The health benefits of vitamin D greatly outweigh the health risks.

Starting in the 1980s, largely observational evidence mounted that solar ultraviolet-B (UVB) irradiance and vitamin D reduce the risk of many types of cancer. Based on observational studies, it is estimated that 1500 and 3600 International Units (IU) of vitamin D3 are required daily for a 50% reduction in risk of colorectal and breast cancer, respectively. A recent randomized, double-blind, placebo-controlled clinical trial that studied post-menopausal women in Nebraska found a 77% reduction in all-cancer incidence between the ends of the first and fourth years, adding strong support to the observational studies.