Carotenoid bioavailability in humans reflects the mammalian nocturnal bottleneck: a position paper.

Recommended levels of dietary carotenoids remain uncertain, although their health benefits have gained renewed interest. One reason is that humans have a unique bioavailability of carotenoids compared to other mammals, which is not well understood. This position paper suggests that humans' bioavailability of dietary carotenoids reflects the mammalian nocturnal bottleneck-a period when the ancestors of modern mammals became nocturnal during the Mesozoic era to avoid predatory dinosaurs. The roles of carotenoids are closely linked to evolutionary processes involving light and vision. Vertebrates developed advanced color vision by using specific xanthophylls in their photoreceptor cells. In contrast, mammals' color vision declined due to their nocturnal lifestyle. Primates later returned to daytime activity and accumulated two xanthophylls, lutein and zeaxanthin, from their diet, in the macular lutea of the eyes, to protect color vision. Additionally, humans uniquely store β-carotene and lycopene in exposed skin. Throughout primate evolution, humans' ability to accumulate dietary carotenoids shifted from xanthophylls toward less polar carotenoids, including β-carotene and lycopene, unlike the high selectivity of xanthophylls observed in birds. Overall, carotenoid deposition in human tissues is not highly specialized, likely due to remnants of the mammalian nocturnal bottleneck. We should consider this when evaluating recommended carotenoid intake.