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.