Tryptophan: functions beyond protein synthesis

Abstract Text: Tryptophan is a limiting essential amino acid for the growth of pigs. In addition to its important role to support growth, Trp also has unique physiological functions when it is metabolized to other compounds such as serotonin, melatonin, niacin, etc in the body. Serotonin is a cerebral neurotransmitter playing a major role in regulating physiological processes such as appetite, stress adaptation, activity, and aggressive behavior. Tryptophan crosses blood-brain barrier, and thus increasing Trp intake is shown to elevate serotonin synthesis in the brain of pigs. However, large neutral amino acids (LNAA) compete for a same type of transporter to cross blood-brain barrier and thus dietary ratio of LNAA to Trp can affect Trp availability for serotonin synthesis in the brain. In a typical production environment, pigs can be under social stress conditions as they go through re-grouping when they are weaned and when they are moved from nursery to finisher pens. Pigs confront with new mates when they are re-grouped in a same pen causing social mixing stress such as fighting and other aggressive behaviors. Pigs with social stress are shown to undergo increased systemic oxidative stress which is negatively related to animal productivity. In our research, increasing Trp intake up to 10.8 g/d elevated serotonin concentration in hypothalamus, reduced salivary cortisol, and enhanced growth of pigs under social mixing stress. Reducing dietary LNAA content from 4.5 to 3.8% provided similar benefits of increased Trp intake when Trp to LNAA ratio was maintained at 0.157. This allowed reducing supplemental levels of Trp from 0.8 to 0.6%. In summary, in a typical pig production, pigs can be under social mixing stress and increasing tryptophan intake seems to help reducing stress and thus improve performance of pigs.

Keywords: pigs, serotonin, social stress, tryptophan