Effects of fasting on muscle protein turnover, the composition of weight loss, and energy balance of obese and nonobese Zucker rats.

The effects of prolonged fasting on the composition of weight loss and the rates of muscle protein synthesis and degradation were compared in obese and nonobese (lean) rats. Lean rats weighting 400 g could survive 60 days of fasting whereas obese rats weighing 550 g could survive 60 days. Weight loss was similar in both phenotypes over the first 10 days of fasting (15 g/day), but the composition of weight loss differed. Obese rats lost nearly twice as much lipid but only one-fifth as much body protein as lean rats. The fasting metabolic rate [kilocalories/(day . kilograms 0.75)] was similar in both phenotypes. This finding indicates a slower decline in metabolic rate during fasting in obese rats, since maintenance requirements are greater in lean rats. In fed rats, the fractional rates of muscle protein synthesis (FRS) and breakdown (FRB) were slightly higher in obese rats. Fasting reduced muscle protein synthesis in both phenotypes. In obese rats, however, the FRS declined more slowly than in lean rats. On the other hand, FRB decreased in fasted obese rats but greatly increased in fasted lean rats. It was concluded that the better protein retention and slower decline in metabolic rate in fasted obese rats were related to their different regulation of muscle protein synthesis and breakdown during fasting.

Comparative amino acid and protein metabolism in obese and non-obese Zucker rats.

A mixture of [U-14C]amino acids was injected intraperitoneally into fed obese and non-obese Zucker rats that were either growing (8 weeks of age) or at maintenance (17 weeks of age). The metabolic fate of the dietary amino acid pool was determined from the distribution of 14C into carcass lean tissue, carcass lipid, CO2 and urine. In a second experiment, urinary and skeletal muscle 3-methylhistidine content was used to compare the rate of muscle protein breakdown between phenotypes at 8 weeks of age. The obese rat deposited a smaller proportion of its dietary amino acid pool in lean tissue compared with its non-obese control during growth and at maintenance. Obese rats incorporated a greater proportion of dietary amino acids into body lipid at both ages and metabolized a greater proportion of dietary amino acid carbon to CO2 during growth. At 8 weeks of age, the obese rat had a higher fractional rate of muscle protein breakdown and was less efficient at retaining amino acids that had been incorporated into muscle. These latter differences were major factors in producing the variation in dietary amino acid utilization and protein accretion between growing obese and non-obese rats. At maintenance, the variation in dietary amino acid utilization between phenotypes was due principally to the smaller body protein mass of the obese animals.

Effects in the rat of environmental temperature, diet dilution, and treadmill running on liver and adipose enzymes and metabolism of 14C-glucose: a multiple regression analysis.

