Women who regularly take multivitamin pills may face a higher risk of breast cancer, a new study has claimed.
The Swedish study, which looked at more than 35,000 women aged between 49 and 83 over a period of 10 years, found that those who regularly took vitamin supplements were 19 per cent more likely to develop a tumour, the Daily Mail reported.
According to researchers at Karolinska Institute in Stockholm, supplements may trigger tumour growth by increasing the density of breast tissue, a known risk factor for cancer.
However, 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.
Although the results were worrying, the researchers called for in-depth studies to determine whether or not multivitamins are safe. "These results suggest multivitamin use is associated with an increased risk of breast cancer. This is of concern and merits further investigation," they told the American Journal of Clinical Nutrition .
Earlier studies have suggested that high doses of folic acid found in multivitamin pills may promote tumour growth. For the latest study, the researchers took account of whether the women smoked, did much exercise, or had a family history of the disease -- all strong risk factors -- but still they found a significant link with multivitamin use.
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.
But in 2007 a study of nearly 3,00,000 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: "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."