test
Annually as part of a metabolic and hormonal panel for those over 35, or if using any GH-axis interventions. Always interpret in the context of age-specific reference ranges and current nutritional status (results may be artificially low during caloric restriction or fasting protocols).
Frequently asked questions
What is a good IGF-1 level for longevity?
Mid-range for age is the longevity-optimal position — roughly 150–220 ng/mL for ages 30–45. Very high IGF-1 is associated with cancer risk; very low with frailty. The J-shaped curve favours avoiding both extremes.
Does higher IGF-1 mean better health?
Not straightforwardly. Higher IGF-1 builds muscle and supports metabolism — good for healthspan. But excess IGF-1 signalling inhibits longevity pathways (autophagy, stress resistance). The animal model evidence consistently shows lower IIS signalling extends lifespan. Mid-range optimisation is the current consensus.
How do I increase IGF-1 naturally?
Protein intake above 1.2g/kg/day, resistance training, quality sleep (maximising slow-wave sleep), and zinc/magnesium adequacy. These are the natural levers without pharmaceutical or peptide intervention.
Is there a connection between IGF-1 and cancer risk?
Observational associations exist between elevated IGF-1 and breast, prostate, and colorectal cancer risk. The mechanism is plausible — IGF-1 promotes proliferation and inhibits apoptosis. Causality not definitively established. Sufficient reason to avoid aggressively maximising IGF-1 without clear clinical indication.