Oral and postoral determinants of food reward.

Tasty high-fat and high-sugar foods induce overeating and obesity in animals. To separate the roles of oral and postoral factors in food preference and consumption, recent studies have used an "electronic esophagus" preparation. With this system, when an animal drinks a flavored solution, it controls the infusion of nutrients directly into its stomach through an implanted catheter. Improving the palatability of the flavored solution increases solution intake, infused nutrient intake, and weight gain. Changing the composition of the infused nutrient (e.g., fat-carbohydrate ratio) can also alter energy intake when flavor cues are held constant. In addition, animals learn to associate flavor cues with the postoral effects of nutrients, which can significantly increase flavor preference and acceptance. In some but not all cases, conditioned preferences are associated with increased hedonic evaluation of the nutrient-paired flavor. Flavor preference conditioning by nutrients can occur in chow-sated animals and is not dependent upon an energy-depleted state. Thus, food reward is determined by oral and postoral stimuli and may override the homeostatic regulation of energy balance.