DOES IT WORK?Selenium and prostate cancer prevention
SELENIUM IS a mineral required in the diet. Broccoli, garlic and onions, when grown in soils rich in selenium, are good sources of selenium, as are seafood, liver and Brazil nuts.
The amount of selenium required for good health is very small (about 50 micrograms per day). However, too much can cause problems. The difference between the two levels is very small, raising concerns about its use as a dietary supplement.
Canada, the US, Japan and Norway are the only countries where dietary intake of selenium is believed to be adequate, primarily because most soils in the world are low in selenium.
Selenium is important for the normal functioning of certain enzymes and proteins. At plasma concentrations of
1mg/L, selenium acts as an antioxidant.
Paradoxically, though, when the concentration goes above 2mg/L, selenium stimulates cellular oxidation. For this reason, selenium is often taken along with other vitamins and antioxidants which are believed to balance one another’s effects.
Antioxidants play an important role in preventing several diseases, including some cancers.
Selenium gained much public attention after a study on nutrition and cancer prevention was published in 1996. The study was designed to examine the impact of selenium supplements on skin cancer.
It was conducted in an area of the US where selenium soil levels were low and skin cancer rates were high. About halfway through the study, the results showed that selenium had no impact on skin cancers.
However, it seemed that men taking selenium had almost two-thirds less prostate cancers, and lower levels of some other cancers. After more than four years of taking selenium supplements, the rate of new cancer diagnosis was one-third lower and the rate of death from cancer was half that of the placebo group.
A wave of hope swept through those examining the role of nutrition in cancer prevention, especially to prevent prostate cancer.
Over the next decade, while selenium supplements were promoted as a means of reducing the risk of prostate cancer, research studies produced conflicting results.
Earlier this year, the much-anticipated study called Select (Selenium and Vitamin E Cancer Prevention Trial) was published.
Enrolling over 35,500 men in their 50s or older, it was the largest randomised controlled trial ever conducted for cancer prevention. Unfortunately, the results showed that men taking selenium supplements (400 micrograms per day for four to seven years) had the same rate of prostate cancer as those taking placebo.
The study also included groups of men taking vitamin E with selenium and vitamin E alone, and found no statistically significant differences between any of the groups. The size and duration of this study makes it unlikely any benefit was missed.
In Select, those taking selenium had a slightly higher risk of diabetes. Although the difference was not statistically significant, other studies have found similar results.
A condition called selenosis can occur when people take more than 1mg selenium per day. Several people have died as a result, usually from heart failure.
If too much selenium is taken, a garlic-like breath odour, white streaks in fingernails and brittle hair usually develop before serious complications occur. In these cases, selenium should be discontinued immediately and medical attention sought.
The Select results may, in part, reflect the medical and pharmaceutical progress being made with prostate cancer. More than 80 per cent of the men in Select had annual prostate-specific antigen (PSA) testing independent of the study.
An elevated PSA level would have led to treatment and reduced the chances of cancer developing.
When planning the study, the researchers anticipated that up to 100 men would die from prostate cancer during the study, yet only one such death occurred. This is welcome news for men, but it makes it increasingly difficult to detect whether supplements add any additional benefit in the prevention of prostate cancer.
The other lesson to learn from Select is that it is increasingly difficult to identify which supplements influence the risk of cancer.
Population studies clearly show that diet influences the risk of cancer. But it may be impossible to identify exactly which supplement plays what role. Diet and metabolism appear to be too complex to allow that.
People can have a selenium deficiency and, if so, they will benefit from selenium supplements. For everyone else, rather than searching for a magical supplement, our focus should be on consuming a diet rich in whole foods that include a wide variety of fruits and vegetables.
Dónal O’Mathúna has a PhD in pharmacy, researching herbal remedies, and an MA in bioethics, and is a senior lecturer in the School of Nursing, Dublin City University