Food is an immensely complex and endlessly varied matrix, rich in polymers of plant and animal origin, some of which resist hydrolysis by human digestive enzymes. The upper gastrointestinal tract is exquisitely adapted for the extraction of energy and nutrients from food, but inevitably some undigested material passes from the ileum into the colon, where the host benefits from the much greater repertoire of hydrolytic enzymes deployed by the resident microflora. Interest in the fate and biological effects of unabsorbed food residues has waxed and waned since at least the 19th century, but the scientific investigation of polysaccharides resisting hydrolysis in the small intestine began in earnest in the 1970s, with the study of dietary fibre. Early definitions of fibre encompassed only plant cell wall polysaccharides and lignin, but efforts to standardise the measurement of these substances in food soon revealed that a significant fraction of dietary starch could also resist hydrolysis by pancreatic enzymes, both in vitro and during normal digestion in humans.1

Starch is the principal storage polysaccharide in plants utilised by humans as food. It consists of varying proportions of two polymers: amylose, a simple linear chain of glucose units linked by α1–4 bonds, and amylopectin, which is a more complex macromolecule containing shorter linear chains and α1–6-linked branches. In plant tissue, the starch polymers are packed into semicrystalline granules, which need to be hydrated and dispersed by cooking before they can be digested. Starch resists digestion for many reasons; in leguminous …