Effect of amino acid imbalance on the utilisation efficiency of digestible energy in rainbow trout (Oncorhynchus mykiss)

In rainbow trout, a wide variance in the utilisation efficiency of digestible energy for growth (k gDE ) has been found, with values ranging from 55% to 92% across different studies. This range may be related to differences in dietary macronutrient composition, but it has also been suggested to be related to differences in the composition of growth. The current study investigated whether an amino acid imbalance would affect the k gDE in rainbow trout. It was expected that this would lead to a lower utilisation of protein which would instead be used for ATP production or for fat deposition. This was tested by comparing an amino acid (AA) imbalanced diet to an AA balanced diet at 4 different feeding levels. The use of these different feeding levels permitted the estimation of the K gDE and the requirement of digestible energy for maintenance (DE m ) by regression analysis on retained energy and digestible energy intake. Feeding the AA imbalanced diet to rainbow trout led to lower growth which was related to both a lower ad libitum feed intake as well as a lower feed efficiency. Furthermore, this diet led to a reduction in protein retention and a higher level of retained fat. However, these differences in the retention of protein and fat did not lead to differences in the k gDE or DE m which averaged 79% and 40 kJ/kg 0.8 per day. This indicates that feed evaluation on a digestible energy basis for rainbow trout is not influenced by an imbalanced amino acid profile.