Natural ?capsule? in food plants: Cell wall porosity controls starch digestion and fermentation

Dietary fibre (DF) is increasingly being recognised as playing an important role in positive metabolic and health effects. Cell walls in the endosperm and cotyledon of plant foods are an important source of DF, and its behaviour in the digestive tract underlies the health-beneficial mechanisms. In the small intestine, the extent to which cell walls restrict enzyme access inside the intact cells largely determines macronutrient digestion followed by release and adsorption of hydrolysed micro-molecules. Porosity, a structural parameter of cell walls, controls the extent of restriction in a size-dependent manner. Plant tissues with macronutrients that are incompletely digested in the small intestine are excursed to colon and become fermentable substrates for microflora; cell walls with higher structural complexity than purified DF tend to result in more complex colonic microbiota. This mini-review summarizes the recent studies about how cell walls impact on the digestibility and fermentability of encapsulated macronutrients (especially starch). The reductionist approach using isolated individual plant cells (as a model) reveals important aspects of the nutritional functionality. Porosity is a useful parameter to evaluate the structure-function relations of plant foods.