Modelling the Dynamics of Phosphorus Digestion in Rainbow Trout

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Mota Damasceno, Flavia

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University of Guelph


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.



Digestibility, Fish, Mathematical modeling, Phosphorus, absorption, excretion, phytate, hydroxyapatite