A new study recently published in the Journal of the National Cancer Institute suggests that taking high doses of selenium and vitamin E supplements may increase the risk of prostate cancer, depending on a man's selenium levels prior to taking the supplements.

The research team, including first author Dr. Alan Kristal of the Public Health Sciences Division of the Fred Hutchinson Cancer Research Center in Seattle, WA, analyzed 1,739 patients with prostate cancer and 3,117 matched controls from the Selenium and Vitamin E Cancer Prevention Trial (SELECT).

According to the investigators, previous research has suggested that men who already have an adequate intake of selenium would not benefit from supplements of the nutrient.

Therefore, the researchers took selenium measurements from the toenails of participants at the baseline of the study.

Selenium is a chemical element most commonly found in seafoods and organ meats, such as liver. Other food sources of selenium include muscle meats, cereals and dairy products.

The National Institutes of Health state that selenium is nutritionally essential for humans and plays roles in reproduction, thyroid hormone metabolism and DNA syntheses, as well as protects against oxidative damage and infection.

According to the Food and Nutrition Board, the recommended dietary allowance for both males and females aged 14 years and over is 55 mcg per day.

For the study, the researchers wanted to determine whether taking daily high doses of vitamin E (400 IU) and/or selenium (200 mcg) may reduce the risk of prostate cancer.

Vitamin E is a group of fat-soluble compounds that act as an antioxidant in the body. The vitamin is commonly found in foods such as nuts, seeds, vegetable oils, green leafy vegetables and fortified cereals.

Dietary supplements 'not necessarily helpful or innocuous'

SELECT began in 2001 and was scheduled to carry on for 12 years. But in 2008, the study was called to a halt on the grounds that no protective effects were found from selenium supplements and vitamin E supplements were thought to increase the risk of prostate cancer.

However, although the men stopped taking the supplements in 2008, the researchers continued following them in order to monitor their prostate cancer risk.

The findings revealed that men who had high selenium levels at the beginning of the study had a 91% increased risk of high-grade prostate cancer. According to the researchers, the levels of selenium for these men became toxic.

The investigators also found that for men with low selenium levels at the baseline of the study, vitamin E increased total prostate cancer risk by 63%, while high-grade prostate cancer risk increased by 111%.

"Many people think that dietary supplements are helpful or at the least innocuous. This is not true," says Dr. Kristal.

"We know from several other studies that some high-dose dietary supplements - that is, supplements that provide far more than the daily recommended intakes of micronutrients - increase cancer risk.

We knew this based on randomized, controlled, double-blinded studies for folate and beta carotene, and now we know it for vitamin E and selenium."

He adds that people taking vitamin E or selenium supplements should stop because there is no evidence that they produce any health benefits - only risks.

Dr. Kristal says that even standard multivitamins - which he says have yet to demonstrate any risk - could be harmful in high doses.

"Taking a broad view of the recent scientific studies, there is an emerging consistency about how we think about optimal intake of micronutrients," he adds.

"There are optimal levels, and these are often the levels obtained from a healthful diet, but either below or above the levels there are risks."

Of late, there have been many studies questioning the health benefits of vitamin supplements. Medical News Today recently reported on a study suggesting that vitamin C and E supplements may hinder athletes' training, while other research suggests that multivitamins are a waste of money and have no health benefits.