Indigestible describes food components that human digestive enzymes cannot break down — dietary fibre above all: cellulose, hemicelluloses, lignin, and resistant starches that pass through the small intestine intact. “Indigestible” is not synonymous with useless: fibre’s indigestibility is precisely its virtue — it adds bulk that prevents constipation, slows nutrient absorption, and feeds the colonic bacteria whose fermentation products nourish the gut lining. Some indigestible components cause trouble in excess — the oligosaccharides of beans ferment into gas — and truly indigestible contaminants (plastic, for instance) are hazards. But in nutrition, indigestibility is often a feature, not a failure.