Our study aimed at investigating the impact of the level of protein in milk formula on intestinal structure, barrier function, and its nervous regulation in normal and LBW neonates using a porcine model. Normal birth weight (NBW) or LBW piglets were fed from d7 to d28 of age either with a high protein (HP) or with an adequate protein (AP) formula or stayed with their mother [mother fed (MF)]. The proximal jejunum and distal ileum were sampled at d28 for morphometry analysis and ex vivo permeability measurement in Ussing chambers. Formula feeding induced a trophic effect on the jejunum and ileum of both NBW and LBW piglets, which exhibited longer villi than MF animals, irrespective of the type of formula. In NBW piglets, intestinal permeability was not altered by formula feeding. On the contrary, LBW piglets fed with HP formula, but not AP, exhibited a greater ileal permeability than MF piglets. Feeding the HP formula also disturbed jejunal and ileal regulation of permeability by acetylcholine and vasoactive intestinal peptide (VIP) in LBW compared with MF LBW piglets. In conclusion, the level of protein in formulas did not modify intestinal structure and function in NBW individuals but dramatically modified intestinal barrier function physiology in LBW individuals. (Pediatr Res 69: 4-9, 2011)