Vitamin D and cancer: current dilemmas and future research needs.

A diversity of scientific literature supports a role for vitamin D in decreasing colorectal cancer incidence, but the available evidence provides only limited support for an inverse association between vitamin D status and the risk of other types of cancer. We need additional studies analyzing the dose-response relation between vitamin D status and cancer risk, the optimal level of 25-hydroxyvitamin D, the length of time required to observe an effect, and the time period of life when exposure is most relevant. Studies of vitamin D receptor polymorphisms have found that not all polymorphisms have the same association with cancer, and the cancer site could further dictate which polymorphisms might be most important; this indicates a need for more research on gene-environment interactions. Several dietary components and the balance between energy intake and expenditure influence vitamin D metabolism. These studies show that scientists need to identify confounders and modifiers of the biological response to vitamin D, including dietary factors, lifestyle factors such as exercise, and race or ethnicity. Transgenic and knockout animals are powerful tools for identifying the molecular targets of bioactive food components. Scientists should therefore make increased use of these models to identify molecular targets for vitamin D. Many research gaps relate to the need to develop predictive, validated, and sensitive biomarkers, including biomarkers that researchers can use to reliably evaluate intake or exposure to vitamin D, assess one or more specific biological effects that are linked to cancer, and effectively predict individual susceptibility as a function of nutrient-nutrient interactions and genetics.