25-hydroxyvitamin D circulates in different fractions of calf plasma if the parent compound is vitamin D2or vitamin D3, respectively

Abstract

Vitamin D has become one of the most discussed nutrients in human nutrition, which has led to an increased interest in milk as a vitamin D source. Problems related to fortifying milk with synthetic vitamin D can be avoided by securing a high content of natural vitamin D in the milk by supplying dairy cows with sufficient vitamin D. However, choosing the most efficient route and form of supplementation requires insight into how different vitamin D metabolites are transported in the body of cattle. There are two forms of vitamin D: vitamin D2(D2) and vitamin D3(D3). Vitamin D2originates from fungi on roughage. Vitamin D3originates either from endogenous synthesis in the skin or from feed supplements. Vitamin D2is chemically different from, and less physiologically active than, D3. Endogenous and dietary D3is chemically similar but dietary D3is toxic, whereas endogenous D3appears well regulated in the body.