IGF-1, or insulin-like growth factor 1, is a peptide hormone made mainly by the liver under the direction of growth hormone, and it carries out most of growth hormone’s building work on bone, muscle and other tissue. It is named for its resemblance to insulin, and it signals into the same pro-growth machinery that Insulin uses. Growth hormone comes in short nocturnal pulses, so one measurement says little; IGF-1 holds steady across the day, which makes it the practical marker for how much growth signal a body is running on. The reference entry on IGF-1 covers the clinical picture in more depth.
What it measures
IGF-1 is measured in serum from an ordinary blood draw, usually by immunoassay, and reported in nanograms per milliliter or nanomoles per liter. Fasting is generally not required, and the level does not swing with meals or time of day the way glucose does.
The number reported is total IGF-1. Almost all of it circulates bound to carrier proteins, chiefly IGF-binding protein 3, which stretches its half-life from minutes to hours; only a small free fraction is active. Clinically the test screens for growth hormone excess and deficiency, folding a day of pulsatile secretion into one stable value. It does not capture IGF-1 made locally inside a tissue, such as a muscle you have just trained.
Typical values
There is no single adult normal range. IGF-1 peaks in puberty, falls steeply through the twenties and declines slowly thereafter, so a result is read against an interval for that age and sex. Assays differ enough that values are not interchangeable between laboratories, which is why many reports add a standard deviation score. Use the range on your own report.
| Where the result sits | What it usually reflects |
|---|---|
| Above the range for age and sex | Excess growth hormone, most importantly acromegaly, or injected growth hormone. Followed up, not acted on alone. |
| Within the range for age and sex | Normal output. Position inside the range is not a fitness score. |
| Below the range for age and sex | Growth hormone deficiency or pituitary disease, but more often low protein or calorie intake, liver disease, uncontrolled diabetes, hypothyroidism or acute illness. |
Why it matters for longevity
This axis sits inside nutrient sensing, a hallmark of aging, and feeds the growth pathway governed by mTOR. In animals the link is consistent: worms, flies and mice with reduced insulin and IGF-1 signaling live longer, and dwarf mice that cannot respond to growth hormone are among the longest-lived rodents studied. People with Laron syndrome, an inherited inability to respond to growth hormone, have very low IGF-1 and are reported to be strikingly protected from cancer and type 2 diabetes, though their lifespan is not clearly extended.
In humans the evidence is observational and messier. Cohorts tend to find a U-shaped relationship, with both high and low IGF-1 associated with higher all-cause mortality rather than a simple “lower is better” line. Higher levels have been associated with modestly higher risk of some cancers; lower levels in older adults with frailty and muscle loss. No trial has shown that deliberately lowering IGF-1 extends human life.
What changes it
Age is the largest influence and you do not control it. After that, food matters most. Protein intake and total calories hold IGF-1 up; long-term calorie restrictors studied in humans had lower IGF-1 mainly when protein intake fell too, not from cutting calories alone. Prolonged fasting lowers it temporarily. Because the liver makes most of it, liver disease pulls it down, as do hypothyroidism, uncontrolled diabetes and undernutrition.
Exercise splits into two answers. Resistance training raises IGF-1 produced locally in the worked muscle; effects on the circulating level are small and inconsistent. Injected growth hormone raises it sharply, which is why anti-aging clinics measure it and why off-label use carries real risks: joint pain, insulin resistance, fluid retention. A result far outside the range for your age deserves a clinician’s attention; one inside it is not a target to move.
Related reading
- Which Blood Tests Reflect Aging and How to Read Them
- Caloric Restriction and Lifespan
- How Much Protein You Need After 40 to Keep Muscle
- Sarcopenia
Disclaimer. This article is for information only and does not replace medical advice. Talk to a qualified clinician before changing anything about your health.