Millions of people who take dietary supplements to ward off cancer may be toying with a “two-edged sword” that might do them harm, experts have warned. A team of US scientists said there was no good evidence that supplement pills and capsules reduced the risk of cancer in healthy individuals, pointing out that antioxidants such as as beta carotene, and vitamins C and E might even have biological effects that promote cancer. Antioxidants are believed to counter the destructive effects of rogue oxygen molecules called free radicals. Oxidative stress by free radicals, which attack cell membranes, proteins and DNA, has been linked to cancer and heart disease. But the US authors, writing in the Journal of the National Cancer Institute, argue that the supposed benefits of antioxidant supplements are largely a myth, saying people were misled by “messages from supplement manufacturers” stressing the health benefits of their products, including cancer prevention. The panel of five experts, led by Dr Maria Elena Martinez, from the University of California at San Diego, wrote: “Undoubtedly, use is driven by a common belief that supplements can improve health and protect against disease, and that at worst, they are harmless. However, the assumption that any dietary supplement is safe under all circumstances and in all quantities is no longer empirically reasonable.” Health supplements are booming in the US, with annual sales estimated at 30 billion dollars (£18.6 billion), said the scientists, who assessed the evidence relating to several supplements including antioxidants, folic acid, vitamin D and calcium. A number of animal, laboratory and observational studies had appeared to show that dietary supplements could lower cancer risk, they said. However, these findings were not confirmed by the “gold-standard” in evidence-based medicine, randomised controlled trials (RCTs). Only a small number of RCTs had been carried out to test the effectiveness of dietary supplements, said the experts – and several of these had reported increased risks. Experimental studies had shown that different tissues with different cancer-triggering pathways may not respond the same way to a particular nutrient. “In fact, a nutrient may be associated with protection in one tissue and harm in another,” said the experts. Dr Carrie Ruxton from the Health Supplements Information Service, which is funded by supplement manufacturers, said: “It is vital to note that in the UK all supplement claims are strongly regulated and companies cannot make claims that supplements prevent cancer. “The primary purpose of food supplements is to ensure nutritional sufficiency; they are not intended to treat, prevent or cure any disease. Indeed it is illegal to claim or imply that they could do any of these things.”

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