Investigation of the effects of dietary supplementation with 25-hydroxyvitamin D 3 and vitamin D 3 on indicators of vitamin D status in healthy dogs

To compare the effects of short-term dietary supplementation with vitamin D and 25-hydroxyvitamin D (25[OH]D ) on indicators of vitamin D status in healthy dogs. 13 purpose-bred adult dogs. 20 extruded commercial dog foods were assayed for 25(OH)D content. Six dogs received a custom diet containing low vitamin D concentrations and consumed a treat with vitamin D (0.33 μg/kg ) plus 1 of 3 doses of 25(OH)D (0, 0.23, or 0.46 μg/kg ) once daily for 8 weeks followed by the alternate treatments in a crossover-design trial. In another crossover-design trial, 7 dogs received a custom diet supplemented with vitamin D or 25(OH)D (targeted content, 3,250 U/kg [equivalent to 81.3 μg/kg] and 16 μg/kg, respectively, as fed) for 10 weeks followed by the alternate treatment. In washout periods before each trial and between dietary treatments in the second trial, dogs received the trial diet without D-vitamer supplements. Dietary intake was monitored. Serum or plasma concentrations of vitamin D metabolites and biochemical variables were analyzed at predetermined times. 25(OH)D concentrations were low or undetected in evaluated commercial diets. In the first trial, vitamin D intake resulted in quantifiable circulating concentrations of 25-hydroxyvitamin D but not 24R,25-dihydroxyvitamin D . Circulating 25(OH)D concentration appeared to increase linearly with 25(OH)D dose. In the second trial, circulating 25(OH)D concentration increased with both D vitamer-supplemented diets and did not differ significantly between treatments. No evidence of vitamin D excess was detected in either trial. Potency of the dietary 25(OH)D supplement estimated on the basis of targeted content was 5 times that of vitamin D to increase indicators of vitamin D status in the study sample. No adverse effects attributed to treatment were observed in short-term feeding trials.