Vitamin D is particularly important for bone health because of the role it plays in calcium homeostasis.
The active form of vitamin D stimulates the synthesis of calcium binding protein in the intestine to facilitate calcium transport across the intestine. Vitamin D also plays a regulatory role in renal calcium reabsorption and in calcium release from bone.
Vitamin D can be obtained from the diet (although only vitamin D-fortified milk and fatty fish provide substantial amounts) or is made in the skin following exposure to sunlight. Deficiency of this vitamin is increasingly recognized as an issue of concern across all age groups of the US population from neonates to the elderly. This deficiency is due to a combination of inadequate dietary intake (dairy products provide the largest dietary contribution to vitamin D intake) and to inadequate sunlight exposure. Rickets is increasing among exclusively breastfed minority infants in the United States. This is thought to be due to the low vitamin D content of human milk combined with insufficient endogenous dermal synthesis. Vitamin D deficiency in adults results in osteomalacia and secondary hyperpar-athyroidism, increasing bone resorption and the risk of osteoporosis.
Lack of sufficient endogenous production of vitamin D in the skin is influenced by geographical location (more northern latitudes have a shorter season during which the wavelength needed for vitamin D synthesis is available), increased use of sunscreen and cosmetics and skin care products containing sunscreen (sunscreens with SPF values of 8 or greater block the dermal production of vitamin D), and lifestyle factors that decrease exposure to sunlight.
Studies suggest that the optimal serum concentration of vitamin D may be markedly higher (>30 ng/ml) than that traditionally used to define vitamin D deficiency (<10-15 ng/ml). If these increased levels are eventually accepted as optimal target concentrations, an even greater fraction of the population will have suboptimal status of this vitamin.
Supplementation with vitamin D and calcium has been found to be effective in decreasing fracture incidence. Several studies in older adults have found significant relationships between vitamin D status (as determined by 25-hydroxyvitamin D concentrations) and both musculoskeletal function and risk of sarcopenia. Combined vitamin D and calcium supplementation in the elderly may also decrease the risk of falling. Individuals with low dairy product intake, those living in northern latitudes, or those with inadequate sunlight exposure may need to rely on supplemental sources of vitamin D to maintain circulating concentrations at optimal levels required to promote bone health.
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