IGF-1

Insulin-like growth factor 1 (IGF-1) is the downstream mediator of most growth hormone action and the most stable serum readout of growth-hormone-axis activity. Unlike growth hormone itself, which pulses wildly across the day, IGF-1 stays relatively flat and reflects integrated GH output over the preceding 24 hours. It carries a U-shaped relationship with mortality and cancer risk that makes it one of the more interesting and most-debated longevity biomarkers.

Optimal range

Range varies by individual.

Test frequency

Annually as part of a comprehensive hormonal and metabolic panel; every six to eight weeks during any GH-axis intervention or major training/nutrition change.

When to measure

Measure as part of a comprehensive hormonal panel at intake to any optimization protocol, especially if you train heavily, eat at the lower end of caloric needs, or are considering peptide therapy. Re-measure annually if values are within range, or six to eight weeks after any change in protein intake, sleep duration, training volume, or supplementation that targets the GH/IGF-1 axis. Avoid measuring within a week of a major illness or surgery, which transiently suppresses values.

How to measure

Standard serum immunoassay or LC-MS, LC-MS is preferred for accuracy, particularly at low values. Cost is $50–$150 retail. No fasting required; values are relatively diurnal-stable. Always interpret against age-adjusted reference ranges, IGF-1 of 180 ng/mL is normal for a 50-year-old, low for a 25-year-old, and high for an 80-year-old. Most labs report age- and sex-adjusted Z-scores alongside the raw value.

Why this biomarker matters

IGF-1 supports muscle protein synthesis, bone density, neurogenesis, and lean mass maintenance, all things you want to preserve into the seventh and eighth decades. Frankly low IGF-1 in older adults associates with sarcopenia, increased frailty, fall risk, and higher all-cause mortality. Adult-onset growth hormone deficiency with low IGF-1 is treated clinically because the morbidity is substantial. The complication is the upper end. Population studies (the laron and lung-cancer cohorts, the GH/IGF-1 axis literature) show that very high IGF-1 across midlife associates with higher cancer incidence, particularly breast, prostate, and colorectal. Mendelian randomization analyses are consistent with a causal role. The implication is that optimal IGF-1 sits in the middle of the age-adjusted reference range, neither suppressed nor maximized. Athletes and bodybuilders using exogenous GH or peptide secretagogues to push IGF-1 toward the upper limit are running an experiment with uncertain long-term consequences.

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