Influence of diet, phytase, and incubation time on calcium and phosphorus solubility in the gastric and small intestinal phase of an in vitro digestion assay.

To determine the influence of incubation time, diet, and particle size on Ca and P solubility in vitro, experimental diets were formulated to contain 0.89% Ca and 0.40% available P (positive control; PC) or 0.76% Ca and 0.27% available P (negative control; NC). The PC was supplemented with 0 or 1,000 phytase units (FTU) of microbial phytase/kg and the NC with 0, 1,000, or 5,000 FTU/kg diet of microbial phytase for a total of 5 experimental diets. In Exp. 1, diets were exposed to simulated gastric digestion containing HCl and pepsin for 42 min, or a small intestinal digestion phase containing NaHCO(3) and pancreatin for 60 min. In Exp. 2, diets were ground to pass a 1- or 2-mm screen and exposed to gastric digestion for 5, 10, or 20 min. Phosphorus and Ca solubility were similarly influenced by diet and digestion phase (Exp. 1), and there was no interaction. Phytase supplementation improved (P < 0.001) Ca and P solubility in both the PC and NC diets (Exp. 1) and increased P (P < 0.001) and Ca (P < 0.001) solubility in the gastric phase of the in vitro digestion model (Exp. 2). Phytase continued to release P in the gastric test over time, which resulted in a diet × time interaction (P < 0.05). Calcium solubility reached an asymptote at 5 min and both Ca and P solubility was reduced (P < 0.05) in diets ground to pass a 2 mm screen compared with diets ground to pass a 1-mm screen. In addition, P and Ca solubility did not change over time in diets not supplemented with phytase. In conclusion, phytase or particle size altered the kinetics of Ca and P release in a non-parallel fashion, which may be associated with the precipitation of Ca with phytate and the sequential dephosphorylation of phytate by a microbial 6-phytase. In the presence of phytase, considerable Ca and P hydrolysis occurred within 5 min of a simulated gastric digestion. However, the solubility of Ca and P reached a plateau in the gastric phase of digestion and no further improvements in solubility are apparent in the small intestine. Therefore, absorption of Ca and P may be complicated by conditions within the gastrointestinal tract, particle size, precipitation with anti-nutrients, and differential rates of delivery to the small intestine.