Low serum phosphorus levels stimulate calcitriol synthesis, whereas high serum phosphorus levels inhibit it. The renal synthesis of calcitriol is tightly regulated by two counter-acting hormones, with up-regulation via parathyroid hormone ( PTH) and down-regulation via fibroblast-like growth factor-23 ( FGF23) ( Galitzer et al., 2008 Bergwitz and Juppner, 2010). Overview of vitamin D synthesis, intake, and activation. DBP also transports vitamin D and calcitriol. 25OHD, the precursor of calcitriol, is the major circulating form of vitamin D it circulates bound to a specific plasma carrier protein, vitamin D binding protein ( DBP). The 1α-hydroxylase gene is also expressed in several extra-renal tissues, but its contribution to calcitriol formation in these tissues is unknown. The second reaction takes place in the kidney, mediated by 1α-hydroxylase (CYP27B1), which converts 25OHD to the biologically active hormone, calcitriol ( 1,25-dihydroxyvitamin D). The first takes place in the liver, mediated by the 25-hydroxylase (most likely cytochrome P450 2R1 ) which forms 25-hydroxyvitamin D (hereafter referred to as 25OHD). Vitamin D, in either the D 2 or D 3 form, is considered biologically inactive until it undergoes two enzymatic hydroxylation reactions. The activation steps involved in converting vitamin D from the diet and cutaneous synthesis are illustrated in Figure 3-1. Experimental animal studies have indicated that vitamin D 2 is less toxic than vitamin D 3, but this has not been demonstrated in humans. When activated, the D 2 and D 3 forms have been reported to exhibit identical responses in the body, and the potency related to the ability to cure vitamin D–deficiency rickets is the same ( Fieser and Fieser, 1959 Jones et al., 1998 Jurutka et al., 2001). The differences do not affect metabolism (i.e., activation), and both forms function as prohormones. The D 2 and D 3 forms differ only in their side chain structure. Both vitamin D 3 and vitamin D 2 are synthesized commercially and found in dietary supplements or fortified foods.
ESSENTIAL ANATOMY 3 FOR ORGANIZATIONS التفعيل SKIN
Vitamin D 2 (ergocalciferol) is largely human-made and added to foods, whereas vitamin D 3 (cholecalciferol) is synthesized in the skin of humans from 7-dehydrocholesterol and is also consumed in the diet via the intake of animal-based foods. The two major forms are vitamin D 2 and vitamin D 3. Vitamin D, also known as calciferol, comprises a group of fat-soluble seco-sterols.
These dual sources of vitamin D make it challenging to develop dietary reference intake values. A unique aspect of vitamin D as a nutrient is that it can be synthesized by the human body through the action of sunlight. Vitamin D, first identified as a vitamin early in the 20th century, is now recognized as a prohormone.
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