Women who take a daily multivitamin pill to ward off illness may actually be increasing their risk of breast cancer, according to a study. Researchers found middle-aged and older women who regularly took supplements were almost 20 per cent more likely to develop a tumour.
They stressed the findings did not prove vitamin pills were to blame for an increase in cancer cases, as it is possible women may be compensating for an unhealthy lifestyle that puts them at increased risk.
However, the experts warned the results were worrying and called for in-depth studies to determine whether or not multivitamins are safe. They believe supplements may trigger tumour growth by increasing the density of breast tissue, a known risk factor for cancer.
Studies suggest taking supplements containing vitamins and minerals may increase breast tissue by more than 5 per cent. It is also possible folic acid found in multivitamin pills could be a factor, as studies suggest high doses may promote tumour growth.
Experts from the Karolinska Institute in Stockholm, Sweden, tracked more than 35,000 women aged between 49 and 83 over a ten-year period. They found those who regularly took multivitamins were 19 per cent more likely to have developed a breast tumour.
Even when researchers took account of whether the women smoked, took much exercise, or had a family history of the disease - all strong risk factors - they still found a significant link with multivitamin use.
They told the American Journal of Clinical Nutrition: 'These results suggest multivitamin use is associated with an increased risk of breast cancer. This is of concern and merits further investigation.'
Researchers stressed that, on an individual basis, the risks to women remain small and the vast majority of vitamin users will not develop cancer. In the study, women did not say what brands of vitamins they took - they simply reported whether or not they took them.
The study could also be flawed as it relies on women to recall whether they took the pills in the past. Studies that ask people to describe past behaviour are vulnerable to a well-known statistical phenomenon called recall bias.
It is estimated nearly a quarter of all UK adults take antioxidant supplements or multivitamins on a regular basis. The market for supplements is worth £500million a year. But in 2007 a study of nearly 300,000 men found those taking supplements more than once a day were 32 per cent more likely to develop an aggressive form of prostate cancer.
And a 2008 Copenhagen University investigation found high doses of vitamin A, vitamin E and beta-carotene appeared to increase the chances of an early death.
Every year around 40,000 women in Britain are diagnosed with breast cancer, the equivalent of more than 100 a day. A woman has a one in nine chance of developing the disease at some point in her life.
Kat Arney, Cancer Research UK's science information manager, said last night: 'Like several other recent studies, this research adds to the evidence that multivitamins may not actually be beneficial for your health.
'Most can get all the nutrients they need from a healthy balanced diet, rich in fruit and vegetables.'
Dr Gilbert Ross, medical director for the American Council on Science and Health, said: 'If you really want to take multivitamins, this study is no reason to stop. Of course, on the other hand, I would advise anyone concerned that there is no good health or medical reason to take multivitamin supplements, except in rare cases of malnutrition.'
The Health Supplements Information Service, funded by supplements manufacturers, said: 'This does not provide any proof that multivitamins are linked to breast cancer.
'Given the low intakes of micronutrients in women across the UK and the continuing lack of improvement in our national dietary patterns, a multivitamin can make an important contribution to the vitamin and mineral intake in this population group.'
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