Vitamin D attracted scientific interest during the widespread epidemic of rickets at the beginning of the 20th century. Since then the skeletal benefits of this vitamin have been well established. However, several non-classical functions of vitamin D have only recently been elucidated: the ability of vitamin D to regulate the cell cycle and immunity, its influence on the production of renin, insulin, cytokines, neurotransmitters and cathelicidine. In the light of these new and diverse physiological functions of the „sunshine vitamin“ the threshold for insufficiency has been raised to 75 nmol/L (30 ng/mL). Using these new criteria for insufficiency, a large proportion of the Estonian population is shown to have suboptimal vitamin D levels in winter.