Vitamin D exists in 2 forms that are important regarding vitamin D status and supply in cattle: vitamin D2 (D2) and vitamin D3 (D3). To become physiologically active, both D2 and D3 must undergo 25-hydroxylation in the liver. The resulting 25-hydroxyvitamin D2 [25(OH)D2] and 25-hydroxyvitamin D3 [25(OH)D3] are measured as indicators of the physiological vitamin D status of cattle. The study used 14 Danish Holstein cows housed without access to sunlight. The cows were orally administered 250 mg (1.0 × 107 IU) of D2 and D3 in a cross-over design with 2 treatment groups and 2 study periods, rendering 4 treatments when carryover effects were taken into account: D2 given first, D2 given last after D3, D3 given first, and D3 given last after D2. Two weeks elapsed between the treatment in the first study period and the treatment in the second study period. Blood samples were collected 0, 3, 6, 14, 17, 20, 23, 26, 40, 48, 70, 94, 166, and 214 h after providing the oral bolus of vitamin to the cows. Comparisons between plasma levels of the metabolites D2, D3, 25(OH)D2, and 25(OH)D3 over time were made by comparing areas under the plasma concentration curves. Oral administration of D3 increased plasma D3 (182.6 ± 17.1 ng/mL; mean ± SEM) and 25(OH)D3 (103.5 ± 10.0 ng/mL) more efficiently than oral administration of D2 increased plasma D2 (49.1 ± 32.6 ng/mL) and 25(OH)D2 (27.9 ± 2.1 ng/mL). The D3 given after an oral dose of D2 was less efficient for increasing plasma concentrations of 25(OH)D3 (61.2 ± 12.0 ng/mL) compared with D3 given without previous D2 administration (103.5 ± 10.0 ng/mL), whereas the plasma concentrations of D3 itself were the same when given first (182.6 ± 17.1 ng/mL) as when given after D2 (200.0 ± 123.9 ng/mL). The same occurred for plasma concentrations of D2 metabolites both if D2 was given first (49.1 ± 32.6 ng/mL) and after D3 (54.7 ± 7.7 ng/mL). In conclusion, D3 given after D2 is less efficient at increasing the plasma status of 25(OH)D3 than D3 given without previous D2 administration.
