Carbohydrate in rainbow trout diets: Effects of the level and source of carbohydrate and the number of meals on growth and body composition

Isonitrogenous rations containing various levels of digestible carbohydrate were fed to groups of trout for 8 weeks. Two levels (15 or 30%) and two sources (glucose or starch) of carbohydrates were used. A given amount of a 30% glucose diet was distributed in either 2, 4 or 6 meals/day, to determine the effect of meal frequency.Growth, feed efficiency and body composition of the different groups were compared. Glycaemia levels, as influenced by the different dietary treatments, were measured immediately and 6 h after a given meal.The 30% glucose diet promoted the best weight gain, feed conversion and protein efficiency. Groups fed the 15% glucose diet gave values between those of the 30% glucose and the 30% starch-fed groups. The additional energy retained in groups fed the highest glucose level (30%) did not modify carcass composition but increased fat deposits in the viscera and glycogen in the liver. Plasma glucose levels indicated that the regulation of glycaemia is effective within a 24-h period, even after a daily intake of 550 mg glucose/100 g body weight. Increasing the number of meals had no discernible effect, the efficiency of the 30% glucose diet being the same in the various groups.It is concluded that, under certain experimental conditions, trout can tolerate up to 30% glucose in the diet and can use it for energy needs.