Gross body composition and total energy metabolism in normal rats

¹ Laboratory of Physiology, National Cancer Institute National Institutes of Health, Bethesda, Maryland

The gross body composition of normal, male rats weighing between 150 and 300 gm was measured utilizing recognized procedures. By determining the nitrogen and calorie content of the individual nonfat and fat fractions obtained by chemical separation of the rat's dry, total carcass, a two-compartment model was established suitable for predicting from the values of milligrams nitrogen per gram and kilocalories per gram the proportions of fat and nonfat substance in the total dry carcass of any individual rat. The theoretical line of average composition which was constructed is described by the equation: mg N/gm = –29.4 Kcal/gm + 274. The study of the total energy metabolism of five growing, male rats was done over a continuous 8-day period. From the experimental data it was possible to compute the values for kilocalories and milligrams of nitrogen retained per gram of dry carcass weight gained by the living animal. The values, when plotted, were in excellent agreement with the theoretical line of average body composition. The results indicate that the theoretical model is satisfactory for estimating the composition of the dry total carcass of the sacrificed animal or the dry carcass weight gain of the living animal.