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In the new study, about 25,000 middle-aged and elderly Swedish women filled out a questionnaire regarding various health and lifestyle factors, including vitamin use, cigarette smoking, alcohol consumption, waist circumference, and use of certain medications. The women were then followed for an average of eight years. After the researchers adjusted for the health and lifestyle factors mentioned above, they found that women who regularly or occasionally took 1,000 mg per day of vitamin C were 25% more likely to have had cataract surgery than women who did not take vitamin C supplements.
The study has an important weakness that should be taken into account: it was an observational study, meaning that the researchers observed how certain behaviors correlated with certain outcomes. Observational studies cannot prove a cause-and-effect relationship because they are rarely, if ever, able to account for all of the differences between people who do and do not engage in the behavior being examined (in this case, taking vitamin C supplements).
Other observational studies have found that the use of vitamin C supplements was associated with a decreased incidence of cataracts, and still others found no association between vitamin C use and cataract risk. The widely divergent results in the earlier studies and the new study highlight how unwise it is to draw conclusions from observational studies.
The most reliable method to determine whether vitamin C supplements influence cataract risk is a double-blind trial, in which people are randomly assigned to receive vitamin C or a placebo. Only one such study has been done to date. In that study, middle-aged and elderly people were given a placebo or a daily supplement that contained 500 mg of vitamin C, 400 IU of vitamin E, and 15 mg of beta-carotene. After an average of six years, the incidence of cataracts did not differ between the supplement group and the placebo group. In addition, among participants who already had a cataract at the start of the study, there was no difference in the rate of cataract progression between groups. So, when considering all of the research, one might reasonably conclude that there is no firm evidence that taking vitamin C supplements influences the risk of developing cataracts.
(Am J Clin Nutr 2009 Nov 18 [E-pub ahead of print])