Protein substitutes used in the treatment of phenylketonuria distinctly modulate gut nutrient absorption and bacterial growth: An in vitro study.

Low phenylalanine (Phe) protein substitutes (PS), either l-amino acids (L-AAs) or casein glycomacropeptide with added amino acids (CGMP-AAs) are the mainstay of phenylketonuria (PKU) treatment. Given the central role of intestinal nutrient absorption and gut microbiota in metabolic regulation, this study investigated how different PS affect nutrient absorption and bacterial growth. It was hypothesized that l-AAs and CGMP-AAs would exert distinct effects. Caco-2 cells, an in vitro model of enterocytes, were used to assess the acute effects of PS and whey protein (WP) on glucose, amino acids, and fatty acids analogues uptake, as well as the longer-term effects on nutrient transporter gene expression. The growth and metabolite production of Escherichia coli, Lactobacillus paracasei, and Shigella flexneri were also assessed. l-AAs reduced glucose absorption and downregulated glucose transporter 2 (GLUT2). Although l-AAs did not affect amino acids or fatty acids absorption, they downregulated l-type amino acid transporter 1 (LAT1) and peptide transporter 1 (PEPT1). CGMP-AAs had no effect on glucose or fatty acid absorption but impaired amino acid absorption and upregulated LAT1. These findings demonstrate formulation-specific effects on nutrient absorption and transporter gene expression. All PS increased bacterial abundance, with l-AAs producing the most pronounced effect relative to CGMP-AAs. l-AAs and WP accelerated the growth rate of all bacterial species, whereas CGMP-AAs selectively promoted the growth of L. paracasei, indicating a potential prebiotic-like effect. In conclusion, PS distinctly modulate intestinal nutrient transport and bacterial growth, underscoring the importance of considering their specific metabolic impact in PKU and the need for further translational research.