Carotenoids are a structurally related group of phytonutrients, some of which possess vitamin A activity in humans. Carotenoids are pigments in nature and produce many of the red, yellow and orange colours in vegetables and some fruits. In plants, carotenoids are oxidised enzymatically to xanthophylls and these two groups share many of the same chemical and physical properties. In particular they are potential antioxidants, and in this role may play an important part in protecting the plant from oxidative stress caused by exposure to high light intensities. In addition, the xanthophylls form part of the chloroplast structure and in times of low light intensity, may allow additional harvesting of light to aid photosynthesis. Carotenoids and xanthophylls are bioavailable in humans and when absorbed they are incorporated into tissues where they may have antioxidant effects similar to those in plants. In this regard, carotenoids and xanthophylls may be able to scavenge singlet oxygen.
Some xanthophylls and carotenoids may accumulate in the lenses of the eye and here they may decrease the degree of oxidative stress. The xanthophylls that predominate in the eye are lutein and zeaxanthin (zeaxanthin is a stereoisomer of lutein). The lack of accumulation of major dietary carotenoids such as β-carotene and lycopene explains the lack of efficacy of these compounds at protecting from eye diseases in many studies. In contrast, studies that have investigated the effects of lutein and shown that improvements are seen in the macular pigment optical density with supplements containing around 30 mg of lutein suggesting that lutein accumulates in the eye. In addition, consumption of spinach, a good source of lutein and zeaxanthin (1 cup per day containing about 0.4 mg lutein and 0.3 mg zeathanthin), also changes the macular pigment optical densities. In studies, macular pigment optical densities remain elevated at study end for weeks despite plasma lutein and zeaxanthin levels returning to baseline.
Some evidence suggests that consumption of foods containing high concentrations of lutein and zeaxanthin may reduce the risk of age-related macula degeneration. Foods such as eggs, spinach and broccoli have been investigated and shown to have some protective effects. For example in one study, consumption of eggs, spinach or broccoli was associated with a 20 % reduction in cataract risk and a 40 % reduction in age-related macular degeneration risk. While this evidence does not prove cause and effect, the presence of high concentrations of both lutein and zeaxanthin in the eyes of subjects consuming these foods, in combination with the known oxygen radical scavenging effects of these compounds, strongly suggests that these compounds are protective of eye related disease. In particular, xanthophylls appear to show the properties necessary to protect the eye from ultraviolet induced oxygen radicals, the exact type of damage that might be expected to cause cataracts and macular degeneration.
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