Modelling the Dynamics of Phosphorus Digestion in Rainbow Trout
The development of nutritional dietary strategies to efficiently meet phosphorus requirement of fish and minimize phosphorus (P) waste output from aquaculture operation depends on a comprehensive understanding of the dynamic digestion of P by the animals. Dietary P is found under different chemical forms in feed (e.g. bone-P, phytate-P, organic- P). Each of these forms has distinct solubility and, therefore, is expected to have different digestion dynamics. This thesis presents: (i) an in vivo trial to assess the dynamic of digestion of dietary P forms postprandially, with emphasis on phytate-P and tricalcium-P; (ii) application of a dynamic model to estimate absorption and endogenous losses of P; (iii) a structural model (path analysis) to study the relationship between parameters important to digestion of P. In Study 1, it was observed that the digestibility of P at the proximal intestine (PI) of rainbow trout was reduced by the presence of phytate-P and tricalcium-P, and calcium (Ca) had a negative effect on Pi absorption. The study provided evidence that the proximal intestine was the primary site for Pi absorption. In Study 2, the validation of a mathematical model evidenced the influence of different forms of dietary P on the rate of absorption and endogenous Pi secretions in different compartments of the gastrointestinal tract (GIT). A high concentration of low-solubility P compounds decreased the absorption rate of P while it increased the endogenous Pi secretion. Finally, Study 3 revealed that gastric pH directly and indirectly explained 16% and 39% of the variance in P digestibility and absorption, respectively. Gastric pH had a negative relationship with dietary P forms demonstrating that P digestion is highly dependent on gastric acid secretion. Ca indirectly affected P digestibility due to its correlation to dietary P forms. The combination of a comprehensive experimental protocol associated with mathematical modelling used in this thesis improved our understanding on the dynamics of P digestion in trout. In addition, our findings could be used to further investigate nutritional strategies to improve the digestibility of P as well as that of other nutrients by different fish species.