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To C or Not to C?

To C or Not to C?: Main Image
The study has an important weakness that should be taken into account: it was an observational study, which cannot prove a cause-and-effect relationship
A recent study published in the American Journal of Clinical Nutrition has received widespread publicity for reporting that the use of vitamin C supplements was associated with an increased risk of developing cataracts. But don't throw away your vitamin C bottle as the new study does not prove that C caused this effect, and that result is inconsistent with previous research that suggests vitamin C either has no effect on cataract incidence or even decreases risk.

Oh say, do you C?

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).

  • Many people who take vitamin C supplements do so because they have heard that vitamin C prevents colds and other infections that they wish to avoid.
  • Others may take vitamin C because they have high serum cholesterol and have read that vitamin C can help prevent heart disease.
  • It is possible that people who are prone to infections or to high serum cholesterol levels have certain weaknesses in their body chemistry that also make them prone to developing cataracts.
  • If that is the case, then taking vitamin C supplements may be an effect, rather than a cause, of the increased cataract risk.

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.

C-ing ahead

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])

An expert in nutritional therapies, Chief Medical Editor Alan R. Gaby, MD, is a former professor at Bastyr University of Natural Health Sciences, where he served as the Endowed Professor of Nutrition. He is past-president of the American Holistic Medical Association and gave expert testimony to the White House Commission on Complementary and Alternative Medicine on the cost-effectiveness of nutritional supplements. Dr. Gaby has conducted nutritional seminars for physicians and has collected over 30,000 scientific papers related to the field of nutritional and natural medicine. In addition to editing and contributing to The Natural Pharmacy (Three Rivers Press, 1999), and the A-Z Guide to Drug-Herb-Vitamin Interactions (Three Rivers Press, 1999), Dr. Gaby has authored Preventing and Reversing Osteoporosis (Prima Lifestyles, 1995) and B6: The Natural Healer (Keats, 1987) and coauthored The Patient's Book of Natural Healing (Prima, 1999).
 
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