Stimulation of fluid intake by nutrients: oil is less effective than carbohydrate.

It has been thought that the ability of nutrients to reinforce ingestion is related to their ability to provide metabolizable energy. This implies that the reinforcing effect of carbohydrate should be similar to that of fat. To test this concept, rats were trained in an apparatus that infused water or nutritive solutions/suspensions into their stomachs every time they drank a sapid solution. Each training trial lasted for 1 day. Successive training trials were interspersed by 1-day periods in which the rats were infused with plain water and offered plain water or a tastant different from the training taste (i.e., taste paired with vehicle infusion). Three different sapid solutions were used: a sweet solution (saccharin), a nonsweet solution (NaCl), and a mixture of sweet and nonsweet. Starch or maltodextrin infusions strongly and consistently stimulated intake of these solutions. Oil infusions also significantly stimulated intake, but feebly and less consistently. Indeed, in the one experiment in which the only sapid fluid offered was saccharin, oil infusions had no significant effect. Two different oil suspensions (one fine and one coarse) were equally ineffective in stimulating saccharin intake. To determine whether some unsuspected flaw in the infusion experiments somehow produced invalid results, an additional experiment was conducted in which rats ingested starch or oil suspensions by mouth. Consistent with the infusion experiments, starch stimulated ingestion to a much greater degree than did oil. It is concluded that intragastric infusion of triglyceride oil is a less potent reinforcer of ingestion than is equicaloric infusion of carbohydrate.