Three active forms of vitamin A: all-trans-retinol, -retinal, and –retinoic acid

There are three active forms of vitamin A: all-trans-retinol, -retinal, and –retinoic acid. Collectively, these are referred to as retinoids.

The vitamin can be either consumed in the diet as esterified retinol, or biosynthesized from b-carotene, a plant isoprenoid especially abundant in carrots.

b-carotene is cleaved in the intestine by a monooxygenase to form two molecules of all-trans-retinal (retinaldehyde), which are then reduced to retinol.

All-trans-retinol is the form which circulates in the blood and which has the highest biological activity.