Vitamin D, known as the "sunshine vitamin," is not a true vitamin because it can be manufactured in the body in the presence of sunlight. It was classified as a vitamin because researchers discovered a fat-soluble factor D in the diet, and exposure to sunlight prevented rickets, a childhood disease characterized by improper bone development. Vitamin D has an inactive form, D3, and an active form.
D3 (Cholecalciferol)
The compound made in your skin from exposure to sunlight is a prohormone called cholecalciferol, or vitamin D3. It is a fat-soluble sterol that controls calcium metabolism in bone formation. Cholecalciferol, commonly shortened to calciferol, is then activated by two enzymatic reactions in the liver and kidneys to become the active form of Vitamin D, calcitriol.
Calcitriol
Calcitriol is synthesized in the kidneys and functions similar to a steroid hormone. It works with parathyroid hormone in the regulation of blood calcium concentrations, which play an important role in the formation and maintenance of bone, kidney, intestine, brain, heart, muscle, skin and immune system tissues.
Mechanism of Action
Dietary vitamin D is absorbed in the intestine wall, carried through the bloodstream, and distributed to the tissues via chylomicrons and vitamin-D-binding protein. Cholecalciferol diffuses from the skin and is picked up by D-binding protein and transported to the liver, where it undergoes the first reaction. Following this conversion, it is released into the bloodstream and taken up by the kidneys, where a second reaction takes place resulting in the active form calcitriol.
Sources, Deficiencies and Toxicities
Natural sunlight is the most readily available source of vitamin D. Food sources include foods of animal origin, dairy products and saltwater fish. In infants and children, rickets results from vitamin D deficiencies. Rickets is characterized by failure of the bones to mineralize, resulting in deformities in the legs, pelvis and spine. In adults, osteomalacia and osteoporosis can result from deficiencies. Calcium and phosphorus are no longer absorbed, and the bones become soft and brittle. Hypercalcemia results from excess dietary vitamin D consumption. This leads to the calcification of tissues, including the kidneys, heart, lungs and blood vessels. The adequate intake of vitamin D for both men and women from birth to the age of 50 is 5 micrograms, or 200 IU per day with a tolerable upper limit of 50 mcg/day, or 2,000 IU.
References
- "Advanced Nutrition and Human Metabolism (third edition)"; James Groff, Sareen Gropper; 2000
- "Basic Nutrition and Diet Therapy": Sue Rodwell Williams; 2001
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