Dietary Leucine Supplementation Decreases Whole-Body Protein Turnover before, but Not during, Immune System Stimulation in Pigs

Background: Immune system stimulation (ISS) adversely affects protein metabolism and reduces pig productivity. Leu has a regulatory role in skeletal muscle and whole-body protein turnover, which may be affected by ISS.

Objective: We sought to determine the effect of supplemental Leu intake on whole-body protein turnover in pigs before and during ISS.

Methods: Pigs [mean ± SD initial body weight (BW): 10.6 ± 1.1 kg] were surgically fitted with jugular vein catheters and assigned to 1 of 3 treatments: 1.36% standardized ileal-digestible (SID) Leu (CON; n = 13); 2.04% SID Leu (LEU-M; n = 8); and 2.72% SID Leu (LEU-H; n = 7). Pigs were infused continuously with 0.66 ± 0.05 mmol ¹⁵N ⋅ kg BW^-1 ⋅ d^-1 to determine whole-body protein kinetics. The study consisted of a 72-h prechallenge period followed by a 36-h challenge period. At the start of the challenge period, ISS was induced in all LEU-M and LEU-H pigs and half of the CON pigs with LPS (ISS+); the remaining CON pigs were administered saline (ISS-).

Results: Whole-body protein synthesis (309, 273, and 260 ± 14 mmol N ⋅ kg BW^-1 ⋅ d^-1 for CON, LEU-M, and LEU-H pigs, respectively) and protein degradation (233, 203, and 185 ± 14 mmol N ⋅ kg BW^-1 ⋅ d^-1 for CON, LEU-M, and LEU-H pigs, respectively) were reduced with increasing Leu intake during the prechallenge period (P < 0.05). ISS reduced whole-body protein synthesis (203 compared with 169 ± 12 mmol N ⋅ kg BW^-1 ⋅ d^-1 for ISS- and ISS+ pigs fed CON, respectively; P < 0.05) and protein deposition (PD) (64.9 compared with 45.0 ± 2.9 mmol N ⋅ kg BW^-1 ⋅ d^-1 for ISS- and ISS+ pigs fed CON, respectively; P < 0.01), whereas ISS did not affect whole-body protein degradation. Leu intake did not affect whole-body protein synthesis or degradation in ISS+ pigs.

Conclusions: Our results indicate that supplemental Leu intake improves the efficiency of PD rather than PD directly in healthy pigs but did not affect whole-body protein turnover during ISS.