Phytate Hydrolysis, Intestinal Microbiota, Microbial Metabolites, and Innate Immune Cell Numbers Are Changed in Growing Pigs Fed Diets with Varying Calcium-Phosphorus Levels and Fermentable Substrates

Effects of dietary calcium-phosphorus (CaP) on the swine immune system and intestinal microbiota are not fully elucidated. The present study assesses effects of diets containing varying CaP levels and fermentable substrates on intestinal CaP net absorption, phytate (myo-inositol hexakisphosphate, InsP₆) hydrolysis, intestinal microbiota, microbial metabolites, and innate immune cell numbers using 32 crossbred pigs (initial BW 54.7 kg). In a 2 נ2 factorial arrangement, pigs were fed either a corn-soybean meal (SBM) or corn-field pea diet with either low (4.4 g Ca/kg; 4.2 g total P/kg) or high (8.3 g Ca/kg; 7.5 g total P/kg; supplemented with monocalcium phosphate) CaP content. Digesta and feces samples were examined for P, Ca, inositol phosphate isomers, and TiO₂. After 3 wk of diet adaptation, feces were collected by rectal stimulation to determine 16S rRNA gene copy numbers by qPCR and bacterial metabolite analyses. Blood was analyzed for immune cell numbers. Jejunal (-9.6 vs. 18.3 %), cecal (-1.8 vs. 26.3 %), and total tract P (3.6 vs. 22.1 %) net absorption was lower (P < 0.01) for the low than high CaP diets. Diet did not affect InsP₆ hydrolysis in digesta. The jejunal InsP₆ concentration was lower (P < 0.05; 21.5 vs. 31.5 µmol/g DM) for the low than high CaP diets. Jejunal (1.6 vs. 1.2 µmol/g DM) and cecal (2.2 vs. 1.8 µmol/g DM) concentration of Ins(1,2,4,5,6)P₅ tended to be greater (P < 0.10) for the SBM than field pea diets. Gene copy numbers of Bacteroides-Prevotella-Porphyromonas were lower (P < 0.05) for pigs fed the high than low CaP diets. The Clostridium cluster IV tended to be greater (P < 0.10) and fecal acetate and propionate concentrations were lower (P < 0.05) for the field pea than SBM diets. Blood granulocyte, neutrophil, and monocyte numbers were lower (P < 0.05) for pigs fed the low CaP diet combined with SBM 3 wk after diet adaptation compared to the other diets. Diets low in CaP increased the abundance of proteolytic Bacteroides-Prevotella-Porphyromonas and in combination with SBM might impair the first line of defence and limited the activation of cellular adaptive immune response, thereby possibly increasing the risk for intestinal disturbances. For the corn-field pea diets, the greater SCFA concentration may indicate increased saccharolytic fermentation activity, which may be favorable for gut health, whereas the abundance of Clostridium cluster IV in feces, that is known to be less healthy, was greater for field pea.