Gut secretions and nutrient absorption responses to dietary phytic acid and phytase in piglets
Phytic acid (PA) reduces nutrient digestibility in pigs and poultry, and has been shown to increase endogenous nutrient losses (ENL) in poultry. However, there is lack of information on the effect of PA on ENL in pigs, and mechanisms by which PA increases ENL. Three experiments were conducted to determine the effects of PA on ENL in pigs and to establish mechanisms by which PA increases the ENL. The first experiment investigated the effect of PA on ileal digestibility and ileal endogenous nutrient flows. Phytic acid decreased the apparent ileal sodium digestibility to a negative value (-18%). The second experiment investigated the effect of PA on gut enzyme activities, histomorphology and sodium-dependent glucose transporter 1 (SGLT1) gene expression. Phytic acid did not affect the gut villous height, villous height to crypt depth ratio, and jejunal SGLT1 gene expression, but decreased gastric pepsin activity and tended to decrease jejunal Na-K-ATPase activity. In the third experiment, the effect of PA on piglet performance and ion uptake in jejunum mounted in Ussing chamber, and jejunal SGLT1 protein level was evaluated. Phytic acid did not affect jejunal SGLT1 protein expression, but lowered piglet performance and jejunal active ion uptake. In conclusion, results from this study show that PA can reduce the apparent ileal digestibility of sodium to a negative value, indicating that PA can increase ileal endogenous sodium loss. The results also show that PA can reduce the pepsin activity and ion uptake in the gut. The reduced pepsin activity implies increased secretion of the enzyme plus hydrochloric acid and hence increased secretion of sodium bicarbonate that neutralizes the acid. The reduced ion uptake by PA implies reduced nutrient absorption. Because sodium is absorbed partly by co-transportation with other nutrients, the reduced ion uptake by PA implies reduced sodium absorption. Thus, it appears that PA increases ileal endogenous sodium flow partly through reduced pepsin activity and ion uptake in the small intestine. Overall, the results show that phytase (a PA-hydrolysing enzyme), which is added in pig diets to improve phosphorus availability, does not only improve phosphorus availability, but alleviates ant-nutritional effects of PA as well.