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Interpretive Summary: Effects of dietary leucine and tryptophan on serotonin metabolism and growth performance of growing pigs   

By: Dr. Emily Taylor

Tryptophan is often a limiting amino acid in corn-soybean meal-based diets of pigs and plays a vital role in appetite regulation. However, this mechanism competes with other large neutral AA such as Val, Leu Ile, Tyr, and Phe. Standard corn and sorghum diets fed to pigs are rich in Leu and may reduce pig feed intake and growth performance. Therefore, the objective of this study was to evaluate excess Trp in high-Leu diets for growing pigs. The authors hypothesize that excess Trp, the precursor for serotonin, will prevent the drops in both plasma serotonin and hypothalamic serotonin concentrations that play an essential role in the mechanism for appetite regulation.

Pigs were assigned to treatments in a 3x3 factorial consisting of 3 levels of dietary Leu (101%, 200%, or 299% standardized ileal digestible (SID) Leu:Lys) and three levels of dietary Trp (18%, 23%, or 28% SID Trp:Lys). Increasing dietary Trp increased hypothalamic serotonin, though increases in average daily gain (ADG) and average daily feed intake (ADFI) were only observed in pigs fed diets containing excess Leu. ADG, ADFI, and hypothalamic serotonin were reduced with increasing dietary Leu levels. Dietary Trp at 299% SID led to an increase in ADG and ADFI. As expected, there were positive effects of Leu concentrations in diets containing Leu, while Trp in the diet negatively affected Leu concentrations. However, plasma Trp concentrations were positively impacted by increased dietary Trp and Leu at a 101% SID Leu;Lys ratio. 

This study provided evidence that supplementing Trp in Leu-heavy diets may help overcome the adverse effects of excess Leu. This demonstrates the importance of Trp in regulating hypothalamic serotonin, and therefore, feed intake and growth performance of pigs.

This article is available in the Journal of Animal Science.