Role of Xanthophylls in Visual Function

Overview

Ecy of lutein supplementation alone, or zeaxanthin supplementation alone, or a combination of both, compared to placebo, on the evolution of macular pigment density (MPOD) measured by fundus photography in XANTH-deficient patients receiving enteral nutrition.

Lutein and zeaxanthin are plant pigments belonging to the carotenoid family, and more precisely to the subfamily of xanthophylls (XANTH), which are oxygenated carotenoids. These phytochemicals are not synthesised by humans and are almost always present in our diet. They are significantly absorbed and transported in our body. They are present in various organs, e.g. the eye and the skin. They accumulate in very high concentrations in the central area of the human retina. Because of this high accumulation, it has been hypothesised that they play one or more specific roles in the eye. This has been confirmed by several studies which have shown that XANTHs are involved in protecting the eye from damage by certain light rays (blue) and age-related macular degeneration. Other studies suggest that XANTH may improve glare tolerance and contrast sensitivity. They may also improve sensitivity to red and green colour discrimination. However, identifying the exact relationship between XANTH and visual function in XANTH supplementation is complicated by two main problems. Firstly, XANTH supplementation does not always increase MPOD (Macular Pigment Optical Density), which is an indicator of the concentration of XANTH in the macula, because there are poor responders to XANTH, i.e. subjects who absorb it very poorly and/or transport it very poorly to the retina. Secondly, MPOD seems to reach a maximum value and a plateau, and increasing the dose of XANTH, or the duration of supplementation, does not increase MPOD further. This is particularly the case in individuals who already have a high MPOD, close to the maximum value, before supplementation because they often consume XANTH-rich foods, and/or because they are strong responders to XANTH. Therefore, although studies suggest a specific role for XANTH in visual function, the problem of initially high MPOD in some individuals, coupled with MPOD saturation, has not allowed previous studies to provide definitive evidence on their role(s) in visual function.

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