Vitamin D Supplementation for the Prevention of Acute Respiratory Tract Infection: A Randomized, Double‐Blinded Trial among Young Finnish Men

Vitamin D is formed in the skin from 7-dehydrocholesterol after activation induced by ultraviolet B (UVB) radiation (290-315 nm). Vitamin D is the precursor of the hormone 1,25(OH) 2 D, which is formed in 2 hydroxylation reactions, first to 25-hydroxyvitamin D, hereafter abreviatd 25(OH)D, in the liver and then to 1.25(OH) 2 D in the kidneys or target organs Vitamin D regulates the calcium and phosphate balance, as well as bone mineralization [2], and vitamin D deficiency leads to secondary hyperparathyroidism, which causes rickets in children and osteoporosis and osteomalacia in adults