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DRI Dietary Reference Intakes Calcium Vitamin D
FIGURE 3-3 The metabolism of vitamin D3 from synthesis/intake to formation of metabolites. The process is the same for vitamin D2 once it enters the circulation.
NOTE: CYP = cytochrome P450 (a large and diverse group of enzymes).
SOURCE: Reprinted with permission from Hector DeLuca.
Production of the CYP27B1 enzyme is stimulated by PTH, which is secreted in response to a lack of calcium. It is also stimulated by the hypophosphatemic action of FGF23 on renal phosphate excretion, but to a lesser extent. When PTH is suppressed, or FGF23, produced by osteocytes, is stimulated, 1α-hydroxylation is markedly reduced (Liu et al., 2007; Quarles, 2008). Furthermore, calcitriol can act as a suppressor of CYP27B1, although the mechanism is not fully understood.
Calcitriol has its strongest metabolic activity in inducing its own destruction by stimulating the 24-hydroxylase enzyme (now known as CYP24A1;