Dietary carotenoids, especially xanthophylls, have attracted significant attention because of their characteristic biological activities, including anti-allergic, anti-cancer, and anti-obese actions. Although no less than forty carotenoids are ingested under usual dietary habits, only six carotenoids and their metabolites have been found in human tissues, suggesting selectivity in the intestinal absorption of carotenoids. Recently, facilitated diffusion in addition to simple diffusion has been reported to mediate the intestinal absorption of carotenoids in mammals. The selective absorption of carotenoids may be caused by uptake to the intestinal epithelia by the facilitated diffusion and an unknown excretion to intestinal lumen. It is well known that β-carotene can be metabolized to vitamin A after intestinal absorption of carotenoids, but little is known about the metabolic transformation of non provitamin A xanthophylls. The enzymatic oxidation of the secondary hydroxyl group leading to keto-carotenoids would occur as a common pathway of xanthophyll metabolism in mammals. This paper reviews the absorption and metabolism of xanthophylls by introducing recent advances in this field.

We reported previously that lysophosphatidylcholine remarkably enhanced β-carotene uptake from bile acid-mixed micelles by human intestinal Caco-2 cells. In the present study, we evaluated how mixed micelle components other than phospholipids, viz., fatty acids, monoolein, and cholesterol, affect carotenoid uptake by Caco-2 cells. Each component influenced the β-carotene uptake in a different way depending on micellar composition. Oleic acid at 200 µM significantly enhanced uptake in the absence of lysophosphatidylcholine. Cholesterol at 40 µM significantly reduced uptake in the presence of lysophosphatidylcholine, while no reduction was found in the presence of 200 µM oleic acid. Facilitated diffusion was suggested partly to mediate uptake in mixed micelles, except for mixed micelles containing 200 µM oleic acid. Uptake mediated by facilitated diffusion was approximately 20% of total uptake. Mixed micellar lipids have the potential to modify intestinal uptake.

Fucoxanthin has an antiproliferative effect on cancer cells, but its detailed structure–activity correlation has not yet been elucidated. To elucidate this correlation, fucoxanthin was degraded by ozonolysis. The degraded compounds of fucoxanthin obtained by ozonolysis were purified by HPLC and analyzed by NMR. The polyene chain of fucoxanthin was cleaved by ozonolysis, and the fucoxanthin was divided into two types of cyclohexyl derivatives, one with a β,γ-epoxy ketone group and the other with an allenic bond. In order to elucidate the structure–activity correlation, Caco-2 cells (human colorectal carcinoma) were treated with fucoxanthin degradation compounds. It was found that the entire structure of fucoxanthin is not essential for its antiproliferative effect and that even a partial structure exerts this effect.

Vitamins D have various biological activities, as well as intestinal calcium absorption. There has been recent concern about insufficient vitamin D intake. In addition to vitamins D₂ and D₃, there are lesser-known vitamins D₄–D₇. We synthesized vitamins D₅–D₇, which are not commercially available, and then evaluated and compared the mixed micelles-solubilized vitamins D uptake by Caco-2 cells. Except for vitamin D₅, the uptake amounts of vitamins D₄–D₇ by differentiated Caco-2 cells were similar to those of vitamins D₂ and D₃. The facilitative diffusion rate in the ezetimibe inhibited pathway was approximately 20% for each vitamin D type, suggesting that they would pass through the pathway at a similar rate. Lysophosphatidylcholine enhanced each vitamin D uptake by approximately 2.5-fold. Lysophosphatidylcholine showed an enhancing effect on vitamin D uptake by reducing the intercellular barrier formation of Caco-2 cells by reducing cellular cholesterol, suggesting that increasing the uptakes of vitamins D and/or co-ingesting them with lysophosphatidylcholine, would improve vitamin D insufficiency. The various biological activities in the activated form of vitamins D₄–D₇ were estimated by Prediction of Activity Spectra for Substances (PASS) online simulation. These may have some biological activities, supporting the potential as nutritional components.