Vitamin D Status in the First 9 Months of Life

Danish people living at northern latitudes (56°N), often with overcast and foggy weather and few sunshine hours are at increased risk of vitamin D insufficiency. Although vitamin D is obtainable from fortified food and oily fish, the major source is the dermal synthesis of the vitamin D through exposure to solar ultraviolet light.

In the 19th century rickets was endemic in northern Europe, and many children developed rickets, a severe bone-deforming disease. Encouragement of sensible sun exposure, supplementation with cod liver oil and fortification of milk with vitamin D, resulted in an almost complete eradication of rickets by the end of the 19th century. At present vitamin D deficiency seems again to be more widespread. The classical outcomes of severe vitamin D deficiency are rickets in growing individuals and osteomalacia in adults. Low vitamin D status also relates to low bone density and increased risk of osteoporotic fractures. Although the incidence of rickets has declined over the last decades, cases attributable to inadequate vitamin D intake and low exposure to sunlight continue to be reported, and maternal vitamin D status may have permanent effects on newborns' health. Pregnant women, newborns, breastfed children, and lactating women are at a high risk of vitamin D deficiency, especially during winter and early spring.

Since fetal plasma 25-hydroxyvitamin D (25OHD) depends on maternal 25OHD, fetal vitamin D status may show seasonal changes parallel to those observed in the mothers. To prevent rickets and vitamin D deficiency in infants, most Western countries, including Denmark, recommend a daily maternal intake during pregnancy and lactation of 10 μg and that breastfed children are given a supplement of 10 μg of vitamin D/day. Despite these recommendations, approximately one third of Danish pregnant and lactating women have vitamin D insufficiency. However, childhood rickets is nowadays rare in Denmark, although it still exists especially among immigrants and mothers with prolonged lactation. Accordingly, in 2010, the Danish National Board of Health extended the recommendations for vitamin D supplementation to the first two years of life.