Food microstructure and protein digestion in the human gastrointestinal tract: an ileostomy study focusing on interindividual variability in gluten digestibility

Abstract

The degree of digestion of gluten proteins is thought to be directly linked to their capacity to elicit immune-mediated responses in predisposed individuals. Levels and timing of detection of gluten epitopes in stool samples exhibit high interindividual variability. However, the reasons behind this variability remain unclear and existing data do not allow distinctions between the effects of the upper gastrointestinal tract and colonic fermentation. This ileostomy study aimed to analyse the structural and biochemical degradation of a gluten-containing meal in the terminal ileum and to investigate interindividual variability. Eleven participants consumed the test meal (oat porridge and a wheat breakfast cereal) and ileal effluent was collected once every hour over eight hours for quantification of soluble protein, free amines, potentially immunogenic gliadin fragments, and insoluble nitrogen. The molecular weight distribution of soluble proteins was assessed by SEC-HPLC. Microstructural features were studied using scanning electron and confocal laser scanning microscopy. Total ileal effluent output exhibited a relatively high interindividual coefficient of variation (CV) (40.2%, range: 169.4 to 553.1 g), similar to protein and free amines (46.0% and 40.2%, respectively). The ratios of protein to ileal effluent and of free amines to protein exhibited lower CVs (26.9% and 18.6%, respectively). In contrast, potentially immunogenic gliadin fractions exhibited markedly higher variability (CV = 69.4%, range: 3.1 to 65.7 mg of gliadin equivalents), even after normalization by the gliadin-to-soluble-protein ratio (CV = 63.1%, range: 0.8 to 15.1 mg g⁻¹ of protein). Amino acid sequences recognized by the R5 antibody were detected in both 60% ethanol and aqueous extracts, with water-soluble components dominating gliadin output for almost all participants. The disproportionately higher variability in gliadin excretion compared to total nitrogen, protein, and free amines, suggests that interindividual differences in gluten hydrolysis originate primarily in the upper gastrointestinal tract, indicating that individual characteristics likely modulate the specific capacity to degrade gluten proteins.