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Increased Risk of Prostate Cancer in Men Consuming High Doses of Vitamin E or Selenium Dietary Supplements

By: Peter Olins, PhD, on November 3, 2011.

A large-scale, controlled clinical study has found no evidence for a beneficial effect of vitamin E dietary supplements in prostate cancer. On the contrary, a significantly increased risk was observed.

Traces of antioxidants (such as vitamins A, C and E) are essential in the human diet. For a longIncreased risk of prostate cancer in men consuming high doses of vitamin E or selenium dietary supplements time, people have wondered whether it might be beneficial to consume more than the minimal requirement. In other words, surely if a little is important for health, then more should be better, right? While many people have made claims about the beneficial effects of antioxidant supplementation (often based on small observational studies), these preliminary results have almost never been verified in controlled studies.

But surely if vitamins are “natural”, they can’t do any harm?

A recent study (Ref. 1) tested the effect of dietary supplements of vitamin E and selenium on the risk of prostate cancer in men (which is one of the most common cancers). This well-controlled study was the largest of its kind, involving over 35,000 men, and extended for a period of 7-10 years. The study was performed at 427 clinical sites in the US, Canada and Puerto Rico. Four different daily “treatments” were tested: 1) 400 International Units of vitamin E; 2) 200 micrograms selenomethionine; 3) vitamin E plus selenium; and 4) a placebo control. The dose of vitamin E (400 IU) was greater than the recommended daily allowance (30 IU), but corresponds to the amount in several readily-available over-the-counter high-dose supplements.

No protective effect was found in any of these groups.  In fact, there was a statistically significant increase in prostate cancer incidence both for vitamin E and selenium supplementation.  For vitamin E, the increased risk was 17%.

Ref. 1: Vitamin E and the Risk of Prostate Cancer  JAMA. 2011; 306(14):1549-1556. doi:10.1001/jama.2011.1437 Klein, EA, et al.

Why have early studies of vitamin consumption seemed promising?

Most early studies on a variety of vitamins and diseases were “observational”, meaning that disease risk was compared for people consuming different amounts of nutrients in their diet. These observations are almost impossible to prove, since there is usually little evidence that a given vitamin is the cause of a reduction in disease risk.  People have different genetics, they lead different lifestyles, and consume a host of different foods.

In attempt to sort out cause and effect, large clinical studies are usually required, in which just a single variable (such as the consumption of extra nutrient) is tested. These are usually expensive and can take years to perform. This is why we need to be very cautious about preliminary observations, especially those that get attention in the news.

In another large study (Ref. 2), a number of nutritional supplements were examined for their potential benefit in preventing lung cancer. No benefit was observed, unfortunately, and extra vitamin E consumption was associated with a slight increase in the risk of lung cancer in smokers.

Ref. 2:  Long-Term Use of Supplemental Multivitamins, Vitamin C, Vitamin E, and Folate Does Not Reduce the Risk of Lung Cancer. American Journal of Respiratory and Critical Care Medicine Vol 177. pp. 524-530, (2008) Slatore, CD, et al.

What does this mean for people with celiac disease?

Celiac patients often have health problems due to poor absorption of nutrients, and it is common for physicians to prescribe nutritional supplements to help overcome this. However, this clinical study indicates that it may be inadvisable to “self-medicate” with large doses of supplements, even though these drugs may be freely available over-the-counter. As always, you should discuss the potential benefits and safety of any nutritional supplements you are taking with your physician.

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