Rats integrate meal cost and postoral changes in caloric density.

Rats can maintain energy intake in response to changes in the caloric density of food by compensatory shifts in the number and size of meals. This study challenged rats' regulation of intake in two ways: manipulating the caloric density covertly to remove orosensory cues and altering baseline meal patterns by increasing the cost of obtaining meals. Rats with gastric catheters were maintained on a complete liquid diet delivered by pumps to a drinking spout and to the stomach; within meals, a rat's licking at the spout controlled the timing and duration of the simultaneous isovolemic intragastric infusions. Prior to each meal, some of the rats were required to press a bar to activate the spout mechanism; when the meal cost was paid, a cue light signaled that licks would deliver food. The light remained on until the rat ceased licking for 10 consecutive min; to resume feeding, the rat had to pay the cost again. The standard diet pumped to the spout was prepared at 1 kcal/g; the intragastrically co-infused diet varied from 0 (water) to 2 kcal/g in 0.5-kcal/g increments. Each concentration was presented for at least 4 days. Separate groups of rats were studied at no cost, low cost (5 bar presses/meal), and high cost (80 bar presses/meal). As the infusate concentration increased, the rats ate fewer meals per day. Meal size by weight decreased as caloric density increased, with he largest meals taken when water was infused. Caloric meal size increased linearly with caloric density for all groups. Meal sizes in the low-cost group were similar to those of the no-cost group, but the latter took more meals per day and, therefore, consumed more total calories. High-cost meal patterns were parallel to those of the other groups, but with fewer, larger meals. All groups maintained a constant daily caloric intake across infusate concentrations. Meal patterns changed rapidly in response to these "covert" (untasted, isovolemic) changes in caloric density. Rats can, thus, integrate the cost of obtaining food and its postingestive benefits in the absence of mediating orosensory cues.