A unit of measurement for the amount of a substance (e.g., vitamin), based on measured biological activity or effect.

Vitamin A 

1 IU is the biological equivalent of 0.3 micrograms (mcg) retinol, or of 0.6 micrograms beta-carotene; 1 nanogram (ng)/ml = 3.5 nanomol (nmol)/l.
 
Vitamin C
1 IU is 50 micrograms (mcg) L-ascorbic acid; 1 ng/ml = 5.5 nmol/l.
  
Vitamin D
1 IU is the biological equivalent of 0.025 micrograms (mcg) cholecalciferol/ergocalciferol; 1 ng/ml = 2.5 nmol/l.

Vitamin E
1 IU is the biological equivalent of about 0.667 mg d-alpha-tocopherol, or of 1 mg of dl-alpha-tocopherol acetate; 1 ng/ml = 2.78 nmol/l.