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Body weight, body composition, and energy metabolism in lean and obese Zucker rats fed soybean oil or butter^1,2

¹ From the Laboratoire de Neurobiologie des Regulations, CNRS, Paris and the Unité INRA-INAPG, Physiologie de la Nutrition et du Comportement alimentaire, Paris.

Background: Dietary fat composition is thought to affect body weight regulation independent of the amount of fat ingested.

Objective: We analyzed the feeding behavior, body weight gain, body composition, and energy metabolism in lean and obese rats fed a diet in which fat was in the form of either butter or soybean oil.

Design: Ten lean (Fa/?) and 10 obese (fa/fa) adult Zucker rats were divided into 4 groups according to a 2 x 2 experimental design. They were fed a normally balanced diet over 11 wk in which 30% of energy was either soybean oil or butter. Food intake, body weight gain, and body composition were measured. Indirect calorimetry was used to study energy metabolism at rest and in relation to feeding and activity.

Results: Food intake increased similarly in lean and obese rats after butter feeding. Body weight gain increased in obese rats and decreased in lean rats after butter feeding. Body weight gain in obese rats was due mainly to an increase in the weight of lean tissues besides muscle, whereas adiposity and distribution of fat between the various pads did not change. Resting metabolic rates and postprandial lipid oxidation increased in butter-fed obese rats. Lipid oxidation during exercise was not significantly different between obese and lean rats. Fat oxidation increased in butter-fed lean rats during treadmill running at moderate intensity.

Conclusions: In obese rats, basal metabolism and postprandial lipid oxidation increased during butter feeding, which appeared to prevent fat accumulation in the long term. In lean rats, butter feeding favored lipid utilization by working muscles, an observation that deserves further investigation in terms of endurance and performance.

Key Words: Dietary fats • saturated fatty acids • unsaturated fatty acids • medium-chain triacylglycerol • long-chain triacylglycerol • obesity • organ mass • respiratory quotient • substrate oxidation • exercise • indirect calorimetry • rats