Vitamin D is classified as a vitamin, although it is naturally produced in the skin under the influence of UV rays from the sun. During the autumn and winter months, when natural sunlight exposure is limited, there is a risk of vitamin D deficiency. Cross-sectional studies have shown that 90% of healthy adult women and men have vitamin D deficiencies, i.e., low levels of 25-hydroxyprovitamin D (25(OH)D)