Vitamin C and early cataracts

Nutrition Research Newsletter,  Jan, 1998  

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Experimental and epidemiologic evidence suggests that vitamin C may be protective against cataract. The oxidation of lens constituents is believed to contribute to the formation of cataract; vitamin C inhibits this oxidation. The body concentrates vitamin C in the lens to a level that greatly exceeds that present in blood plasma, and lens vitamin C concentrations respond to changes in vitamin C intake. Vitamin C has been shown to protect against experimentally induced cataracts in animal models, but the relevance of these experiments is questionable, since there are no animal models of age-related cataract in species that require a dietary source of vitamin C. Some, but not all, epidemiologic studies have associated higher vitamin C intakes with reduced risks of cataract, but most of these studies have had a retrospective design, and the possibility that participants may have changed their dietary intake or reporting after learning that they had cataracts cannot be ruled out.

In a previous report, long-term vitamin C supplementation was associated with a lower rate of cataract extraction among the participants in the Nurses' Health Study, a long-term prospective study of more than 100,000 US women. This report from the same study examines the relationship between early age-related lens opacities and long-term vitamin C supplement use in a subset of the study participants. A total of 247 women aged 56-71 years underwent ophthalmologic examinations to detect early lens opacities. Since early opacities do not affect visual acuity, it is unlikely that the subjects were aware of their lens status before their examinations. As part of their participation in the larger study, the women had provided information on several prior occasions on their dietary habits and supplement use. Women with unusually high or low vitamin C intakes were deliberately oversampled for the substudy.

The use of vitamin C supplements for 10 years or more was associated with a 77% lower prevalence of early lens opacities and an 83% lower prevalence of moderate lens opacities, as compared with nonuse of vitamin C supplements. This association was not affected by adjustment for other antioxidant nutrients or other factors that are believed to influence cataract risk. Use of vitamin C supplements for less than 10 years was associated with a nonsignificant decrease in the risk of moderate opacities and no decrease in early opacities.

The results of this analysis confirm other evidence associating the use of vitamin C supplements with reduced risk of cataract. In spite of this association, the authors of the study could not conclude that supplements would be needed to protect the lens. Since eye tissues may saturate with vitamin C at intakes between 150 and 250 mg/day--a level of intake achievable by diet alone--it may be possible to obtain enough vitamin C to protect the lens without the use of supplements. An editorial accompanying the study raises other reasons for caution about vitamin C supplementation, pointing out that the possibility that high doses of vitamin C may have adverse effects on the eye and on other aspects of health has not been ruled out.

Paul F. Jacques, Allen Taylor, Susan E. Hankinson et al, Long-term Vitamin C Supplement Use and Prevalence of Early Age-related Lens Opacities, American J Clinical Nutrition 66(4):911-916 (Oct 1997) [Reprints: Allen Taylor, Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University, 711 Washington Street, Boston, MA 02111. Internet: Taylor_cl@hnrc.tufts.edu]

Julie A. Mares-Perlman, Contribution of Epidemiology to Understanding Relations of Diet to Age-related Cataract [Editorial], American J Clinical Nutrition 66(4): 739-740 (Oct 1997) [Reprints: Julie A. Mares-Perlman, Department of Ophthalmology and Visual Sciences, University of Wisconsin-Madison, 610 North Walnut Street, 405 WARF, Madison, WI 53705-2397. Internet: maresp@epi.ophth.wisc.edu]

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