Conditioned flavor avoidance, preference, and indifference produced by intragastric infusions of galactose, glucose, and fructose in rats.

The postingestive reinforcing and satiating effects of intragastric (i.g.) infusions of 16% galactose, glucose, and fructose were compared in adult female rats. In Experiment 1, food-restricted rats were trained to drink (30 min/day) flavored solutions (the CS+Gal and CS+Glu) paired with intragastric (i.g.) infusions of galactose and glucose; other flavors (the CS-) were paired with IG water infusions. In subsequent choice tests, the rats strongly preferred (91%) the CS+Glu to the CS-, but avoided the CS+Gal (21%) in favor of the CS-. In Experiment 2, the rats were trained with a CS+Fru paired with i.g. fructose infusions and a CS- paired with i.g. water. In the choice test they consumed similar amounts of CS+Fru and CS- (CS+Fru preference = 51%). In other choice tests they preferred the CS+Glu (>80%) to CS+Fru and CS+Gal, and the CS+Fru (81%) to CS+Gal. Satiation tests were performed in Experiment 3 by adapting the rats to drink a 3% sugar + 0.2% saccharin solution paired with i.g. water infusions (30 min/day). On different test days 16% sugar instead of water was infused. IG galactose, glucose, and fructose produced comparable reductions in sugar+saccharin intake in the first test session. These findings demonstrate that, while the three sugars had similar satiating effects, they differed substantially in their postingestive flavor conditioning effects. The glucose and fructose results confirm prior data indicating that only glucose generates potent postingestive reinforcing stimuli. The galactose-induced flavor avoidance indicates that this sugar has a negative postingestive consequence. This may be due to the slow and incomplete hepatic metabolism of this sugar in adult rats. Conceivably, galactose intolerance may contribute to the lactose avoidance in adult animals.