Flavor preferences conditioned by intragastric sugar infusions in rats: maltose is more reinforcing than sucrose.

Prior research indicates that glucose conditions much stronger flavor preferences in rats than does fructose. This could occur because intestinal absorption of fructose is much slower than that of glucose and because fructose malabsorption may have aversive consequences. Fructose absorption is facilitated when glucose is also present in the gut. The present study therefore compared the flavor conditioning effects of maltose (a glucose + glucose disaccharide) to those of sucrose (a glucose + fructose disaccharide). In Experiment 1, rats had different flavors paired with intragastric infusions of 32% maltose (CS+M), 32% sucrose (CS+S), and water (CS-) 23 h/day. In subsequent two-bottle tests, both CS+ solutions were strongly preferred to the CS-, but the CS+M was also preferred (78%) to the CS+S. Experiment 2A revealed that the rats also learned to prefer a CS+M to a CS+S when 16% sugar infusions were used. In Experiment 2B, the same rats preferred a flavor paired with 16% maltose to a flavor paired with 8% maltose. They did not reliably prefer a flavor paired with 16% sucrose to a flavor paired with 8% maltose. These results demonstrate that the postingestive actions of maltose are more reinforcing than those of sucrose. This indicates that fructose is less reinforcing than glucose even when malabsorption is not a factor. In contrast to their preference for the CS+M over the CS+S, the rats preferred sucrose to maltose when drinking the sugars by mouth. Therefore, sugar preferences mediated by oral taste receptors differ from those conditioned by postoral nutrient detectors.