Biomarker 101: Insulin-like growth factor 1 (IGF-1)

IGF-1 sits in the middle of an unresolved argument about growth, ageing and longevity — which makes it interesting, and makes overconfident interpretation of it unwise.

What IGF-1 is

Insulin-like growth factor 1 is a hormone produced mainly by the liver in response to growth hormone. Growth hormone and IGF-1 work as a pair: growth hormone sends the signal, and IGF-1 carries out much of the effect on bone, muscle and other tissue.

Why it's measured instead of growth hormone

Growth hormone is released in pulses and swings dramatically through the day, responding to sleep, food and exercise. A single measurement of it is close to uninterpretable.

IGF-1 is far more stable, so it serves as a practical proxy for average growth hormone activity. That stability is the entire reason it appears on panels at all.

How levels change across life

IGF-1 is low in early childhood, rises through childhood, peaks sharply during puberty, and then declines steadily through adult life. By later adulthood levels are a fraction of their adolescent peak.

That decline is a normal part of ageing rather than a condition, which matters when reading a result: a level appropriate for a fifty-year-old would be low for a teenager, and reference ranges are age-banded accordingly.

What it's used for clinically

The established uses are in assessing growth hormone disorders — either deficiency or excess — usually alongside dynamic testing that stimulates or suppresses growth hormone to see how the system responds. These are specialist investigations rather than something a single IGF-1 result settles.

IGF-1 and longevity research

This is where IGF-1 attracts attention, and where the evidence is genuinely contested.

In animal models, reduced growth hormone and IGF-1 signalling is associated with extended lifespan — a finding that is consistent enough across species to be taken seriously. In humans the picture is considerably murkier. Some observational research associates higher IGF-1 with better muscle mass and physical function; other research associates higher levels with increased risk of certain cancers. Lower levels have been linked with both favourable and unfavourable outcomes depending on the population studied.

No target range for longevity has been established, and there is no evidence that deliberately manipulating IGF-1 improves outcomes in healthy people. Anyone presenting an optimal IGF-1 level for healthy ageing is going beyond what the evidence supports.

What moves IGF-1

Age is the dominant factor. Beyond that, nutritional status affects it — sustained low energy intake lowers IGF-1 — as do liver conditions, since the liver produces it. Some hormonal conditions influence it in either direction.

What IGF-1 won't tell you

• It isn't a biological age marker. Despite how it is sometimes presented, no validated interpretation exists for that purpose.
• It doesn't have an optimal range for healthy people. Reference ranges describe what is typical for your age, nothing more.
• It doesn't replace growth hormone testing. Assessing the axis properly requires dynamic testing.
• The longevity evidence is unsettled. Animal findings have not translated into human recommendations.

The bottom line

IGF-1 is a stable proxy for growth hormone activity with well-established clinical uses and a genuinely unresolved role in ageing research. It is worth understanding, and not worth optimising toward a number nobody has established.