JOHN G. HADDAD JR., M.D.
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Two sources provide vitamin D to man: photoconversion of 7-dehydrocholesterol to cholecalciferol (vitamin D3) during skin exposure to effective ultraviolet light, and intestinal absorption of dietary vitamin D. In addition to the absorption of dietary vitamin D, the small intestine is the site for absorption of vitamin D and metabolites which enter the intestine via the bile. The vitamin and its metabolites are transported in blood via a specific binding protein that uniquely has both high-affinity and high-capacity characteristics. Vitamin D is transformed in the liver to 25-hydroxyvitamin D (25-OHD), which then undergoes 1-hydroxylation to form 1,25-dihydroxyvitamin D (1,25-(OH)2D) in
HADDAD JG. Vitamin D Economy in Gastrointestinal Disease. Ann Intern Med. 1977;87:629–630. doi: https://doi.org/10.7326/0003-4819-87-5-629
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Published: Ann Intern Med. 1977;87(5):629-630.
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