The effects on food intake and body weight in lean and obese Zucker rats were evaluated following substitution in the diet with either 1) poorly absorbable lipid (hydrogenated soybean oil) for corn oil, or 2) nonabsorbable carbohydrate (fiber) for glucose. Lean Zucker rats compensated for the reduced caloric availability of the high-fiber and hydrogenated oil diets by increasing food consumption. In contrast, obese rats did not respond significantly to these dietary alterations and failed to attain caloric balance during the 16-day study. These differences in caloric compensatory responses were reflected in body weight gains. There were no differences in the amount of weight gained by lean rats fed either the high-or low-fiber diets because of compensatory hyperphagia in the high-fiber group. Lean rats fed the hydrogented oil diets gained less weight than controls fed corn oil diets, even in the presence of compensatory hyperphagia, because of the enhanced fecal excretion of water and metabolites caused by the poorly absorbed fat diet. As a result of a delayed and incomplete response to reduced caloric availability, obese rats fed the high-fiber and hydrogenated oil diets gained significantly less weight than the obese rats fed low-fiber and corn oil diets.