The essential amino acid L-tryptophan is not restricted to its role in protein synthesis. The metabolism of L-tryptophan leads to biologically active compounds including serotonin, melatonin, tryptamine, quinolinic acid and kynurenic acid. It is the precursor of coenzymes NAD and NADP, and can partly replace niacin as an essential nutrient. These metabolic pathways are the reason why L-tryptophan plays such an important role in the regulation of feed intake, immune response, and behavior. 

The multi-function amino acid.

In addition to piglets and lactating sows, L-tryptophan can be limiting in diets for fattening pigs. The supplementation of L-tryptophan allows low crude protein diets to be formulated for fattening pigs (e.g. 15%  and 13% CP in grower and finisher diets, respectively).