Human milk (HM) is a complex fluid containing a myriad of bioactive compounds, providing the nutritional requirements of infants and ensuring their optimal growth and development [[1], [2], [3]]. However, a high rate of infants receiving infant formula (IF) is still worldwide reported [4]. Despite the optimization of IF manufacturing over the last few decades [5], differences in IF nutritional quality related to milk origin (e.g., bovine milk vs. HM) and IF manufacturing practice [[6], [7], [8]] still exist, with consequences on digestion kinetics [[9], [10], [11], [12], [13]], altogether leading to metabolic discrepancies between breastfed and IF-fed infants [[14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]].

Although the underlying mechanisms of communication between the intestinal microbiota and the gut-brain axis are not fully understood, vagal, immune and endocrine pathways are associated with reciprocal exchanges of signals from the gut and the brain, and of metabolites produced by the intestinal microbiota [27,28]. Within this complex system of interactions, early postnatal life nutrition plays a central role via nutrient interaction with intestinal epithelial cells and microbiota, priming the balance between pathways, thereby modulating infant development. However, the dietary impact on metabolic signatures of key organs involved in the gut-brain signal exchange is poorly documented in this sensitive time window for infant development.

In a preclinical study, we have recently demonstrated that the microbiota-gut-brain axis development is modulated by infant nutrition [19]. The intestinal microbiota profile associated with HM-feeding correlated with an improved maturation of the intestinal epithelial barrier, immune system and endocrine functions, and with a modulation of intestinal and brain tryptophan (Trp) metabolism as well as with several other brain functions in the early postnatal period of life [19].

To better understand the underlying mechanisms of intercommunication between the intestine and brain regions, the present study aimed to compare metabolomic profiles of colonic digesta, plasma, liver and 6 regions of the brain, between HM- and IF-fed Yucatan minipiglets used as a human infant model. The suckling pig is a well-established and suitable animal model for human infants, with a digestive system that closely mimics that of humans and similar brain development [29]. The relationship between dietary protein reaching the gut microbiota and inducing the release of metabolites and their bioavailability in the brain has been demonstrated in the pig model [[30], [31], [32], [33]]. In addition, the Yucatan minipig has the advantage of consuming less food (and therefore less milk) than the conventional pig and provides access to a sufficient amount of samples (tissue and digesta) for different analysis. A targeted metabolome approach using tandem mass spectrometry coupled to liquid chromatography (LC-MS/MS) was performed and allowed the quantification of 45 metabolites, including proteinous amino acids (AAs), Trp-derived metabolites, polyamines, neurotransmitters and other nitrogenous compounds. Correlations with our previously published data [19] were also carried out (gene expression and gut microbiota).