Modelling of calcium, phosphorus, and zinc fluxes in the gastrointestinal tract of growing pigs

Improving phosphorus (P) utilization is a key strategy to enhance the sustainability of pig production. Microbial phytase is widely used in pig nutrition to improve phytate P (PP) degradation and P availability, but its efficacy is strongly influenced by the digestive processes governing mineral solubilization along the gastrointestinal tract. To map the determinants of phytase efficacy, a mechanistic model describing the digestive fate of Ca, P, PP and zinc (Zn) in growing pigs was developed. The model represents the stomach as a single, well-mixed compartment and the small intestine as 180 sequential segments to capture spatial gradients in digestion and absorption. Dietary and endogenous pools of Ca, P, PP and Zn were represented in soluble and insoluble forms. Gastric phytate solubilization was modelled as a pH-dependent process, phytate hydrolysis by microbial phytase followed Michaelis–Menten kinetics, and intestinal mineral absorption as combined saturable and passive processes. The model was implemented in Python and run over 1440 min. Five dietary scenarios were simulated based on experimental diets: two Ca levels (low Ca, LCa; control Ca, CCa), two phytase levels (0 or 500 FTU/kg), and one additional CCa diet with 2 000 FTU/kg phytase. The model predicted a phytase dose-response for PP digestibility, increasing from 22.5% without phytase to 80.7–81.1% with 500 FTU/kg and 88.8% with 2 000 FTU/kg. Simulated total tract digestibility of P increased from 50.2–55.3% to 69.2–68.0% with 500 FTU/kg, Ca digestibility from 61.7–64.4% to 67.2–69.6%, and Zn digestibility from 47.3–50.0% to 77.0–77.8%. 500 FTU/kg of phytase released approximately 1.1 g/kg digestible P, close to the provided equivalency of 1.2 g/kg by the supplier, whereas digestible Ca release was lower under LCa conditions (0.6 vs 1.2 g/kg expected). Sensitivity analysis indicated that PP digestibility was mainly driven by phytase kinetics and gastric liquid retention time, whereas Ca digestibility was primarily influenced by Ca absorption parameters. Model evaluation showed good predictive performance for P (n = 230; R² = 0.91; RMPSE = 8.9%) and PP (n = 29; R² = 0.97; RMPSE = 11.7%), moderate performance for Ca (n = 156; R² = 0.48; RMPSE = 12.4%), and poor performance for Zn (n = 119; R² = 0.43; RMPSE = 76.9%). Overall, the model captured the main mechanisms governing P and Ca digestion and highlighted the key roles of gastric pH, phytate hydrolysis, and intestinal complex formation in determining mineral availability in pigs.