The biochemical mechanisms that mediate the selective uptake, concentration, and stabilisation of the of the macular carotenoids are unknown. In lower animals, such as lobsters and cyanobacteria, specialised carotenoid-binding proteins perform these tasks. Much less is known about carotenoid-binding proteins derived from vertebrates, yet it has been hypothesised that comparable carotenoid-binding proteins may have a similar role in the human macula. In the human blood stream, high-density lipoprotein (HDL) is the major carrier of lutein and zeaxanthin, while carotenes are preferentially carried by low-density lipoprotein (LDL) [
58]. In the mammalian eye, it has been reported that retinal tubulin binds macular carotenoids [
59], possibly as a site for passive deposition in the tissue. Further postulations included the assumption that human macular membranes could be a rich source of specific binding proteins for the macular carotenoids, especially since many plant and invertebrate carotenoid-binding proteins are known to be membrane associated [
60,
61]. In a recent report, xanthophyll-binding proteins (XBP) were partially purified and isolated from the human macula and retina and it was shown that lutein and zeaxanthin bind specifically to these proteins [
62]. Available evidence further suggests the presence of tissue competition for plasma carotenoids. The concept that adipose tissue and retina may compete for dietary lutein has been suggested [
44], and the interactions between carotenoids during intestinal absorption has also been investigated [
63]. It has hypothesised that if adipose tissue and liver compete with the retina for dietary lutein as suggested by observations in human subjects [
44], macular pigment may be more effectively increased through supplementation with zeaxanthin than with lutein (preferentially absorbed by fat). There is no evidence in literature suggesting that zeaxanthin can be converted to lutein in serum or retina, but the reverse has been proposed by some authors [
64,
65]. Dietary lutein may serve as a precursor for the very high concentrations of zeaxanthin found in the primate fovea, [
64] and conversion of lutein to mesozeaxanthin has been suggested [
66].