Flavor preferences conditioned by intragastric fructose and glucose: differences in reinforcement potency.

Many prior conditioning studies indicate that fructose, unlike glucose, has minimal postingestive reinforcing effects. Using a new training procedure, food-restricted rats were trained in alternate 20-h/day sessions with one flavored solution (CS+F) paired with intragastric (IG) infusions of 16% fructose and another flavor (CS-) paired with IG water. In subsequent two-bottle tests they showed a robust (85%) preference for the CS+F over the CS-. A third flavor (CS+G) was then paired with IG 16% glucose, and it was strongly preferred to the CS+F. When retrained 30 min/day with new flavors paired with IG fructose, glucose, or water the rats learned only a CS+G preference. When training was extended to 20 h/day, a CS+F preference developed. New rats trained 20 h/day with two-bottle access to CS+F and CS- paired with IG fructose and water failed to acquire a CS+F preference. Other rats rapidly developed a strong preference when trained with concurrent access to CS+G and CS- paired with IG glucose and water. These data indicate that both fructose and glucose generate postingestive reinforcing signals, but that the fructose signals are weaker and/or delayed relative to those produced by glucose.