As in the conditioning of appetite for protein in the rat, human preference for and intake of a food at lunch was increased when the flavor of that food was paired with an adequate supply of protein, following a breakfast lacking in protein. Men and women rated their preferences for two flavors in tasted foods (soup and cornflour dessert) on test days before and after a day when one flavor was eaten in very low protein food and another day with a different flavor eaten in food containing protein, but with minimal sensory differences between these foods. Subjects given a low-protein drink preload preferred the protein-paired flavor, while those receiving a high-protein drink did not. In a second experiment, preferences were measured by intake as well as ratings, and the difference in amount of protein between high- and low-protein lunches was increased. By both measures, relative preference for high-protein-paired dessert flavors increased from before to after pairing. The increase in intake preference ratio for the protein-paired flavor was abolished by a high-protein preload. Thus, people have a learning mechanism whereby a lack in protein intake comes to cue the selection of protein-rich foods that are not known to be such, and/or loading with protein might trigger avoidance specifically of a high-protein diet.