Serum 25-hydroxyvitamin D 3 and 24 R ,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 D3 (25(OH)D3 ) concentrations when dogs were supplemented with oral vitamin D3 (D3 ). In the present study, we determined the relative potency of oral 25(OH)D3 compared with D3 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 D3 supplementation at 2·3 µg/kg body weight0·75 , while three dogs received a molar equivalency as 25(OH)D3 . The supplements, dissolved in ethanol, were applied to a commercial treat for consumption. Serum 25(OH)D3 and 24 R ,25-dihydroxyvitamin D3 (24 R ,25(OH)2 D3 ) were analysed weekly using a validated HPLC method. Both supplementations increased ( P  ≤ 0·01) serum 25(OH)D3 concentrations. However, oral 25(OH)D3 resulted in greater ( P  < 0·0001) concentrations than D3 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)D3 concentrations were not appearing to plateau. Thereafter, a washout period of 1 month separated the cross-over. Following 25(OH)D3 , but not D3 supplementation, serum 24 R ,25(OH)2 D3 concentrations increased ( P  ≤ 0·02), 3 to 5 weeks after initiating supplementation. Vitamin D status, as indicated by serum 25(OH)D3 and 24 R ,25(OH)2 D3 concentrations, is more rapidly and efficiently increased in adult dogs by oral supplementation of 25(OH)D3 than D3 .