The effects of two environmental temperatures (T; 16 degrees and 31 degrees), five diet dilutions (D; 0%, 12.5%, 25%, 37.5% and 50%), and five daily treadmill running periods (E; 10 minutes, 40 minutes, 70 minutes, 100 minutes, and 130 minutes) upon enzyme activities of liver and adipose tissue of male rats were observed. Liver enzymes studied were glucose-6-phosphatase (G6Pase), 6-P-gluconate dehydrogenase (6PGD), glucose-6-phosphate dehydrogenase (G6PD), fructose diphosphatase (FDPase), NADP-isocitrate dehydrogenase (ICDH), and malic enzyme (ME). Adipose tissue (epididymal fat) enzymes (6PGD, G6PD, and ME) were studied as well as the in vitro incorporation of the 14C of [U-14C] glucose into liberated 14CO2 and into the triglycerides, free fatty acids, and total lipids by adipose tissue slices. Equations describing regression surfaces for these responses (expressed as units/100 g body weight) could contain significant linear coefficients of the independent variables (T, D, and E), their first order interactions, and quadratic coefficients for D and E. Significnat regression coefficients for activities of liver enzymes associated with increased lipogenesis (6PGD, G6PD, and ME) produced response surfaces with conformations generally concave downward. All enzymes possessed positive and negative linear and quadratic coefficients for D which caused response surfaces to be concave downward with respect to that variable. Also, 6PGD and G6PD (positive linear and negative quadratic coefficients for E) exhibited response surfaces concave downward with respect to E. Additionally, 6PGD showed greater activity at 31 degrees than at 16 degrees while G6PD showed no effect of temperature on activity. Liver ICDH, probably important in supplying reducing equivalents for fatty acid synthesis, evidenced response surfaces almost identical to those for 6PGD. Significant regression coefficients for activity of liver enzymes associated with increased gluconeogenesis (FDPase and G6Pase) produced for FDPase a response surface concave downward with respect to both D and E with greater values at 31 degrees than at 16 degrees; but for G6Pase non-concave surfaces with lesser values at 31 degrees than at 16 degrees. Significant regression coefficients for activities of adipose enzymes associated with increased lipogenesis produced for 6PGD a response surface concave upward due to negative linear and positive quadratic coefficients for both D and E. For G6PD and ME regression surfaces were concave upward with respect to E, but these were modified by positive and negative linear coefficients, respectively, for D. Significant regression coefficients for incorporation of the 14C of glucose into triglycerides and free fatty acids of adipose tissue slices and their production of 14CO2 yielded response surfaces concave upward with respect to E (negative linear and positive quadratic coefficients). In addition, the surface for free fatty acids was concave upward with respect to D. The 14CO2 production was greater at 16 degrees than at 31 degrees...

Effects in the rat of environmental temperature, diet dilution and treadmill running on voluntary food intake, body composition and endocrine organ mass: a multiple regression analysis.

The effects upon weight gain, food intake, carcass composition, and weight of pituitary, adrenals, and testes of male rats subjected to three environmental temperatures (T; 16.0 degrees, 23.5 degrees, and 31.0 degrees), five diet dilution rates (D; 0%, 12.5% 25.0%, 37.5%, and 50.0%), and five daily treadmill running periods (E; 10 minutes, 40 minutes, 70 minutes, 100 minutes, and 130 minutes) were studied by multiple regression techniques. Equations describing regression surfaces could contain significant linear and/or quadratic coefficients of the independent variables (T, D, and E) and their first order interactions. Body weight attained was reduced by increased D and T. Total food intake increased linearly with increases in D, but this increase was smaller for the rats in the higher environmental temperature since there was a significant DT interaction. Total food intake was decreased by increases in T. The intake of food minus diluent was maintained until D attained a value of 37.5%; beyond this point the rats at all three temperatures failed to maintain digestible energy intake. The data strongly suggested that the energy intake was not limited by the gastrointestinal tract capacity. E did not significantly affect food intake. Significant regression coefficients were found for the dependent variables of body dry matter, nitrogen, ash, ether extract, and energy and of the weight of depot fat pads, adrenals, and testes, as follows: (1) For D, there was no linear coefficient for body nitrogen. All remaining linear coefficients were negative except for body ash. The only quadratic coefficients (both positive) were for adrenal and testes weights (2) For E, all linear coeffcients were negative except those for body nitrogen, body ash, and adrenal weight. There were not quadratic coefficients for body nitrogen, body ash, adrenal weight, and testes weight. The remaining quadratic coefficients were all positive; (3) For T, there was no quadratic coefficient for body energy. The remaining linear coefficients were positive except those for fat deposit size, body ether extract, and adrenal weight. The remaining quadratic coefficients were negative except those for fat deposit size, body ether extract, and adrenal weight; (4) For the DE interaction, no coefficients were significant; (5) For the DT interaction, there were no significant coefficients for body nitrogen, fat depot size, or adrenal weight. The remaining coefficients were positive except for body ash; (6) For the ET interaction, the only significant coefficients were negative ones for body ether extract and body energy and a positive one for testes weight.