Protein nutrition and damaging behaviour in pigs

Supplementation of low-protein diets with amino acids (AA), such as tryptophan (Trp), has proven effective in reducing tail-biting (TB), a common welfare problem in pig production. Trp is a precursor of serotonin, a key neurotransmitter involved in mood, aggression, stress, and feed intake. Trp competes with large neutral amino acids (LNAA) to enter the brain by using the same transporter to cross the blood-brain barrier. Increasing the dietary Trp level compared to LNAA, raises the ratio of Trp:LNAA in plasma and enhances Trp transport into the brain, potentially boosting serotonin production and reducing harmful behaviours like TB. While most Trp is absorbed in the intestine, some reaches the colon, where it is metabolized by the gut microbiota. The microbiota plays a crucial role in the bidirectional interactions between the gut and the nervous system, known as the microbiota-gut-brain axis, which influences mood and behaviour. Moreover, dietary fibre (DF) plays a crucial role in maintaining gut microbiota diversity and has effects on nutrient absorption. The bulkiness properties of DF can affect on the gastrointestinal transit time and increase viscosity, leading to prolonged retention in the gut and enhanced nutrient digestibility by allowing more exposure to digestive secretions and contact with epithelium cells in the intestine. In addition, DF is partially fermented by gut microbiota in the large intestine, producing short-chain fatty acids that serve as energy sources, while also promoting satiety. It has been hypothesized that certain fibres may potentially reduce harmful behaviours. Therefore, this research aims to investigate the impact of Trp supplementation on TB and its interaction with other LNAA, while also exploring how DF influences protein absorption, behaviour, and satiety in pigs.