The Selenium and Vitamin E Cancer Prevention Trial involving 35, 000 U.S. and Canadian men was halted in October when researchers determined that selenium was not protecting the men from prostate cancer and may have been causing diabetes in some of them.
In winter, experts say, people in the northern half of the U.S. don't receive enough UVB rays from the sun to make vitamin D, regardless of whether they use sunscreen.
In 1980 researchers at Johns Hopkins University noticed that northern states in the U.S. had higher colon cancer death rates than southern ones and theorized that vitamin D might be responsible.
Very recently the Institute of Medicine of the prestigious U.S. National Academy of Science published a review of the scientific literature and noted that vitamin D may be linked to heart disease, immune function, cancer prevention and diabetes.