Provitamin A carotenoids

• Carotenoids are solubilized in micelles in the intestinal lumen and absorbed by passive diffusion in the enterocytes. Approximately 5 g fat is necessary for absorption.
• Absorption: 10-20 %, decreases with higher intake.
• β-carotene is reformed to vitamin A when cleaved by the enzyme 15-15'-β-carotenedioxygenase.

• 2 µg β-carotene in oil -> 1 µg retinol activity equivalent (RAE)

• 6 µg β-carotene from foods -> 1 µg b-carotene in oil
• That is: 12 µg β-carotene from foods -> 1 RAE
• 24 µg mixed carotenoids -> 1 RAE

Previously retinol equivalents were used, with different conversion factors for carotenoids: 6 µg β-carotene or 12 µg mixed carotenoids corresponded to 1 µg retinol = 1 retinol equivalent (RE)

A normal serum level can occur alongside almost empty liver stores.