Utilization of Dietary Urea in Rainbow Trout

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Abstract
Experiments were conducted to examine the potential utilization of dietary urea by rainbow trout [Salmo gairdnerii]. A control diet and 2 diets supplemented with 1 and 3% of urea were fed to fish. Postprandial levels of urea and NH3 in blood plasma, and postprandial excretion of these metabolites were followed during 24 h. Apparent digestibility of urea in rainbow trout was very high (> 98%). Maximum values of urea levels in plasma were reached 6 h (32.3 .+-. 10.2 .mu.g/ml) after a meal in the control fish and, respectively, 6 h (83.4 .+-. 18.4 .mu.g/ml) and 8 h (250.3 .+-. 96.1 .mu.g/ml) after a meal in trout fed 1 and 3% urea diets. Peaks of urea excretion rates appeared 7-9 h after meal, coinciding with the highest circulating urea concentration. Total daily urea excretion amounted to 5.53, 10.43 and 33.80 mg urea N/100 mg N intake in trout fed the control, 1 and 3% urea diets, respectively. The dietary urea is readily absorbed in the digestive tract of trout but is totally excreted thus leading to no beneficial effect on N balance. This excretion of urea also takes place passively without any increase in energy demands.