Diet-induced obesity attenuates anticipation of food access in rats.

Rats were housed in hanging cages and given ad lib access to food (control), approximately 20 g food/day (lean), or a palatable high-fat diet (fat). After body weights diverged, rats were transferred to activity wheels. When food access was reduced to 2 h/day, all control and lean rats displayed anticipatory activity (AA), while only two of eight fat rats anticipated the meals. Baseline activity levels and nonanticipatory wheel running were only marginally reduced in fat rats. In a second experiment, conducted entirely in activity wheels, rats were maintained on curtailed food or a high-fat diet until body weights diverged. Food access was then reduced to 2 h/day and the diets were reversed. Lean rats given restricted access to the high-fat diet gained weight and four of eight rats showed AA. Fat rats switched to chow lost weight and all eight rats displayed AA. The results indicate that body weight changes induced by diet manipulations result in a striking and rather selective reduction in the anticipation of daily meals. This effect is ascribed to a modulation of the output of a circadian pacemaker that entrains running-wheel activity to daily meals.