Karasov 9723793 A widespread notion is that sugar (D-glucose) is mainly actively absorbed by the intestine. The Pi's have evidence, however, that birds can absorb considerable amounts of the other form of glucose, L-glucose, which is thought to be absorbed passively, not actively. The proposed studies test the assumption that L-glucose is absorption passively by birds, and the inference that most D-glucose absorption is passive. The studies will also determine how passive absorption by the avian intestine varies with size of water-soluble molecules, and whether the movement of absorbed solutes occurs across or between intestinal cells. The findings will be important to physiologists, evolutionary biologists, toxicologists, and ecologists. A finding that most sugar is absorbed passively would be important because it would challenge prevailing physiological ideas about nutrient absorption as well as current analyses in evolutionary digestive physiology that attempt to match the physiological capacity to absorb nutrients with nutrient intake. A high intestinal permeability permitting passive absorption might permit toxins to be absorbed from plant and animal material in the intestine. This scenario would beg research on the ecological cost associated with exposure and detoxication of natural toxins, and perhaps the sensitivity of wild animals to the new generation of water-soluble herbicides and pesticides produced by humans.
