VITAMIN D

Vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol) are stored in body fat. The vitamin D precursors produced in yeast and plants (ergosterol) and animals (7-dehydrocholesterol) are converted to vitamin D by exposure to ultraviolet light (290-315 nm). Vitamin D (either vitamin D2 or vitamin D3) is metabolized in the liver to 25-hydroxyvitamin D and then to 1, 25-dihydroxyvitamin D in the kidney. 1, 25-Dihydroxyvitamin D is considered to be the biologically functioning form of vitamin D. The major functions of vitamin D are to increase the efficiency of intestinal calcium absorption and to mobilize calcium stores from bone in order to maintain the serum calcium and phosphorus concentrations within the normal physiological range.

Deficiencies: In humans, deficiency symptoms include rickets in children, osteomalacia in adults, muscle weakness, bony deformities, neuromuscular irritability causing muscle spasms of the larynx (laryngospasm) and hands (carpopedal spasm), generalized convulsions and tetany.

Clinical uses: Vitamin D is useful for preventing and treating vitamin D-deficiency bone diseases (rickets in children and osteomalacia in adults). 25-Hydroxyvitamin D3 is useful for treating disorders, such as severe liver failure, in which vitamin D cannot be metabolized to 25-hydroxyvitamin D. The active form of vitamin D (1, 25-dihydroxyvitamin D3) and its analogs are useful for treating metabolic bone disorders due to inborn and acquired disorders in the metabolism of 1, 25-dihydroxyvitamin D2. These have recently been shown to also be valuable in treating the skin disease psoriasis.

Diet recommendations: The Institute of Medicine (IOM) for the National Academy of Sciences issued a report in August 1997 regarding new dietary guidelines for vitamin D. After careful review of the literature, the IOM concluded that it was not possible to determine a recommended dietary allowance (RDA) for vitamin D from the literature but rather to recommend an adequate intake (AI). Based on the available literature and assuming some exposure to sunlight, an AI for ages 0 - 50 years was set at 200 IU (5 mg)/day. The IOM panel recognized that vitamin D insufficiency and deficiency are prevalent in adults over the age of 50 years and set the AI for adults 51 - 70 years as 400 IU (10 mg)/day and for adults > 71 years, 600 IU (15 mg)/day. There was no compelling data to increase the vitamin D requirement either during pregnancy or lactation. A Tolerable Upper Limit level for vitamin D for ages 0 - 12 months was set at a limit of 1,000 IU (25 mg)/day. For older children and adults, including pregnant and lactating women, the UL was set at 2,000 IU (50 mmg)/day.

Food sources: Good food sources are milk properly fortified with vitamin D, fatty fish such as salmon and mackerel, cod liver oil, fish liver oil, some breads and cereals, and some egg yolks.

Toxicity: Excessive quantities of vitamin D (in excess of 5,000-10,000 IU/day) can cause hypercalcemia, hypercalciuria, kidney stones, and soft tissue calcifications.

Recent research: Epidemiological evidence suggests that there may be a correlation with increased exposure to sunlight with decreased risk of colon, breast and prostrate cancer. Whether this is due to increased production of vitamin D in the skin remains unknown. 1, 25-dihydroxyvitamin D3 is a potent antiproliferative agent for tumor cells and normal cells that possess a vitamin D receptor. 1,25-dihydroxyvitamin D3 also has been shown to be of value in treating osteoporosis, especially in patients who are calcium deficient.

For further information:

Holick, M. F. (1994) Vitamin D-new horizons for the 21st century. Am. J. Clin. Nutr. 60: 619-630.

DeLuca, H. F. (1988) The vitamin D story: a collaborative effort of basic science and clinical medicine. FASEB J. 2: 224-236.

Prepared By:

Michael F. Holick, M.D., Ph.D.
Professor, M-1013
Boston University School of Medicine
80 E. Concord Street
Boston, MA 02118-2394
Phone: 617-638-4545
FAX: 617-638-8882
Email: mfholick@bu.edu

Ronald L. Horst, Ph.D.
Supervisory Research Physiologist
National Animal Disease Center
P.O. Box 70
2300 Dayton Avenue
Ames, IA 50010-0070
Phone: 515-239-8312
FAX: 515-239-8669
Email: rhorst@nadc.ars.usda.gov