Taking too many vitamins 'raises prostate cancer risk'

Overdosing on vitamins can increase the risk of deadly prostate cancer, new research suggests.

US scientists investigated the impact of vitamin use on cancer in 295,344 men enrolled in a national diet and health study.

After five years, they found a surprising link between rates of advanced and fatal prostate cancer and men who regularly took multivitamins.

Those who used the supplements more than once a day were significantly more likely to suffer dangerous advanced forms of the disease than men who took no multivitamins.

The correlation was strongest for men with a family history of the disease, and who also took selenium, beta-carotene or zinc supplements.

No similar association was seen between heavy multivitamin use and localised prostate cancer.

The explanation for the findings, published in the Journal of the National Cancer Institute, remains a mystery.

Dr Karia Lawson and colleagues from the National Cancer Institute in Bethesda, Maryland, wrote: “Because multivitamin supplements consist of a combination of several vitamins, and men using high levels of multivitamins were also more likely to take a variety of individual supplements, we were unable to identify or quantify individual components responsible for the associations that we observed.

"Our findings of a markedly increased prostate cancer risk among men using multivitamin supplements is of concern and warrants further research.”

A total of 10,241 of the men studied were diagnosed with prostate cancer. Of these, 8,765 had localised cancers and 1,476 advanced cancers.

During a separate follow-up period of six years, 179 men died from the disease.

Men taking large amounts of multivitamins were 1.32 times more likely to develop advanced prostate cancer and nearly twice as likely to have a fatal condition as those who did not use multivitamins.

The vitamins and minerals found in fruits and vegetables are known to be essential to good health and long life.

However, some analyses of pooled trial data have suggested that beta-carotene, vitamin A and vitamin E supplements may shorten life rather than extend it.

The jury is still out on vitamin C and selenium. But researchers have shown that in high enough doses, vitamin C can damage instead of protect DNA.

The greatest controversy surrounds antioxidants, which mop up the destructive free radical molecules at the root of many diseases and ageing.

Two experts writing in an accompanying editorial in the Journal of the National Cancer Institute pointed out that moderate numbers of these reactive molecules may actually help the body get rid of unwanted cells.

Dr Goran Bjelakovic, from the University of Nis in Serbia, and Dr Christian Gluud, from Copenhagen University Hospital in Denmark, added: “If administration of antioxidant supplements decreases free radicals, it may interfere with essential defensive mechanisms for ridding the organism of damaged cells, including those that are pre-cancerous and cancerous.

“Thus, antioxidant supplements may actually cause some harm.”

The amounts of antioxidants needed to offer protection are not known and may differ between individuals, they stressed.