Human milk (HM) is considered as the “gold standard” for infant nutrition. Breastfeeding is recommended up to 6 months by the WHO and up to 4 months by the French PNNS. Despite these recommendations, breastfeeding rates in France decrease remain too low. The Epiphane study (2015) showed that only 21% of the 3 months-old infants were still predominantly breastfed. Hence, HM substitutes such as Infant formulas (IF) are required to feed infant who are not breastfeeding. IF composition aims to mime HM composition. HM amino acids bioavailability is the digested and absorbed peptidic and free amino acids fraction used for metabolism. However, the true availability of tryptophan is not known with certainty. Tryptophan is an essential amino acid that cannot be synthesized by the organism and need to be provided by the diet. It is involved in protein synthesis and in production of beneficial metabolites for infant growth, particularly for brain development (e.g.: serotonin (behaviour hormone)) and intestinal maturation (e.g.: indole). Tryptophan, as an essential amino acid, is the limiting factor for establishing the protein value of standard IF (based on bovine whey proteins). Consequently, the knowledge improvement on tryptophan digestibility in HM and in IF will help to optimise the IF protein content. Furthermore, a better knowledge of the impact of the nature of milk (HM and IF) on tryptophan metabolism and production of derived metabolites is required to improve the growth and physiological properties of IF. The main objective of the thesis is to determine the amino acids, and specifically tryptophan, digestibility in HM and IF using an in vivo infant model, the Yucatan mini-piglet. The second objective is to provide in-depth knowledge on the role of tryptophan in infant gut and brain development according to the nature of milk. The presentation will first introduce the context of the study and then, the first results of the thesis will be presented.