Summary of roundtable discussion on vitamin D research needs.

We summarize the discussions of a roundtable following the conference "Vitamin D and Health in the 21st Century: an Update." The roundtable participants offered additional information on vitamin D research needs from a critical, impartial, and interdisciplinary perspective. Although the group recognized the progress to date, they found that the available evidence on the relation of 25-hydroxyvitamin D, dietary intake, status, functional health, and adverse outcomes has significant limitations because most studies have been short term, have failed to consider important confounders such as baseline vitamin D status and body mass index, and did not study key populations. To meet these data gaps, the roundtable identified several overarching research needs: 1) long-term, high-quality dose-response studies with relevant outcomes, including bone health, other functional outcomes (such as immune function, autoimmune disorders, and chronic disease prevention), and adverse outcomes (such as hypercalcemia and hypercalcuria), especially in understudied population groups such as dark-skinned individuals, infants, adolescents, reproductive-aged women, and pregnant and lactating women; 2) further research to understand the relation of 25-hydroxyvitamin D threshold values to relevant functional outcomes in each life stage and in racial and ethnic groups; 3) further research to understand the metabolic partitioning, fate, and mobilization of key vitamin D metabolites at recommended and greater than recommended intakes to assess the availability of stored vitamin D, relative contributions of endogenously produced and dietary vitamin D, and impact of important confounders (such as body mass index) on vitamin D status; and 4) further research to define the maximal, long-term vitamin D intake to ensure safety for all humans.