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 [1]. Vitamin D regulates the calcium and phosphate balance, as well as bone mineralization [2] , and vitamin D deficiency leads to

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... y 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 mg/dL) and mar-