Intestinal adaptations of rainbow trout to changes in dietary carbohydrate.

Although omnivores are able to alter the structure and functions of their digestive system in response to changes in dietary carbohydrate content, it is unclear whether carnivores are capable of such adaptive flexibility. Hence we recorded growth rates, intestinal morphometrics and histology, and nutrient uptake rates and concentrations of disaccharidases in the intestines of a carnivorous fish, the rainbow trout, fed different levels and types of carbohydrate. The trout is unable to adaptively regulate digestive system structure and function to increase glucose availability in response to increasing levels of dietary carbohydrates, even to easily digestible forms such as glucose. Paradoxically, a reduction in the concentrations of enzymes associated with carbohydrate digestion in response to elevated levels of easily digested carbohydrates suggests that carnivores may actually try to repress carbohydrate digestion when glucose is available in high quantities. Thus the lower levels of carbohydrate in the diet of trout throughout their evolution has resulted in a reduced ability to phenotypically regulate the digestion of carbohydrates.