Unravelling the multi-scale structural organisation of in vivo ileal digesta from diets containing protein-seaweed polysaccharide blends.

Abstract

Seaweed-derived polysaccharides such as agar, alginate, carrageenan, and cellulose are increasingly used as functional ingredients in food products, yet their influence on the gastrointestinal digestion process remains poorly understood. This study investigated how different fibres modulate the digestion and nanoassembly process of two model food proteins, casein and whey protein isolate, under physiologically relevant conditions using an in vivo pig model. Ileal digesta were analysed through compositional, rheological, microstructural, and nanostructural characterisation. The results demonstrated that the proteins were extensively hydrolysed and largely absorbed before reaching the ileum in all the formulations. However, some peptidic fragments were particularly resistant to digestion when agar was added to WPI. The seaweed polysaccharides were not digested and showed different types of structures, with alginate generating dense network-like structures, while carrageenan and agar generated more homogeneous digesta. Cellulose was associated with the presence of more ordered nanomicellar structures, likely involving bile salts. This behaviour suggests alterations in bile salt organisation and availability, potentially related to changes in diffusion, retention or reabsorption. These findings suggest that seaweed polysaccharides do not impair protein digestibility, but they affect the micro- and nanostructural organisation of ileal digesta, primarily by modulating viscosity and colloidal assembly. Therefore, it is highly relevant to understand the impact of dietary fibres on the intestinal transport of nutrients, providing the basis for designing functional foods with improved nutritional quality.