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Vitamin D

by Larry E. Johnson, MD, PhD

Vitamin D has 2 main forms:

  • D 2 ( ergocalciferol )

  • D 3 (cholecalciferol): The naturally occurring form and the form used for low-dose supplementation

Vitamin D 3 is synthesized in skin by exposure to direct sunlight (ultraviolet B radiation) and obtained in the diet chiefly in fish liver oils and salt water fish. In some developed countries, milk and other foods are fortified with vitamin D. Human breast milk is low in vitamin D, containing an average of only 10% of the amount in fortified cow’s milk. Vitamin D levels may decrease with age because skin synthesis declines. Sunscreen use and dark skin pigmentation also reduce skin synthesis of vitamin D.

Vitamin D is a prohormone with several active metabolites that act as hormones. Vitamin D is metabolized by the liver to 25(OH)D, which is then converted by the kidneys to 1,25(OH) 2 D (1,25-dihydroxycholecalciferol, calcitriol , or active vitamin D hormone). 25(OH)D, the major circulating form, has some metabolic activity, but 1,25(OH) 2 D is the most metabolically active. The conversion to 1,25(OH) 2 D is regulated by its own concentration, parathyroid hormone (PTH), and serum concentrations of Ca and phosphate.

Vitamin D affects many organ systems (see Table: Actions of Vitamin D and Its Metabolites), but mainly it increases Ca and phosphate absorption from the intestine and promotes normal bone formation and mineralization. Vitamin D and related analogs may be used to treat psoriasis, hypoparathyroidism, and renal osteodystrophy. Vitamin D's usefulness in preventing leukemia and breast, prostate, and colon cancers has not been proved.

Actions of Vitamin D and Its Metabolites




Promotes bone formation by maintaining appropriate Ca and phosphate concentrations

Immune system

Stimulates immunogenic and antitumor activity

Decreases risk of autoimmune disorders


Enhances Ca and phosphate transport (absorption)


Enhances Ca reabsorption by the tubules

Parathyroid glands

Inhibits parathyroid hormone secretion


Stimulates insulin production

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