What it means
Insulin-like growth factor 1 is a peptide made mainly by the liver in response to growth hormone, and it carries out most of GH's anabolic effects: muscle protein synthesis via mTOR, bone growth, cell proliferation. It is a stable, once-a-day measure of GH activity, which is why it is the lab test for the GH axis rather than GH itself (which pulses).
Levels peak in adolescence and fall about 15% per decade; protein intake, insulin, sleep and training raise them, fasting and calorie restriction lower them.
Why it matters for longevity
Reduced IGF-1 signalling extends lifespan in every lab organism, and people with rare IGF-1 deficiency (Laron syndrome) almost never get cancer or diabetes. Yet low IGF-1 in ordinary older adults predicts frailty, sarcopenia and cardiovascular death. Both are true, which is why the relationship is U-shaped and the practical question is where on the curve to sit.
What the evidence says
Burgers 2011 meta-analysis (12 cohorts) and the EPIC and UK Biobank data show a U-shaped curve: all-cause mortality lowest in the middle of the range and higher at both extremes. High IGF-1 is associated with breast, prostate and colorectal cancer (pooled relative risk around 1.2–1.5 for top versus bottom quintile). Low IGF-1 is associated with heart failure, frailty and ischaemic stroke.
Interventions: protein intake above 1.2 g/kg raises IGF-1 by 10–20% (Levine 2014 argued for lower protein in mid-life on this basis, with higher intake after 65); fasting lowers it within days; secretagogues raise it within weeks. No trial has targeted IGF-1 for a longevity outcome.
How to measure or use it
- A standard blood test, any time of day, no fasting needed; reported with an age-adjusted range.
- A commonly used working target is the middle of the age-adjusted range — roughly 150–250 ng/mL for adults over 40 — rising toward the upper third if muscle or bone is the priority after 60.
- Repeat at 6–8 weeks after any change to protein, fasting or a GH-axis compound; above-range readings are a stop signal.
Related
- Hormones after 40 — the GH/IGF-1 axis in the longevity picture
- CJC-1295 / ipamorelin (research page) — where IGF-1 is the control marker
- Muscle mass and ageing — the anabolic side of the trade-off
Frequently asked questions
What is a good IGF-1 level?
Mid-range for your age is the usual target — roughly 150–250 ng/mL over 40. Both extremes carry risk; the curve is U-shaped.
Does protein raise IGF-1?
Yes, by about 10–20% at intakes above 1.2 g/kg. After 65 that is probably protective; in mid-life, opinions differ.
Is high IGF-1 dangerous?
Persistently high levels are associated with several cancers and higher mortality. Anything that raises IGF-1 should have a ceiling and a retest.
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