ABSTRACT
Vitamin D has long been known as the major regulator of calcium homeostasis, and adequate serum vitamin D levels are required for bone mineralization. Vitamin D maintains calcium balance through actions in the intestines, bones, kidneys, and parathyroid glands (Feldman et al. 2007). The extraskeletal benecial effects of vitamin D are increasingly recognized. The vitamin D receptor (VDR) has been demonstrated in nearly all tissues throughout the body. VDR binding results in local tissue-specic calcitriol synthesis, including many cancers. Calcitriol is the active
13.1 Vitamin D Background .........................................................................................................265 13.1.1 Vitamin D Synthesis and Forms ...............................................................................265 13.1.2 Vitamin D Degradation ............................................................................................266 13.1.3 Increased Vitamin D Supplement Sales and Volume of Testing ..............................266 13.1.4 Vitamin D Mechanisms of Action ............................................................................ 267 13.1.5 Serum Vitamin D Measurement and Sufciency ..................................................... 267 13.1.6 D2 versus D3 Supplementation .................................................................................. 267
13.2 Vitamin D Clinical Studies ..................................................................................................268 13.2.1 Epidemiologic Studies of Vitamin D and Cancer Incidence ....................................268 13.2.2 Women’s Health Initiative ........................................................................................268
