Metabolic changes associated with ingestion of different macronutrients and different meal sizes in rats

Indirect calorimetry was used to investigate whole-body metabolic changes occurring during and after feeding in the rat. Measurement of respiratory exchange allowed the derivation of respiratory quotient (RQ = CO2 produced/O2 consumed) and energy expenditure [EE = O2 consumed (364 + 113RQ)], giving an estimate of the energy substrate (fat, carbohydrate, or protein) being utilised and the total substrate oxidation occurring, respectively. Concurrent measurement of locomotor activity (ACT) allowed the changes in EE attributable to feeding (postprandial thermogenesis) to be generally dissociated from those attributable to activity. Experiment 1 examined alterations in RQ, EE, and ACT following brief (< 5 min) calorically matched meals of Froot Loops (86% carbohydrate), casein (protein), lard (fat) and lab chow (mixed macronutrient) and following the ingestion of 0.1% saccharin (which is calorically inert). Rapid (< 5 mins) and sustained increases in RQ occurred following Froot Loop and lab chow meals indicating increased utilization of carbohydrates as an energy substrate. Casein caused little change in RQ, while lard caused a drop in RQ approximately 30 min following ingestion, indicating increased fat utilization following absorption of the lard meal. Saccharin caused little change in RQ, suggesting that a sweet taste is not sufficient to alter substrate utilization. Increased EE was seen following the Froot Loops and lab chow meals in the absence of corresponding increases in ACT, suggesting a postprandial thermogenic effect of these meals. In Experiment 2, the metabolic changes accompanying a large (5 g) and small (0.5 g) Froot Loops meal were compared. The large meal resulting in a larger, more rapid and more sustained increase in RQ than the small meal. In addition, there was an increase in EE in the absence of corresponding increases in ACT following the large meal suggesting the presence of postprandial thermogenesis. It is concluded that both meal macronutrient content and meal size are important determinants of postprandial substrate utilization and thermogenesis in rats.