In 1945, Wald reported that the macula lutea of humans and certain other primates contains a yellow pigment and suggested that this macular pigment (MP) was a carotenoid. By analysing the spectral sensitivity of the human foveal and peripheral photoreceptors, he estimated the light absorbed/transmitted by the macular pigment, and used the term macular pigment optical density (more recently associated with the acronym MPOD). At the same time he observed a great variation in the intensity of macula pigmentation among subjects – from foveas with no perceptible pigmentation to very intense coloured ones.1 Research conducted in recent decades has confirmed that the MP is composed by three distinct carotenoids lutein (L), zeaxanthin (Z) and meso-zeaxanthin (MZ).2,3 MP reaches highest concentration (per mm2 of tissue) at the centre of the fovea decreasing rapidly with distance from the epicentre. The concentrations of L, Z and MZ also vary with eccentricity – in the adult retina, while the three macular carotenoids are present in similar concentrations at foveal centre, as the eccentricity from the fovea increases, lutein becomes the predominant pigment with a decreasing amount of Z, and with MZ, approaching undetectable levels.3 The role of the MP in the human eye is well described and is based on the biological and optical characteristics of its main components.4–7 It acts as an antioxidant by quenching reactive oxygen species and as a filter for damaging high energy/short-wavelengths of visible light (blue light). These functions support the protective role of the MP in the retina, a tissue particularly susceptible to oxidative stress and the more recently explored beneficial effects on visual function.
Macular Pigment Optical Density and the Diet
Studies conducted in monkeys raised on a xanthophyll-free diet from birth have shown that the macular pigment is of dietary origin and that L and Z supplementation can replenish the macular pigment. Moreover, those results indicate that, while the accumulation of L and Z in serum and retina stems from the ingestion of these two xanthophylls with the diet, MZ originates exclusively from the conversion of L in the macula. MZ is not present in serum.8,9 Additional observations conducted in thesexanthophyll-free animals indicate the presence of drusen in the igment epithelium.8 A recent publication addressed the effect of acute blue-light exposure in xanthophyll-free animals lacking the macular pigment in comparison to control animals fed stock diet containing xanthophylls.10 The presence of the macular pigment rendered control animals less susceptible to blue light-induced damage in both the fovea and parafovea although more damage was observed in the parafoveal area than in the fovea. The xanthophyll-free animals, who exhibited no MP, experienced similar damage in both the foveal and parafoveal areas resulting from blue-light exposure as well as more damage than control animals. Supplementation of the xanthophyll-free animals with L or Z increased macular pigment density and decreased foveal susceptibility to blue-light induced damage to control levels. In the author’s words, “L and Z when provided in the diet and deposited as macular pigment, provided foveal protection from acute blue-light photochemical damage. It seems probable that these nutrients would also protect the macula against chronic blue-light-induced damage”.
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