(RxWiki News) Typically the news about vitamin supplements is good, with studies often finding they help prevent diseases and medical conditions. This time, though, the findings aren't so positive.
In a trial that included about 35,000 men from the U.S., Canada and Puerto Rico, those participants who were randomly assigned to receive vitamin E supplements daily had a significantly increased risk of prostate cancer.
"Taking vitamin E might increase the risk of prostate cancer."
Now, Eric A. Klein, M.D., of the Cleveland Clinic, and colleagues examined the long-term effect of vitamin E and selenium on the risk of prostate cancer in relatively healthy men. Essentially, they picked up where SELECT left off. That trial, held between August 2001 and June 2004, included men from some 430 study sites who had a prostate-specific antigen (PSA) measure below a certain level and a rectal examination that was not suspicious for prostate cancer.
The men were randomly assigned to one of four treatment groups:
- About 8,750 men received selenium at 200 micrograms (mcg) daily
- About 8,740 men received vitamin E at 400 International Units (IU) daily
- About 8,700 men received both supplements daily
- Nearly 8,700 menreceived a placebo daily
The planned followup for SELECT was a minimum of seven years and a maximum of 12 years. The analysis reflects the final data collected by the study sites on their participants through early July 2011.
The lifetime risk of prostate cancer in the U.S. is estimated at 16%. While most cases are found at an early, curable stage, treating the disease is expensive, and many patients suffer urinary, sexual and bowel-related side effects.
Early evidence had suggested that the mineral selenium and vitamin E might reduce a man's prostate cancer risk. But as early as December 2008, one study, the Selenium and Vitamin E Cancer Prevention Trial (SELECT), had found no reduction in the risk of prostate cancer with either selenium or vitamin E supplements. This same study also found a very minor increase in prostate cancer risk with vitamin E.
Since the initial report, some 520 additional prostate cancers have been diagnosed: 113 in the placebo group, 147 in the vitamin E group, 143 in the selenium group, and 118 in the combination group. The researchers found that the rate of prostate cancer detection was greater in all treatment groups when compared with placebo but was statistically significant only in the vitamin E alone group: a 17% increased rate of prostate cancer detection.
Compared with the placebo group, in which some 530 men developed prostate cancer, 620 men in the vitamin E group developed prostate cancer, as did 575 in the selenium group and 555 in the selenium plus vitamin E group. The difference in rates of prostate cancer between vitamin E and placebo became apparent during the participants' third year in the trial. The elevated risk estimate for vitamin E was consistent across both low- and high-grade disease.
The study authors write, "Given that more than 50% of individuals 60 years or older are taking supplements containing vitamin E and that 23% of them are taking at least 400 IU/[day] despite a recommended daily dietary allowance of only 22.4 IU for adult men, the implications of our observations are substantial."
The increased risk of prostate cancer in the vitamin E group of this trial became apparent only after extended followup. This finding suggests that health effects from these supplements may continue even after the intervention is stopped, emphasizing the need for long-term followup.
The researchers note, too, that the findings of this and other studies point to the importance of large-scale, population-based, randomized trials to accurately assess the benefits and harms of vitamins and minerals as dietary supplements.
"The observed 17% increase in prostate cancer incidence demonstrates the potential for seemingly innocuous yet biologically active substances such as vitamins to cause harm," the authors note.
They argue that these and other similar findings point to the need for consumers to be skeptical of health claims for unregulated over-the-counter products in the absence of strong evidence of benefit demonstrated in clinical trials.
The study appears in the October 12 issue of JAMA.