New clinical trials dampen anti-cancer hopes for selenium

Although laboratory research looked very promising, in large-scale clinical trials the dietary supplement selenium appears to be disappointing as an anti-cancer agent.

Dietary supplements are often seen as part of a healthy lifestyle, but sometimes the benefits are debatable.

It would help combat prostate cancer. It had the power to fend off skin melanomas. It was even purported to rid humans of dangerous toxins entering the food chain through the oceans' fish. But now it appears that the almost magical powers of the supplement selenium could be nothing more than a myth.

New research from Germany's Institute for Transdisciplinary Health Research, in Berlin, casts serious doubt over long-established claims made for the powers of selenium. The Cochrane Systematic Review of 55 previous studies focused on evidence of links between selenium exposure and cancer risk. Re-assessing the results of observational studies has led the researchers to conclude that selenium alone cannot be cited as the cause of a lower cancer risk in case subjects. A further review of randomised, controlled trials also failed to confirm that taking selenium supplements reduced cancer risk.

"It's a really sad thing about selenium," concedes Dr Helen Rippon, head of research management at the UK Prostate Cancer Charity (www.prostate-cancer.org.uk). "All the initial laboratory data seemed to suggest it could be particularly useful in combating prostate cancer, but when it came down to the really big human-based trials it just didn't cut it."

Until the latest findings, the news for selenium, a mineral found naturally in Brazil nuts, red meat, fish, and grains, had been very positive. It became popular as a supplement in the late 1990s because researchers believed it could help prevent cancer - prostate in particular.

As recently as 2009, one study hailed selenium for it's ability to protect Inuits in northern Canada from the potentially harmful effects on their white blood cells from PCB toxins in fatty fish. At the other end of the earth, Australian researchers reported in the April 2009 Abstract of Cancer Epidemiology, Biomarkers & Prevention that high levels of selenium in the blood could halve a person's risk of skin cancer.

In Britain that year, scientists even discussed "sowing" selenium into the food chain - as an additive to farm fertilisers - to boost the population's intake of a nutrient that the media reported could "reduce the risk of heart disease and certain types of cancer".

But later in 2009 a study of more than 35,000 men published in the Jama, the journal of the American Medical Association, showed that taking a 200 microgram supplement of selenium a day did not protect against prostate cancer, nor any other cancer for that matter. Even worse were follow-up reports from the US suggesting that taking selenium pills over a number of years could upset the body's ability to metabolise sugar and so increase the risk of diabetes in some men in particular.

Rippon believes that selenium is just one of a number of dietary supplements that have hit the health headlines in the past more through media hyperbole and marketing than creditable research. "So many of the claims about supplements add up to nothing," she says. "But because human nature dictates that we want a shortcut approach to things, even our health, there's always a big push in press whenever a possible 'cure-all' pill is hinted at.

"In truth, only a balanced, nutritious diet will help reduce the risk factors associated with many of these killer diseases. It's boring so it's not newsworthy I guess, but eating healthily with a variety of fresh fruit and vegetables is the key."

In the German review of selenium studies some of the most reliable data suggested that organic selenium might reduce the risk of non-melanoma skin cancer. But even then, the authors concluded that among people who are adequately nourished, there was no convincing evidence that adding supplements would prevent cancer.

The latest findings on the failings of selenium supplements are mirrored by results from a trial of 300 men with a high risk of prostate cancer by Canadian urologists. Published this month, the study also found that a combination of selenium, vitamin E and soy in a supplement had no significant effect on the risk of developing the illness.

According to Rippon, the results of supplement trials are often seized upon and linked to possible "cures" before there is any credible evidence. "Just because something kills cancer cells in a Petri dish in a laboratory doesn't mean it will work elsewhere. Washing-up liquid could kill cells in a dish but that doesn't mean you want people to take it.

"What I'd like to see is for the supplement-makers to pour millions of pounds into long-term cohort trials stretching over 20 or 30 years and looking at how people taking supplements fare in the long term compared with those who don't. Then we'd really have something to work with."