Effect of digestible energy on nitrogen and energy balance in rainbow trout

Abstract

The utilization by rainbow trout of high protein diets, incorporating 30% of either gelatinized (diet AG) or natural starch (diet AC) was studied. Compared with a commercial diet, both these diets were more efficiently utilized. Food and protein utilization were higher in trout fed the diet AG than in those fed the diet containing the raw starch. Body composition was similar in both groups except for fat content which was elevated in those fed the natural starch. Relative liver weight and post-prandial plasma glucose levels were higher in those fed the diet containing gelatinized starch than in those fed raw starch. Patterns of post-prandial ammonia and total free amino nitrogen levels in the plasma did not differ between the two groups but the levels were consistently higher in those fed diet AC. A bimodal circadian rhythm was found with a maximum at 6 and 9 h and a second peak at 17 and 20 h after the morning meal in trout fed AC and AG diets, respectively. In fasted trout, only one maximum was observed, corresponding to the second peak in fed animals. Nitrogen excretion, following a meal, was much lower in trout fed the AG diet than in those fed either the AC diet or the control diet. Balance studies confirmed the more efficient utilization of protein and energy by trout fed the more digestible starch than by those in the other groups.