Total Metabolic Rate. Body0Density and Tissue Protein Changes' in Acute Fasting of Rats

Measurements of oxygen consumption, total body density, tissue density and tissue protein content were made on two groups of rats: the first group was the normal control group and the second group was starved for 6 days with free access to water. After the measurement of oxygen consumption animals were sacrificed and the determination of lean body mass (LBM) was made by means of underwater weighing. Comparisons were made between the control and fasted group. The following results were obtained. 1. Total energy expenditure of rat decreased as the fasting progressed. This decrease was parallal to decrease in lean body mass. 2. Although the decrease in oxygen comsumption was parallel to the decrease in gross body weight, the change of lean body mass correlated more closely with the oxygen consumption. 3. Subsequently, oxygen consumption expressed on the basis of both gross body weight or-lean body mass did not decrease as compared to the control value. Oxygen consumption expressed on the basis of body surface area decreased as the fasting progressed. Body surface area was not a good reference expressing oxygen consumption in fasting of the rats. 4. Total body density increased in the fasted group. This reflected the decrease in fat when fasting continued. 5. Tissue density of liver and muscle (gastrocnemius) decreased and that of skin increased in the fasted rats. 6. Protein contents of tissues showed variable changes in acute fasting: liver showed no change, muscle showed a decrease and skin showed an increase. 7. Tissue protein content, which is one component of lean body mass, influenced the changes of tissue density. 8. Total body density was the sum of tissue density

Blood Volume of the Fasted Rats.

Blood volume determinations by means of T-1824 dilution were made on two groups of rats: the first group was the normal control group consisted of 16 rats, the second group was starved for 5 days with free access to water and consisted of 18 rats. After the measurement of blood volume animals were sacrineed and the determination of lean body mass was made by means of underwater weighing. Comparisons were made between the control and the fasted group. The absolute value of plasma and blood volume was found to decrease relative to the initial body weight before the beginning of fasting in the fasted group. Plasma and blood volume. however, expressed on the basis of the final body weight or lean body mass was found to increase in the fasted group. In fasting, body weight, plasma volume and lean body mass decreased with different rate. The rate of decrease was largest in body weight and smallest in lean body mass. Therefore, plasma or blood volume relative to the final body weight showed the greater increase than the value expressed on the basis of lean body mass. In expressing plasma or blood volume in the fasted rat lean body mass was the best reference