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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 D2 originates from fungi on roughage. Vitamin D3 originates either from endogenous synthesis in the skin or from feed supplements. Vitamin D2 is chemically different from, and less physiologically active than, D3. Endogenous and dietary D3 is chemically similar but dietary D3 is toxic, whereas endogenous D3 appears well regulated in the body.

Use of synthetic feed additives, including synthetic vitamin D3 (D3) in the feed for cows and other ruminants, is not consistent with the international principles of organic farming. If dairy farmers wish to produce in accordance with the organic principles, production animals would be left with only their endogenous production of D3 from summer sunlight as a source of D3. To examine the impact of supplemental synthetic D3 from the feed on the D3 status of dairy cattle in organic production in Nordic countries, 20 high-yielding dairy cows and 30 dairy steers were divided into two groups: one supplemented with synthetic D3 in the feed and one not supplemented with synthetic D3. Vitamin D3 status of the animals was assessed by measuring the concentration of the liver-derived 25-hydroxyvitamin D3 (25OHD3) in plasma. Results showed that 25OHD3 concentration in plasma from dairy cattle as well as from steers decreased during winter for both supplemented and unsupplemented groups. Unsupplemented cows and steers had approximately 2 ng 25OHD3 per ml plasma during winter, whereas supplemented animals had between 10 (cows) and 30 (steers) ng/ml. During summer and autumn there was no additive effect of supplementing with synthetic D3 since unsupplemented and supplemented animals had the same D3 status at this time of year. In all cows summer concentrations of 25OHD3 were 20-25 ng/ml and in all steers 40-50 ng/ml plasma. The decrease in vitamin D3 status during winter indicates that cows and steers are able to store D3 only to a limited extent. The results also show that cows or steers fed supplemental D3 according to Swedish recommendation throughout the year are not able to maintain their summer value of 25OHD3 during winter.