Integrated Protein–Lipid Digestion Profiles of Human Milk, Infant Formulas, and Dairy Ingredients Under Infant In Vitro Digestion

Human milk (HM) represents the physiological reference for infant digestion, whereas infant formulas (IFs) differ in protein composition and lipid architecture. This study compared the gastric and intestinal digestion of HM, five commercial IFs, whole milk powder (WMP), and protein ingredients using a standardized infant in vitro model. Protein hydrolysis was assessed through digestibility and residual subunit patterns, and lipid digestion was evaluated through free fatty acid release and fatty acid profiling. Gastric digestion revealed pronounced differences driven by intrinsic structure and processing, whereas intestinal digestion reduced these disparities, although distinct subunit patterns persisted. Lipid analyses showed clear source-dependent separation before digestion and partial convergence during intestinal lipolysis. Several IFs containing structured lipid and vegetable-oil systems showed intestinal fatty acid or free fatty acid profiles closer to HM than WMP, whereas samples with stronger bovine-fat signatures remained more distinct. Overall, the results demonstrate that digestion-product patterns provide mechanistic insight into structure-driven digestion behavior in complex dairy-based matrices, beyond compositional comparison alone.