METABOLIC CHANGES ASSOCIATED WITH INGESTION OF DIFFERENT MACRONUTRIENTS AND DIFFERENT MEAL SIZES IN RATS

Indirect calorimetry was used to investigate whole-body metabolic chan ges occurring during and after feeding in the rat. Measurement of resp iratory exchange allowed the derivation of respiratory quotient (RQ = CO2 produced/O-2 consumed) and energy expenditure [EE = O-2 consumed ( 364 + 113RQ)], giving an estimate of the energy substrate (fat, carboh ydrate, or protein) being utilised and the total substrate oxidation o ccurring, respectively. Concurrent measurement of locomotor activity ( ACT) allowed the changes in EE attributable to feeding (postprandial t hermogenesis) to be generally dissociated from those attributable to a ctivity. Experiment 1 examined alterations in RQ, EE, and ACT followin g brief (<5 min) calorically matched meals of Froot Loops (86% carbohy drate), 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 followi ng Froot Loop and lab chow meals indicating increased utilization of c arbohydrates as an energy. substrate. Casein caused little change in R Q, while lard caused a drop in RQ approximately 30 min following inges tion, indicating increased fat utilization following absorption of the lard meal. Saccharin caused little change in RQ, suggesting that a sw eet taste is not sufficient to alter substrate utilization. Increased EE was seen following the Froot Loops and lab chow meals in the absenc e of corresponding increases in ACT, suggesting a postprandial thermog enic effect of these meals. In Experiment 2, the metabolic changes acc ompanying a large (5 g) and small (0.5 g) Froot Loops meal were compar ed. The large meal resulting in a larger, more rapid and more sustaine d increase in RQ than the small meal. In addition, there was an increa se in EE in the absence of corresponding increases in ACT following th e large meal suggesting the presence of postprandial thermogenesis. It is concluded that both meal macronutrient content and meal size are i mportant determinants of postprandial substrate utilization and thermo genesis in rats.