Vitamin D — Solar Rays, the Milky Way, or Both?

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Abstract
Adequate vitamin D stores are essential for normal skeletal mineralization and growth. The vitamin is now known to be a prohormone that undergoes successive hydroxylation in the liver and kidney. The first hydroxylation yields 25-hydroxyvitamin D (25(OH)D), and the second the active hormone, 1,25-dihydroxyvitamin D (l,25(OH)2nd). This hormone travels in blood to intracellular receptors that regulate gene transcription and thereby stimulates intestinal absorption of calcium and enterocyte differentiation, regulates osteoblast and hematopoietic-cell functions, and suppresses the activity of the parathyroid gland.1 Without adequate reserves of the vitamin, insufficient l,25(OH)2D is produced, leading to poor calcium absorption, soft (undermineralized) . . .