To anyone glibly dismissing this study could I draw your attention to the fact that this is one of the LARGEST and longest running health studies ever carried out in history. It is a masive study of 38,000 women! Yes, you read that right - 38,000.
Researchers from the University of Minnesota examined data from more than 38,000 women taking part in the Iowa Women's Health Study, an ongoing study with women who were around age 62 at its start in 1986. The researchers collected data on the women's supplement use in 1986, 1997 and 2004...Women who took supplements had, on average, a 2.4 percent increased risk of dying over the course of the 19-year study, compared with women who didn't take supplements
I believe iron is the main culprit. The vast majority of people have healthy levels of iron yet many multivitimins and breakfast cereals contain huge quantities of the stuff. After the menopause (or hysterectomy) women need considerably less iron once their periods stop, yet this is precisely when many start to take supplements, often encouraged by well meaning health advisers and doctors.
Excess iron is now known to be associated with almost all neurological disease. It is also known that cancer cells utilise iron as a primary growth factor. Once in the body excess iron cannot be removed.
The vitamins and minerals in supplements are usually in an unnatural forms that often don't occur or are consumed in nature, for example, the iron in your cornflakes is a kind of iron filings dust that you would never eat in nature. It can get absorbed, stored or excreted in an abnormal fashions which we are begining to discover.
A similar longditudinal, prospective study (following about 90,000 people) a couple of years ago discovered that taking iron supplements increases the chances of Parkinson's Disease by 30%http://aje.oxfordjournals.org/content/168/12/1381.abstract