Immunomodulatory, behavioral, and nutritional response of tryptophan application on poultry

Tryptophan is an essential amino acid for all animals that was discovered through casein hydrolysis. The use of tryptophan as feed additives has been attracting the attention of many nutritionists because it cannot be synthesized enough in an animal's body. Tryptophan or precursor to the vitamin niacin in the diet is important, and its supplementation for poultry is determined to improve the amino acid balance and promote the poultry's growth performance through enhancing appetite, feed efficiency, and protein synthesis. Moreover, poultry in different growth phases, breeding, and conditions require various amounts of tryptophan. In addition, supplemented tryptophan also improves the immune response or the immunomodulatory activity of poultry to various diseases through the kynurenine pathway, especially diseases in the bursa. Furthermore, tryptophan also has a strong relationship with lysine (the ideal tryptophan/lysine ratio) in improving growth performance. However, tryptophan deficiency could affect the behavioral responses (e.g. pecking behavior and poultry stress) because tryptophan serves as a precursor for the neurotransmitter serotonin and the pineal hormone melatonin in the diet. This paper tried to summarize all information about applying tryptophan in the diets and illustrate the roles of tryptophan in the poultry industry.