Vitamin D is essential for bone health, being necessary for both the intestinal absorption of calcium and a correct skeletal mineralization. Vitamin D is normally synthesized in the skin upon exposure to ultraviolet rays (UVB). If there is insufficient exposure to natural sunlight, the serum 25-hydroxyvitamin D level—the best clinical indicator of the vitamin D status—drops down. In such cases, vitamin D supplements are indicated to avoid insufficient bone mineralization. In some countries, vitamin D fortified foods are available and such additional dietary intake is normally recommended. For example, the recommended dietary intake of vitamin D is 280 IU/day for children aged 3 years or more in the United Kingdom, and 200 IU/day for those aged 0 to 18 years in the United States.
There are no recommendations for supplementation during adolescence in many countries, in spite of serious evidence of vitamin D deficiency in those with dark skin who wear concealing clothing and live at latitudes where solar UVB is inadequate for cutaneous vitamin D synthesis during the winter months (50).
Young people with chronic illness may be at high risk of vitamin D deficiency because of reduced outdoor activities or the necessity to avoid sunlight exposure (e.g., in systemic lupus erythematosus or dermato-myositis). Liver, kidney, and intestinal dysfunction linked to many diseases can affect the metabolism and function of vitamin D. Finally, some drugs (e.g., corticosteroids, anticonvulsants, heparin, cyclosporine A, tacrolimus) can interfere with vitamin D metabolism. All these conditions can increase the risk of a true vitamin D deficiency, or of inappropriately low levels of circulating vitamin D metabolites.
Low serum vitamin D levels can induce an increase in parathyroid hormone secretion (secondary hyperparathyroidism), which can in turn influence bone health.
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