Intragastric glucose but not fructose conditions robust flavor preferences in rats

Prior work indicates that glucose and fructose differ in their postingestive reinforcing effects. The present study investigated this phenomenon by training rats to associate the intake of flavored water with intragastric (IG) infusions of 16% sugar solutions. In experiment 1, rats had one flavor [conditional stimulus (CS)] paired with IG sugar infusions (CS+; e.g., cherry) and another flavor paired with IG water (CS-; e.g., grape) 23 h/day; Chow was available ad libitum. In subsequent choice tests, rats infused with glucose displayed a strong preference (89%) for the CS+ flavor, whereas rats infused with fructose showed only a small and nonreliable CS+ flavor preference (62%). When next trained to associate one flavor (e.g., orange) with IG glucose and another flavor (e.g., strawberry) with IG fructose, rats in both groups developed a significant preference (81%) for the glucose-paired flavor. In experiment 2, food-deprived rats were trained 2 h/day to associate a CS+ flavor with IG sugar and a CS- flavor with IG water infusions. The glucose-reinforced rats displayed a near-total preference (95%) for the CS+ flavor, whereas fructose-reinforced rats showed a much smaller CS+ preference (67%). The preference findings indicate that the postingestive consequences of glucose are much more reinforcing than those of fructose. It appears that food conditioning is mediated by chemospecific actions of nutrients rather than their general satiating or energy repleting effects.