Whole-body protein metabolism and plasma profiles of amino acids and hormones in growing barrows fed diets adequate or deficient in lysine

Eighteen growing barrows (body weight: 26.6 +/- 1.4 kg) were assigned (day 0) to one of three levels of dietary lysine: to be deficient (0.45%; L1), t o achieve (0.75%; L2), or to exceed (0.98%; L3) National Research Council ( NRC) (1988) lysine requirements las-fed basis), according to a completely r andomized block design. Nitrogen balance was measured over a 6-d period (da ys 10 to 16). On day 16, pigs were moved into respiratory chambers and rece ived a 6-h continuous i.v. infusion of (NaHCO3)-C-13 (1.66 mu mol h(-1) kg( -1), preceded by a priming dose of 2.35 mu mol kg(-1)) on day 17. The follo wing day, pigs received an i.v. infusion of L-[1(-13)C]leucine (11.07 mu mo l h(-1) kg(-1), preceded by a priming dose of 11.07 mu mol kg(-1)). Average daily gain and nitrogen retention increased (P < 0.10) by 36 and 20%, resp ectively, as dietary lysine content increased from L1 to L3. Leucine irreve rsible loss rate increased (P < 0.05) by 14% from diets L1 to L3, while leu cine oxidation remained constant (P > 0.10) between diets. In consequence, fractional oxidation decreased (P < 0.05) by 20%, from L1 to L3. Protein sy nthesis and degradation increased (P < 0.05) from diets L1 to L2, but the v alues for the diets L2 and L3 were similar. Insulin levels tended to increa se from L1 to L3 (P < 0.10), while that of triiodothyronine decreased from diets L1 to L3 (P < 0.05). Daily feed intake, nutrient digestibility, energ y metabolism and plasma concentrations of insulin-like growth factor 1 and growth hormone were not affected (P > 0.10) by treatments. In conclusion, t he improvement in protein gain associated with increasing lysine supplement ation to achieve lysine requirement involved a general stimulation of whole -body turnover, protein synthesis being increased to a larger extent than p rotein degradation.