Protein appetite demonstrated: learned specificity of protein cue preference to protein need in adult rats

After food had been withheld for 4 hours, casein was infused into the stomach of rats while they drank a distinctively flavored non-nutritive fluid. On testing under the same food-deprivation, the rats showed conditioned preference for the flavor. When protein was infused during the deprivation period before the test, they did not prefer the flavor, although they did when equicaloric carbohydrate had been pre-infused. These results demonstrate for the first time a learned hunger specifically for protein