OPPOSITE EFFECTS OF VARIATIONS IN FOOD-INTAKE ON CARBOHYDRATE AND FAT OXIDATION IN AD-LIBITUM FED MICE

Daily carbohydrate and fat oxidation rates were determined during 160 consecutive days in 10 female CD1 mice. The animals had free access to a running wheel and to one of two synthetic diets whose composition resembled either lab chow or a mixed western diet. Carbohydrate oxidation was found to be correlated positively with daily variations in food intake, matching carbohydrate intake closely, regardless of the diets' macronutrient contents (R = .83 and .72, respectively). By contrast, fat oxidation was correlated negatively with daily variations in food intake on each diet, being inhibited by carbohydrate intake (R = -.65). Such results reflect the priority given in metabolic regulation to the maintenance of carbohydrate balance over the maintenance of fat balance, as well as over energy balance, which was linked very tightly to the latter (R = .91 and .95). Fat oxidation appeared to be determined by the gap between overall energy expenditure and the sum of carbohydrate plus protein energy ingested (R = .9), rather than by the amounts of fat consumed. These findings thus illustrate fundamental differences in the control of carbohydrate and fat metabolism, which must be taken into account to understand body weight regulation and maintenance.