Two new studies reveal a regular diet of pills is no guarantee that men will be immune from prostate cancer. Richard McComb finds out why.
The widely held belief that vitamins and antioxidant supplements help to protect men against prostate cancer has been dealt a major blow by two clinical studies.
One looked at the effect of vitamin E and selenium on the risk of prostate cancer in more than 35,500 men. The other, involving almost 15,000 male physicians, examined whether vitamins E and C could help prevent prostate and overall cancer.
In both cases scientists found no evidence of any significant protective effect.
The results, published in the Journal of the American Medical Association, appear to contradict the widespread belief that taking vitamins or antioxidant minerals such as selenium can reduce cancer risk. Although some previous studies have suggested that some of these supplements may have anti-cancer properties, definitive proof has been lacking.
In the larger Selenium and Vitamin E Cancer Prevention Trial (Select), 35,533 men from the US, Canada and Puerto Rico, were randomly assigned to take either selenium, vitamin E, both selenium and vitamin E, or an inactive placebo.
African-American participants were aged 50 or over and other men aged 55 or older at the start of the study. People of African descent are known to be more susceptible to prostate cancer.
Although a follow-up period of up to 12 years was originally planned, the study was stopped early by an independent monitoring committee due to the lack of benefit observed. In the end, patients were followed up for between 4.17 and 7.33 years.
Among the four groups, no statistically significant difference was seen in five-year incidence rates of prostate cancer, which ranged from 4.43% to 4.93%. In fact, the placebo group had the lowest incidence of cancer. The authors, led by Dr Scott Lippman, from the University of Texas in Houston, wrote: “Select has definitively demonstrated that selenium, vitamin E or selenium plus vitamin E did not prevent prostate cancer in the generally healthy, heterogeneous population of men in Select.
“These data underscore the prudence that is needed in considering recommendations to use agents for the prevention or control of disease in the absence of convincing clinical trial results.”
Selenium especially is believed to protect against prostate cancer. Previous research had suggested that men with high blood levels of the mineral were half as likely to develop the disease as those with low levels.
The other investigation, called the Physicians’ Health Study II, included 14,641 male physicians from the US aged 50 or older. Of these, 1,307 had a prior history of cancer. Participants were randomly selected to receive different combinations of vitamin C, vitamin E and a non-active placebo.
During an average follow-up period of eight years, 1,943 cancers were confirmed among the men including 1,008 cases of prostate cancer.
Neither vitamin reduced the risk of prostate or total cancer by any significant degree.
The researchers led by Dr Michael Gaziano, from the Brigham and Women’s Hospital and Harvard Medical School, Boston, wrote: “These data provide no support for the use of these supplements in the prevention of cancer in middle-aged and older men.”
In an editorial in the journal, Dr Peter Gann, from the University of Illinois in Chicago, described the findings as “disappointing news”.
Dr Gann added: “It may be time to give up the idea that the protective influence of diet on prostate cancer risk ... can be emulated by isolated dietary molecules given alone or in combination to middle-aged and older men.
“On the other hand, non-pharmacological dietary prevention of prostate cancer is probably more complex and may involve certain inconvenient truths. Fortunately, no dietary change this profound is likely to be beneficial for prostate cancer alone.
“If it requires whole foods, extracts, or dietary patterns, it may be necessary to give up the reductionist need to know which molecule is most responsible and perhaps give up the notion of placebo controls as well.”