Wednesday, August, 26th at 8:57 AM

Why are Vitamins A, E and D measured in IU rather than grams, mg, or mcg?

Some vitamins – specifically A, E and D – are expressed as a unit of biological activity (IU, International Units), rather than as weight such as grams, milligrams or micrograms. Occasionally, it is helpful to know the weight of a nutrient expressed this way.

Here are some of the common conversions:

Vitamin A
1 IU = 0.3 mcg all-trans retinal = 0.3 mcg retinol = 0.344 mcg retinyl acetate = 0.55 mcg retinyl palmitate = 3.6 mcg Beta-Carotene
1 mcg Retinol = 3.34 IU of vitamin A activity
1 mg of all-trans Beta-Carotene = 1667 IU of Vitamin A activity
1 mcg Beta-Carotene = 1.67 IU of Vitamin A activity
1 mcg dietary Beta-Carotene = 0.167 mcg retinal

Retinol Equivalents (RE) = the Vitamin A activity in foods 1 RE = 1 mcg all-trans retinal = 1 mcg retinal = 3.33 IU Retinol 1 RE = 6 mcg all-trans Beta-Carotene = 6 mcg Beta-Carotene 1 RE = 12 mcg other provitamin A carotenoids

Vitamin E
1 IU = 0.67 mg of d-alpha-tocopherol or 0.45 dl-alpha-tocopherol
1 mg = 1.49 IU d-alpha-tocopherol (natural vitamin E; RRR-alpha-tocopherol)
1 mg = 1.10 dl-alpha-tocopherol (synthetic vitamin E; all-rac-alpha-tocopherol)

Vitamin D
1 IU = 0.025 mcg of cholecalciferol (Vitamin D3)
1 mcg Vitamin D (cholecalciferol) = 40 IU