Effect of dietary protein intake on plasma leucine flux, protein synthesis, and degradation in sheep.

Combined experiments of an isotope dilution method of [1-(13)C]leucine with open circuit calorimetry and a nitrogen (N) balance test were applied to determine the effect of dietary crude protein (CP) intake on plasma leucine flux and protein synthesis and degradation in four sheep. The experiment was conducted in a 3 x 4 Latin rectangle design of three 3-week periods. Dietary CP intake was 5.6, 7.7, and 10.8 g/(kg(0.75) x d). Metabolizable energy intake was 120% of requirement for all dietary treatments. [1-(13)C]Leucine was intravenously infused for 8 h and blood and breath samples were collected during the latter 2-h period of infusion. Isotopic enrichments of plasma [1-(13)C]leucine, alpha-[1-(13)C]ketoisocaproic acid, and exhaled (13)CO(2) were determined. For the N balance test, N digestibility, N excretion in urine, and protein balance (N x 6.25) increased with increasing dietary CP intake. Rates of plasma leucine turnover, protein synthesis, and degradation changed toward reduction with increased dietary CP intake. It is likely that in sheep, high CP intake enhances protein deposition with reduced protein degradation rather than increased protein synthesis.