In the insectivorous Chilean mouse-opossum, Thylamys elegans (Didelphidae), acclimated to eating pulp of fruits with sucrose added and live insects, we studied the activity of intestinal disaccharidases, digestive efficiency, and the effect of each diet on the energy budget. We observed activity modulation of the enzymes sucrase, maltase, and trehalase as well as changes in substrate affinities correlated with dietary carbohydrates. Although this species is able to hydrolyze and assimilate sugars such as sucrose from fruits, it is nevertheless unable to obtain sufficient energy to satisfy its euthermic daily energy requirements and to survive with an exclusive diet of fruits. This situation contrasts with that observed in individuals feeding on insects, for they were able to satisfy their maintenance energy requirements with an exclusive diet of insects. The preferences observed in the mouse-opossum for insects over fruits is explained in terms of trade-offs between factors such as digesta transit time, food digestion (including sugars hydrolysis), and food characteristics.