Genetic obestiy: estrogenic influences on the body weight and food intake of lean and obese adult Zucker (fa/fa) rats

The effects of chronic estrogen withdrawal and subsequent hormone replacement on the feeding and body weight of adult lean and genetically obese Zucker rats were investigated. Following confirmation of a delay in the vaginal canalization of the fatty rat, subgroups of each genotype received either ovariectomy or sham surgery (Experiment 1). One hundred days later all subjects were injected subcutaneously (SC) with 1.0 microgram of estradiol benzoate (EB) daily for 16 treatment days (Experiment 2A). A second series of daily 2.0 microgram EB injections was administered intraperitoneally (IP) for 1 week (Experiment 2B). The first experiment revealed that ovariectomy produced overeating and similar weight gains in both genotypes. In the second experiment, SC hormone treatment completely reversed ovarian obesity in lean animals but failed to alter the food intake or weight gain of fatty rats. IP administration of EB depressed the feeding of fatty and lean animals to a comparable degree but a reduction in weight gain was observed only in the lean rats. These findings are discussed in light of current theories of estrogenic modulation of energy balance.