Sources and Metabolism of Vitamin D

Humans get vitamin D from exposure to sunlight, from their diet, and from dietary
supplements .1-4 A diet high in oily fish prevents vitamin D deficiency.
3 Solar ultraviolet B radiation (wavelength, 290 to 315 nm) penetrates the skin and converts 7-dehydrocholesterol to previtamin D3, which is rapidly converted to vitamin D3 .
1 Because any excess previtamin D3 or vitamin D3 is destroyed by sunlight
(Fig. 1), excessive exposure to sunlight does not cause vitamin D3 intoxication.
2 Few foods naturally contain or are fortified with vitamin D. The “D” represents
D2 or D3 (Fig. 1). Vitamin D2 is manufactured through the ultraviolet irradiation
of ergosterol from yeast, and vitamin D3 through the ultraviolet irradiation of 7-dehydrocholesterol from lanolin. Both are used in over-the-counter vitamin D supplements,but the form available by prescription in the United States is vitamin D2.
Vitamin D from the skin and diet is metabolized in the liver to 25-hydroxyvitamin
D (Fig. 1), which is used to determine a patient’s vitamin D status1-4; 25-hydroxyvi-
tamin D is metabolized in the kidneys by the enzyme 25-hydroxyvitamin D-1α-
hydroxylase (CYP27B1) to its active form, 1,25-dihydroxyvitamin D.
1-4 The renal production of 1,25-dihydroxyvitamin D is tightly regulated by plasma parathyroid hormone levels and serum calcium and phosphorus levels.
1-4 Fibroblast growth factor 23, secreted from the bone, causes the sodium–phosphate cotransporter to be internalized by the cells of the kidney and small intestine and also suppresses 1,25-dihydroxyvitamin D synthesis.
5 The efficiency of the absorption of renal calcium
and of intestinal calcium and phosphorus is increased in the presence of 1,25-dihy-
droxyvitamin D (Fig. 1).
2,3,6 It also induces the
ex pression of the enzyme 25-hydroxyvitamin
D-24-hydroxylase (CYP24), which catabolizes both
25-hydroxyvitamin D and 1,25-dihydroxyvita-
min D into biologically inactive, water-soluble
calcitroic acid.
2-4