Submitted to: Biokyowa Technical Review
Publication Type: Review Article
Publication Acceptance Date: 11/1/2002
Publication Date: 10/28/2002
Citation: Kerr, B.J., Guzik, A.C., Southern, L.L. 2002. Tryptophan: effects on neutotransmitters, behavior, meat quality, and the results of current requirement studies in nursery pigs. Biokyowa Technical Review. p. 1-20.
Technical Abstract: Tryptophan is one of most interesting and complex amino acids. The lack of sensitive methods for tryptophan determination in proteins and biological fluids, low content in common feedstuffs, low product availability, high cost of crystalline tryptophan, and the high degree of variability associated with scientific studies have made determination of tryptophan requirements for animals difficult. Feedstuffs of relatively high tryptophan content include eggs, milk, soybeans, potatoes, and cereals such as rice, barley, wheat, rye, and oats. Tryptophan is completely lacking in gelatin and its content in yeast and corn is extremely low (National Research Council, 1998). In general, tryptophan is the third or equally second limiting amino acid in typical corn-soybean meal diets for pigs. Many researchers have provided requirement estimates for tryptophan in each phase of growth, but there has been a lot of variation in the protein content of the diets used and the expression of requirements on a total versus digestible basis. The literature cited in this review in combination with the described research at the Louisiana State University Agricultural Center suggests that the requirement for tryptophan and the tryptophan:lysine ratio is slightly below National Research Council (1998) recommendations when using corn-soybean meal-pea based diets. Diets containing barley, wheat and corn gluten meal, however, may require higher tryptophan:lysine ratios as suggested by the research at Louisiana State University and also in Europe. In addition, tryptophan is one of the limiting essential amino acids in protein metabolism and serves as the precursor of several neurotransmitters as well as for the vitamin, nicotinic acid. By its involvement in so many pathways, tryptophan and its metabolites regulate neurobehavioral effects such as appetite, sleeping-waking-rhythm, impulsivity, aggression, sexual behavior, and pain perception. Research, however, is lacking on the effect of supplemental tryptophan on meat quality and carcass characteristics. Based on the influence of tryptophan on serotonin production, this may be an area for tryptophan supplementation if future research indicates the level and length of time supplemental tryptophan is needed to decrease stress before slaughter. Research results described in this review provides nutritionists at swine production facilities and feed companies data on dietary levels of tryptophan necessary to optimize growth performance and to minimize nitrogen excretion in pigs. This review also suggests that the use of tryptophan for behavior modification and improved meat quality is not consistent at this time.