Bioavailability of β-Carotene in Humans

Abstract

Naturally occurring carotenoids have attracted much attention owing to their potential usefulness in cancer prevention. However, little is known about the absorption and metabolism of β-carotene, especially 9-cis β-carotene. We investigated stereoisomers of β-carotene in plasma and blood cells of healthy male volunteers after ingesting daily either 60 mg of synthesized all-trans β-carotene preparation, a naturally occurring β-carotene derived from Dunaliella bardawil, or placebo for 44 weeks. Basal levels of 9-cis β-carotene in plasma, platelets, and mononuclear cells were 10%, 20%, and 25% of those of the all-trans form, respectively. The plasma levels reached a maximum after 2 weeks of administration and plateaued thereafter in the subjects who took the β-carotene preparations. The all-trans β-carotene level in the subjects given the synthesized all-trans form was almost twice that for the Dunaliella preparation. The plasma 9-cis level was found to be higher in the all-trans β-carotene group than in the Dunaliella group, despite no intake of the 9-cis form in the all-trans group and the higher intake of the 9-cis form in the Dunaliella group. This finding suggests that isomerization of the all-trans form to the 9-cis form may occur in the body either during or after absorption. The CD4/CD8 ratio increased after 9 months of all-trans β-carotene administration, whereas natural killer cells, virgin T cells, memory T cells, and cytotoxic T cells remained unaltered throughout the study.