Flavor–nutrient learning is less rapid with fat than with carbohydrate in rats

Flavor–nutrient learning occurs when the post-ingestive consequences of a food are associated with its flavor. As a signal of the food's energy density, flavor–nutrient associations have the potential to contribute to the regulation of meal size. While all calorie sources (fat, carbohydrate, protein, ethanol) can support flavor–nutrient learning, prior research has found that flavor–nutrient associations based on fat may require higher nutrient concentrations and more rigorous experimental protocols than are required to train carbohydrate (cho)-based associations. To further explore potential macronutrient-specific differences in flavor–nutrient learning, the present study compared the time course of acquisition of cho- and fat-based associations. Rats were trained to associate distinctive flavors with high-density (3.2 kcal/mL) and low-density (0.2 kcal/mL) orally-consumed solutions, either fat (corn oil emulsion) or carbohydrate (sucrose). For each nutrient, both within- and between-group designs were used to assess (via two-bottle preference testing) whether flavor–nutrient learning had occurred after 2, 4, or 6 training trial pairs. Rats trained with carbohydrate demonstrated preferential intake of the low-density paired flavor after only 2 training pairs; in contrast, rats trained with fat required 6 training pairs. These findings demonstrate differential rapidity of acquisition flavor–nutrient associations. The longer time course of acquisition of fat-based flavor–nutrient associations may be yet another mechanism by which high-fat foods promote overeating.