Carotenoid and retinoid metabolism: insights from isotope studies

Use of isotopes as tracers has had an important role in elucidating key features of vitamin A and retinoid metabolism in animal models and humans. Their use has shown that β-carotene absorption is variable, and that the appearance of β-carotene and its metabolites in the blood by time since dosing follows characteristic patterns. Retinol formed from β-carotene shows a different pattern, as does lutein. In this article, we summarize and discuss insights and some surprises into the absorption and metabolism of vitamin A, β-carotene, and lutein that were gained with the use of isotope tracers in humans, rats, and cells as models.