At this time, much about the relationship between vitamin
D and breast cancer remains unknown. Relatively few epide-
miologic studies have addressed the association, and only
a handful of these have been prospective, used biochemical
measurements of vitamin D, or been large enough to permit
analyses stratified by other factors. Despite many inconsis-
tencies, the potential exists that vitamin D may modestly
reduce the risk of breast cancer. Many questions clearly
remain, including those concerning the utility of assessing
vitamin D through diet and sunlight exposure, the most
appropriate timing of assessment, the relationship between
the two important plasma metabolites, and potential modi-
fying effects of factors such as age, menopausal status, and
tumor characteristics. Given that vitamin D status is fairly
easily modifiable through increased sunlight exposure and/
or dietary modification, further study is necessary to deter-
mine if vitamin D may have important potential for breast
cancer prevention.