Microbial phytase and 25-hydroxy-vitamin D3 fed to growing pigs increase digestibility of calcium and phosphorus and influence plasma vitamin D metabolites and serum bone biomarkers, but effects are not always additive.

The objective was to test the hypothesis that calcifediol [25(OH)D3] and microbial phytase have additive effects on the standardized total tract digestibility (STTD) of Ca and P, serum bone biomarkers, and plasma vitamin D₃ metabolites when fed to growing pigs. Sixty barrows (initial body weight: 25.98 ± 2.01 kg) were housed individually in metabolism crates and assigned to a randomized complete block design with three blocks, 5 diets, and 12 replicate pigs per diet. The positive control (PC) diet was formulated to meet Ca and P requirements of growing pigs. Four additional diets contained 75% of the required Ca and P and were used in a 2 × 2 factorial design with 0 or 50 μg/kg of 25(OH)D3 and 0 or 500 units of phytase per kg diet. Pigs were fed experimental diets for 13 d that included a 5-d adaptation period and a 5-d fecal collection period. Fecal samples were analyzed for Ca and P, and STTD of Ca and P was calculated. Blood samples were collected on days 1 and 13 to measure bone alkaline phosphatase, osteocalcin, type 1 collagen, and fibroblast growth factor 23. Analyzed plasma vitamin D3 metabolites included 25(OH)D3, 24,25 dihydroxycholecalciferol [24,25(OH)2D3], and 1,25 calcitriol [1,25(OH)2D3]. Results indicated that the STTD of P was greater (P < 0.05) in the PC diet than in the diet containing 75% of the required Ca and P and no microbial phytase or 25(OH)D3. The STTD of Ca and P increased (P < 0.001) in pigs fed diets containing phytase, and STTD of Ca and P tended to increase if 25(OH)D₃ was added to the diet, but only in the absence of phytase (interaction; P < 0.10). On day 13, osteocalcin, which is a biomarker for bone tissue synthesis, was increased (P < 0.05) if 25(OH)D₃ or phytase was added to diets, but the other bone biomarkers did not differ among treatments. Plasma 25(OH)D₃ and 24,25(OH)₂D₃ increased (P < 0.05) if diets contained 25(OH)D₃ and(or) phytase, indicating increased metabolic activity of vitamin D3. Plasma 1,25(OH)₂D₃ was greater (P < 0.001) in pigs fed the diet with 75% of the required Ca and P and no phytase or 25(OH)D₃ than in pigs fed the PC diet, but microbial phytase decreased (P < 0.001) plasma 1,25(OH)₂D₃. In conclusion, microbial phytase and 25(OH)D₃ increased Ca and P digestibility and serum osteocalcin, and vitamin D₃ status was improved with the addition of 25(OH)D₃ to the diet, but effects of 25(OH)D₃ and microbial phytase were not always additive.