Vitamin D plays a role in many physiological functions. Vitamin D3
can be synthesized in the skin via chemical reactions utilizing energy from
ultraviolet B (UV-B) rays. It goes through two
hydroxylation reactions in the liver and kidneys, converting it to the most
activated form of vitamin D. The efficiency of these reactions tends to
decrease with age. In addition, sunscreen usage, environmental conditions, vegetarian
diet, low intake of vitamin D fortified foods and other factors may all reduce
vitamin D intake or production.*
Vitamin D is essential in promoting calcium homeostasis. Vitamin D
promotes intestinal calcium and phosphorus absorption and reduces urinary
calcium loss. These are essential mechanisms for maintaining healthy
calcium levels in the body and for healthy bone composition, which has been supported by clinical studies involving supplementation of both vitamin D and calcium, as well as vitamin D alone. Vitamin D may also provide cardiovascular
health in some individuals, which may be attributed to its effect on calcium
metabolism or possibly by helping to maintain healthy plasma renin function.*
A role for vitamin D in supporting colon health by promoting healthy cellular
function has also been suggested. Vitamin D is also believed to provide
general cellular support potential, including for the breast and prostate, in
part by helping to maintain healthy angiogenesis balance, supporting immune cell
activity and maintaining healthy cell metabolism.*
Preliminary evidence suggests that vitamin D may also play a role in
maintaining healthy glucose metabolism, due to the presence of vitamin D receptors on
the islet cells of the pancreas.*
Animal studies also indicate the presence of vitamin D receptors in areas of
the brain involved in both memory and cognition. Additional studies
indicate that vitamin D may help support healthy neural tissue, however more research is needed to understand the specific roles of vitamin D.*
Furthermore, vitamin D may play a role in promoting a healthy immune system, in part
by maintaining healthy cytokine activities and promoting the function of immune cells.*
NOTE: Hypercalcemia is associated with chronic vitamin D intakes of
50,000-200,000 IU per day. Published cases of vitamin D toxicity all
involved an intake in excess of 40,000 IU per day. Hypercalcemia may cause
anorexia, nausea, and vomiting. Prolonged hypercalcemia may result in the
calcification of soft tissues, including the kidney, heart, and lungs. Use
of vitamin D in conjunction with digoxin should be closely supervised by a
physician.*
