Flavor preferences conditioned by sugars: rats learn to prefer glucose over fructose.

The reinforcing effects of glucose and fructose were compared using flavor preference conditioning paradigms. Female rats given access to flavored glucose and flavored fructose solutions developed preferences for the glucose- over the fructose-paired flavor, as well as for unflavored glucose over unflavored fructose. This effect was obtained with 8% and 32% solutions, and with nondeprived and deprived rats. In most cases, the glucose preference was not immediate but appeared only after the rats had one-bottle experience with the two sugars. Female rats also preferred a glucose-paired flavor over a flavor that was paired with a noncaloric saccharin solution. A similar but somewhat weaker preference was displayed for a fructose-paired flavor over a saccharin-paired flavor. Both male and female rats acquired preferences for flavored chow that was paired with the intake of a 32% glucose solution over flavored chow paired with 32% fructose intake. Together, these results indicate that glucose has more potent postingestive reinforcing effects than does fructose. This may explain why rats consume more glucose than fructose in one-bottle acceptance as well as two-bottle preference tests.