Bioavailability of Dietary Carotenoids: Intestinal Absorption and Metabolism

Various carotenoids with diverse structures are present in foods and have been reported to have beneficial effects on human health. Owing to their hydrophobicity, however, poor solubilization in the aqueous milieu of the digestive tract restricts their intestinal absorption. Fats and oils were found to increase the solubilization of carotenoids into mixed-micelles, which would therefore enhance their bioavailability. The uptake of carotenoids solubilized in the micelles by intestinal cells are thought to be mediated by simple diffusion and/or facilitated diffusion through scavenger receptors. Lipids that constituted the mixed-micelles affected the uptake of carotenoids. In particular, lysophosphatidylcholine significantly enhanced uptake. Highly polar carotenoids, meanwhile accumulated in mice but not in humans, suggesting discriminate absorption and metabolism in the latter. The metabolism of provitamin A carotenoids is well known to be mediated by the central cleavage enzyme. Recently, another cleavage enzyme was found to cleave various carotenoids asymmetrically into apocarotenoids. Xanthophylls were found to be oxidized to unstable keto-carotenoids in mice. These metabolic conversions are thought to affect the levels of bioavailable carotenoids in tissues. In this article, the solubilization into mixed-micelles, intestinal absorption and oxidative metabolism of carotenoids are reviewed to understand the factors that determine the bioavailability of carotenoids ingested from fruit and vegetables.