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Oxidative Theory of Cataract Formation
The oxidative hypothesis of cataract formation holds that reactive oxygen species can damage lens proteins and fiber cell membranes, and that nutrients with antioxidant actions can protect against these changes. Results of laboratory and animal studies generally support this theory, though findings from epidemiologic studies have varied.
Results of a new prospective study from Brigham and Women's Hospital and Harvard Medical School add to the body of evidence suggesting that lutein, zeaxanthin and vitamin E may help delay cataract formation (1). The study was published in the January issue of Archives of Ophthalmology.
Study Design and Methods
Dietary intake was assessed at baseline in 39,876 female health professionals by using a detailed food frequency questionnaire. Information on antioxidant nutrient intake from food and supplements was obtained from a total of 35,551 women who were free of a diagnosis of cataract.
The main outcome measure was cataract, defined as incident, age-related lens opacity responsible for a reduction in best-corrected visual acuity in the worse eye to 20/30 or worse based on self-report, and confirmed by medical record review.
A total of 2031 cases of incident cataract were confirmed during a mean 10 years of follow-up.
Women with the highest daily intake of lutein and zeaxanthin, averaging about 6.7 mg, had an 18% lower risk of developing cataract compared to women consuming the least, or a mean of 1.2 mg a day. The multivariate relative risk of cataract was 0.82 (95% confidence interval, 0.71-0.95; test for trend, P =.04) for combined intake of lutein and zeaxanthin.
Comparing women in the extreme quintiles for vitamin E from food and supplements combined, a 14% risk reduction was noted. Those in the highest quintile consumed a mean of 262 mg vitamin E daily, while those in the lowest quintile consumed an average 4.4 mg. The multivariate relative risk of cataract was 0.86 (95% confidence interval, 0.74-1.00; test for trend, P =.03).
No relationship was observed between cataract risk and intakes of other carotenoids and antioxidants, including beta-carotene, beta-cryptoxanthin, lycopene and alpha-carotene.
lutein and zeaxanthin are the only carotenoids detected in the human lens, where they filter harmful short-wave blue light. The presence of oxidation products of lutein and zeaxanthin in the lens further supports a functional role for these carotenoids in maintaining lens clarity. Findings from observational epidemiologic studies also generally support a possible beneficial effect of lutein and zeaxanthin.
The association of vitamin E with lower cataract risk in this study comes in the wake of an intervention trial that reported no benefit of 600 IU vitamin E (every other day) on cataract in women (2).
Though speculative, it is possible that moderate amounts of antioxidants such as vitamins C and E and lutein along with B-vitamins, might have beneficial effects beyond that of a single nutrient administered at high levels. This notion is consistent with the findings from several prospective epidemiologic studies reporting a lower occurrence of cataract in regular users of multi-vitamin and mineral supplements (see EduFacts Volume 8, Number 4). A recent analysis of AREDS for example, linked consistent use of a multi-nutrient supplement with a lower risk of cataract even though the high dose antioxidants studied in that trial showed no benefit (3).
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