What IGF-1 is
Insulin-like growth factor 1 is a hormone produced primarily by the liver in response to growth hormone (GH) signaling. It mediates many of GH's anabolic effects on tissue growth, body composition, and metabolism.
Why IGF-1 vs GH
- GH is pulsatile (4-5 pulses overnight), single GH measurement is unreliable
- IGF-1 is more stable in blood, gives integrated picture of GH activity
- IGF-1 is the practical clinical marker
Optimal ranges by age
IGF-1 declines with age. Reference ranges adjust for age:
- Age 20-30: 200-400 ng/mL
- Age 30-40: 180-300 ng/mL
- Age 40-50: 140-260 ng/mL
- Age 50-60: 120-220 ng/mL
- Age 60-70: 100-200 ng/mL
Optimal target: upper half of age-adjusted range.
What affects IGF-1
Raising:
- Adequate sleep (deep sleep specifically)
- Resistance training
- Adequate protein intake
- Adequate calorie intake (low-calorie suppresses)
- Insulin status
- GH supplementation (if indicated)
Lowering:
- Caloric restriction / extended fasting
- Sleep deprivation
- Aging
- Liver disease
- Certain medications
High vs low concerns
Low IGF-1 in older adults:
- Associated with frailty
- Reduced lean mass
- Slower recovery
- Increased fracture risk
Very high IGF-1 (above age-adjusted ranges):
- Some studies suggest cancer growth signaling concern
- Acromegaly (rare; from pituitary tumor)
- Excessive GH supplementation
Natural optimization
Most patients can optimize IGF-1 through:
- Quality sleep (most powerful)
- Resistance training 2-3x/week
- Adequate protein
- Avoid extreme caloric restriction
- Adequate sleep architecture
GH supplementation is reserved for confirmed deficiency cases.
The clinical pearl: IGF-1 is the practical GH marker. Optimization through sleep and training is preferable to direct supplementation for most patients. Track baseline and follow-up.
Bottom line
IGF-1 reflects growth hormone activity. Age-adjusted optimal ranges. Optimization through sleep, training, and protein. Very high or very low both have implications.