METHODOLOGICAL EVALUATION OF INDIRECT CALORIMETRY DATA IN LEAN AND OBESE RATS

1. The applicability of current indirect calorimetry formulae to the s tudy of energy and substrate balances on obese rats has been evaluated . The energy consumption of series of 60-day rats of Wistar, lean and obese Zucker stock were studied by means of direct and indirect calori metry, and by establishing their energy balance through measurement of food intake and retention. Calorimetric studies encompassed a 24 h pe riod, with gas and heat output measurements every 2 or 5 min, respecti vely, for direct and indirect calorimetry. 2. The analysis of fat comp osition (diet, whole rat, and synthesized and oxidized fat) showed onl y small variations that had only a limited effect on the overall energ y equation parameters. 3. A gap in the nitrogen balance, which represe nts a urinary N excretion lower than the actual protein oxidized, resu lted in significant deviations in the estimation of carbohydrate and l ipid oxidized when using the equations currently available for indirec t calorimetry. 4. Analysis of the amino acid composition of diet and r at protein as well as of the portion actually oxidized, and correcting for the nitrogen gap allowed the establishment of a set of equations that gave better coincidence of the calculated data with the measured substrate balance. 5. The measured heat output of all rats was lower t han the estimated values calculated by means of either indirect calori metry or direct energy balance measurement; the difference corresponde d to the energy lost in water evaporation, and was in the range of one -fifth of total energy produced in the three rat stocks. 6. Wistar rat s showed a biphasic circadian rhythm of substrate utilization, with al ternate lipid synthesis/ degradation that reversed that of carbohydrat e, concordant with nocturnal feeding habits. Zucker rats did not show this rhythm; obese rats synthesized large amounts of fat during most o f the light period, consuming fat at the end of the dark period, which suggests more diurnal feeding habits. Lean Zucker rats showed a simil ar, but less marked pattern. 7. The results obtained indicate that lea n and obese rats can be studied using the same indirect calorimetry fo rmulae provided that there is an adequate measure of protein oxidation and the composition of diet does not differ.