Leucine, Val, and Ile are categorized as the branched-chain AA (BCAA) because of the structural similarity of their side chains. All 3 BCAA share the enzymes that are involved in the first 2 steps of their catabolic pathway. The first step is a transamination step catalyzed by BCAA transaminase (BCAT) and producing branched-chain α-keto acids (BCKA). The second step is an irreversible degradation step catalyzed by branched-chain α-keto acid dehydrogenase (BCKDH). In the second step, Leu has been considered a key regulator as its BCKA metabolite (α-keto isocaproate) stimulates activation of the BCKDH complex in the liver. When excess Leu in diets is offered to pigs, degradation of all 3 BCAA may increase because of increased activity of BCAT and BCKDH. Leucine and Trp are both categorized as large neutral AA, and they share a common uptake pathway across the blood-brain barrier. Therefore, it is possible that excessive Leu may result in reduced Trp uptake into the brain due to competition for transporters, resulting in reduced serotonin synthesis.

Corn-based proteins usually have high concentrations of Leu and diets based on corn, soybean meal, and 20 or 30% distillers dried grains with soluble, therefore, usually contain between 150 and 200% of the required concentration of Leu. Sorghum protein contains even more Leu than corn protein and diets based on sorghum and sorghum DDGS will have high concentrations of Leu. Because of the potential negative impact of Leu on Val, Ile, and Trp metabolism, it is possible that negative impacts of Leu in practical diets are observed.

Therefore, the objective of this experiment was to test the hypothesis that excess dietary Leu may negatively affect N balance, growth performance, plasma urea N (PUN), plasma and cerebral serotonin, tissue BCAA, serum and tissue BCKA, and expression of genes related to BCAA metabolism in growing pigs.

Experimental Desing

Forty barrows (initial body weight: 30.0 ± 2.7 kg) were housed individually in metabolism crates and allotted to 5 dietary treatments (8 replicates per treatment). The 5 treatments were based on identical quantities of corn, soybean meal, wheat, and barley and designed to contain 100, 150, 200, 250, or 300% of the requirement for standardized ileal digestible Leu by adding increasing concentrations of crystalline _L-Leu to the diets. The body weight of pigs was recorded at the start and at the end of each period, and daily feed provisions were recorded as well. Urine and fecal samples were collected for 5 d following 7 d of adaptation to the diets to measure N balance and biological value of the diets. At the end of the experimental period, blood and tissue samples were collected to analyze PUN, plasma and cerebral serotonin, tissue BCAA, serum and tissue BCKA, and relative gene expression.

Results

The ADG, ADFI, and G:F decreased (linear, P < 0.05) as dietary Leu increased (Table 1). A trend (linear, P = 0.082) for decreased N retention and decreased (linear, P < 0.05) biological value of protein in diets was also observed. The PUN increased (linear, P < 0.05) as dietary Leu increased. A quadratic reduction (P < 0.05) in plasma serotonin and a linear reduction (P < 0.05) in cerebral serotonin were observed with increasing dietary Leu. Concentrations of BCAA in liver increased (linear, P < 0.001), whereas concentrations of BCAA in skeletal muscle decreased (linear, P < 0.05). Concentration of α-keto isovalerate (BCKA from Val) was reduced (linear and quadratic, P < 0.001) in liver, skeletal muscle, and serum, and α-keto-β-methylvalerate (BCKA from Ile) was reduced (linear and quadratic, P < 0.001) in skeletal muscle and serum. In contrast, α-keto isocaproate (BCKA from Leu) increased (linear, P < 0.05) in liver and skeletal muscle, and also in serum (linear and quadratic, P < 0.001) with increasing dietary Leu. Expression of mitochondrial BCAA transaminase and of the E1α subunit of BCKDH increased (linear, P < 0.05) in skeletal muscle as dietary Leu increased.

Key Points 

• Excess dietary Leu reduced ADG, ADFI, and G:F of growing pigs. The reduction was most pronounced if Leu exceeded 200% of the requirement.
• Nitrogen retention and biological value of N decreased with increasing dietary Leu because of stimulated catabolism of Val and Ile.
• The PUN concentration in pigs fed increasing levels of dietary Leu increased due to the reduced availability of Val and Ile.
• Plasma and cerebral serotonin decreased as dietary Leu increased.
• No changes in expression of genes related to the BCAA catabolic pathway were observed, whereas changes in BCAA and BCKA concentrations were observed as dietary Leu increased.

Table 1. Growth performance, N balance, and concentrations of plasma urea N and serotonin of growing pigs fed diets with graded levels of standardized ileal digestible (SID) Leu relative to requirement¹ (as-fed basis)²

¹NRC (2012)

²Each least squares mean represents 8 observations.

³Biological value was calculated as [N retained/(N intake – N output in feces)] × 100.