The eye is at particular risk of oxidative damage due to high oxygen concentrations, large amounts of oxidizable fatty acids in the retina, and exposure to ultraviolet rays. In Western countries, age-related macular degeneration (AMD) is the leading cause of blindness among older people. Cataracts are also widespread among the elderly and occur when the lens is unable to function properly due to the formation of opacities within the lens. These develop when proteins in the eye are damaged by photooxidation; these damaged proteins build up, clump, and precipitate. It has been proposed that antioxidants may prevent cellular damage in the eye by reacting with free radicals produced during the process of light absorption.
The results of intervention trials in this area have also been mixed. The Age-Related Eye Disease Study in the United States investigating the effects of combined antioxidant vitamins C (500 mg), E (400IU), and ^-carotene (15 mg) with and without 80 mg zinc daily for 6 years showed some protective effect (a reduction in risk of approximately 25%) on the progression of moderately advanced AMD but no benefit on the incidence or progression or early AMD or cataracts. The Lutein Antioxidant Supplementation Trial, a 12-month study of 90 patients with AMD, found significant improvements in visual function with 10mg/day lutein (one of the major carotenoids found in the pigment of a normal retina) alone or in combination with a number of other antioxidant nutrients. The Roche European Cataract Trial, providing a combined daily supplement of ^-carotene, vitamin C, and vitamin E among adults with early signs of age-related cataract, showed a small deceleration in the progression of cataract after 3 years.
However, the Linxian trial found no influence of vitamin supplementation on risk of cataract; the ATBC trial found no reduction in the prevalence of cataracts with vitamin E, ^-carotene, or both among male smokers; and the Health Physicians Study of more than 22 000 men showed no benefit from 12 years of supplementation with ^-carotene (50 mg on alternate days) on cataract incidence. In fact, current smokers at the beginning of this trial who received the supplement experienced an increased risk of cataract (by approximately 25%) compared to the placebo group. The Vitamin E, Cataract and Age-Related Maculopathy Trial also reported no effect of supplementation with vitamin E for 4 years (500 IU/day) on the incidence or progression of cataracts or AMD.
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