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

Abstract

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), to achieve (0.75%; L2), or to exceed (0.98%; L3) National Research Council (NRC) (1988) lysine requirements (as-fed basis), according to a completely randomized block design. Nitrogen balance was measured over a 6-d period (days 10 to 16). On day 16, pigs were moved into respiratory chambers and received a 6-h continuous i.v. infusion of NaH13CO3 (1.66 µmol h−1 kg−1, preceded by a priming dose of 2.35 µmol kg−1) on day 17. The following day, pigs received an i.v. infusion of L-[1-13C]leucine (11.07 µmol h−1 kg−1, preceded by a priming dose of 11.07 µmol kg−1). Average daily gain and nitrogen retention increased (P < 0.10) by 36 and 20%, respectively, as dietary lysine content increased from L1 to L3. Leucine irreversible loss rate increased (P < 0.05) by 14% from diets L1 to L3, while leucine oxidation remained constant (P > 0.10) between diets. In consequence, fractional oxidation decreased (P < 0.05) by 20%, from L1 to L3. Protein synthesis and degradation increased (P < 0.05) from diets L1 to L2, but the values for the diets L2 and L3 were similar. Insulin levels tended to increase 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, energy metabolism and plasma concentrations of insulin-like growth factor 1 and growth hormone were not affected (P > 0.10) by treatments. In conclusion, the improvement in protein gain associated with increasing lysine supplementation to achieve lysine requirement involved a general stimulation of whole-body turnover, protein synthesis being increased to a larger extent than protein degradation. Key words: Pigs, protein, metabolism, hormone, lysine, leucine