Serum 25-hydroxyvitamin D(3) and 24R,25-dihydroxyvitamin D(3) concentrations in adult dogs are more substantially increased by oral supplementation of 25-hydroxyvitamin D(3) than by vitamin D(3)

We previously found a weak response in serum 25-hydroxyvitamin D(3) (25(OH)D(3)) concentrations when dogs were supplemented with oral vitamin D(3) (D(3)). In the present study, we determined the relative potency of oral 25(OH)D(3) compared with D(3) for increasing vitamin D status in dogs with low serum 25(OH)D concentrations. Four male and three female, 4-year-old, intact, lean, genetically related, Chinese-crested/beagle dogs were studied in a randomised, single cross-over trial. After feeding a low-vitamin D diet (<4 IU/100 g) for 30 d, four dogs received daily D(3) supplementation at 2·3 µg/kg body weight(0·75), while three dogs received a molar equivalency as 25(OH)D(3). The supplements, dissolved in ethanol, were applied to a commercial treat for consumption. Serum 25(OH)D(3) and 24R,25-dihydroxyvitamin D(3) (24R,25(OH)(2)D(3)) were analysed weekly using a validated HPLC method. Both supplementations increased (P ≤ 0·01) serum 25(OH)D(3) concentrations. However, oral 25(OH)D(3) resulted in greater (P < 0·0001) concentrations than D(3) by week 1, with a difference of 173 % (P < 0·0001) by week 2. The supplementation period was limited to 14 d after serum 25(OH)D(3) concentrations were not appearing to plateau. Thereafter, a washout period of 1 month separated the cross-over. Following 25(OH)D(3), but not D(3) supplementation, serum 24R,25(OH)(2)D(3) concentrations increased (P ≤ 0·02), 3 to 5 weeks after initiating supplementation. Vitamin D status, as indicated by serum 25(OH)D(3) and 24R,25(OH)(2)D(3) concentrations, is more rapidly and efficiently increased in adult dogs by oral supplementation of 25(OH)D(3) than D(3).