ACUTE ADAPTATION TO ISOCALORIC HIGH-FAT FEEDING PREDICTS MAINTENANCE OF BODY COMPOSITION IN FEMALE RATS 243

Abstract

The goals of this study were to; 1) examine the acute effect of increased fat intake on oxygen consumption (VO2) and respiratory exchange ratio (RER) in rats fed isocalorically, and 2) relate short-term changes in RER to long-term changes in body fat (BF). We hypothesized that the 24-hr RER observed after 7 days on an isocaloric high-fat (iso-HF) diet would predict BF changes with chronic elevation of dietary fat. Forty-four mature female Sprague-Dawley (SD) rats were fed a low-fat diet (12% Kcal fat) ad libitum for 3-wk, then divided into groups matched for weight and food intake. 24-hr VO2 and RER were measured by indirect calorimetry during 8-hr of fasting and 16-hr of food access. Rats were then meal-fed diets providing 12, 18, 24, 30, or 36% Kcal as fat (food quotient, FQ =.92,.90,.88,.87, and.85, respectively) in amounts equal to baseline energy intake. All diets provided 21% Kcal from protein and increased fat replaced carbohydrate. VO2 and RER were assessed after 7 days on iso-HF, and BF and fat pads were measured at 16 wk. Baseline RER was characteristically low during fasting and elevated during feeding; baseline 24-hr RER was similar to FQ (p > 0.05). After 1 week of iso-HF feeding, VO2 was unchanged while a lower RER during feeding produced a decrease in 24-hr RER that was proportional to the change in FQ (r2 = 0.99, p = 0.001). The change in RER reflects acute adaptation and maintenance of substrate balance with iso-HF, which would predict no change in BF, and indeed there was no effect of fat intake on body mass, BF or fat pad weights after 16 wk. The results of this study demonstrate that body composition is maintained in mature female SD rats despite elevated fat intake when fat utilization acutely adapts to match fat intake, as is predicted by early changes in RER with increased dietary fat.