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

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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)2D, 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)2D in the kidneys or target organs [1]. 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 [3, 4].

Vitamin D status is determined by measuring the serum concentration of 25(OH)D, the major circulating form of the hormone [5]. The emerging consensus is that vitamin D insufficiency be defined as serum 25(OH)D levels of <80 nmol/L [6, 7]. Diet is the most important source of vitamin D in northern latitudes during the wintertime, because sunlight exposure during this time is inadequate for inducing the endogenous production of vitamin D. Vitamin D deficiency is common in all age groups in Finland from October through March [8]. As a public health policy, vitamin D fortification of liquid milk products (0.5 µg/dL) and margarines (10 µg per 100 g) has been implemented in Finland since February 2003.