Effect of Diet, Body Type and Sex on Voluntary Intake, Energy Balance and Body Composition of Zucker Rats

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Abstract
Young lean and obese Zucker rats of both sexes were fed a high protein (PRO), high carbohydrate (CHO), or high fat (FAT) diet ad libitum for a 22-day period. The isoenergetic diets were formulated to supply 60% of the digestible energy from one nutrient and 20% from each of the other two nutrients. The body balance technique was used to determine the efficiency of metabolizable energy utilization and total heat production. Heat increment was calculated utilizing values obtained in separate experiments for heat of basal metabolism and voluntary activity. Rats fed the CHO and FAT diets consumed significantly more food than rats fed the PRO diet. Obese rats consumed more food and metabolized it much more efficiently than their lean littermates. However, fed either diet, lean and obese rats consumed food to a constant heat increment. Sex had no effect on food intake when intake was expressed in g per unit metabolic body size (Wtkg0.75). The data provide the first evidence that both lean and obese Zucker rats regulate their food intake by sensing the heat increment of the diet rather than its gross or digestible energy content. It would appear, therefore, that obese Zucker rats do not have a defect in their food intake regulatory mechanism. Body composition analysis revealed no differences between the sexes of lean rats but obese females had significantly more fat and less protein than obese males. Rats fed the PRO diet had significantly less fat and dry matter, and generally more protein in their bodies than rats fed the CHO and FAT diets. Although the high protein diet significantly reduced food intake in both lean and obese rats, it did not prevent the deposition of excess fat in the genetically obese rat. Obese rats gained significantly more fat and less protein on each diet than their lean littermates.