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

We previously found a weak response in serum 25-hydroxyvitamin D (25(OH)D) concentrations when dogs were supplemented with oral vitamin D (D). In the present study, we determined the relative potency of oral 25(OH)D compared with D 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 supplementation at 2·3 µg/kg body weight, while three dogs received a molar equivalency as 25(OH)D. The supplements, dissolved in ethanol, were applied to a commercial treat for consumption. Serum 25(OH)D and 24,25-dihydroxyvitamin D (24,25(OH)D) were analysed weekly using a validated HPLC method. Both supplementations increased ( ≤ 0·01) serum 25(OH)D concentrations. However, oral 25(OH)D resulted in greater ( < 0·0001) concentrations than D by week 1, with a difference of 173 % ( < 0·0001) by week 2. The supplementation period was limited to 14 d after serum 25(OH)D concentrations were not appearing to plateau. Thereafter, a washout period of 1 month separated the cross-over. Following 25(OH)D, but not D supplementation, serum 24,25(OH)D concentrations increased ( ≤ 0·02), 3 to 5 weeks after initiating supplementation. Vitamin D status, as indicated by serum 25(OH)D and 24,25(OH)D concentrations, is more rapidly and efficiently increased in adult dogs by oral supplementation of 25(OH)D than D.