Undigested describes food — or parts of food — that passes through the digestive system without being broken down and absorbed. Some of this is normal and healthy: dietary fibre is meant to resist digestion, adding bulk and feeding gut bacteria. Undigested food becomes a concern when it signals malabsorption — visible fat, starch, or muscle fibres in stool suggest the gut is failing to do its job, as in pancreatic insufficiency or coeliac disease. Hurried eating and poor chewing also leave more work undone. The distinction matters: fibre’s resistance is by design, while the escape of nutrients the body should have captured points to disease worth investigating.