Foul-smelling stools

Stool naturally has an odour, produced by the gases and compounds gut bacteria make as they break down food, so some variation is normal and often simply reflects what you have eaten (foods high in sulfur, garlic, or a lot of protein), a supplement, or a course of antibiotics. A short-lived change with a stomach or gut infection (gastroenteritis) is also common and settles. What is more meaningful is a persistently and markedly foul smell, particularly when it comes with other changes — pale, bulky, greasy stools that are hard to flush, diarrhoea, or weight loss. This combination suggests malabsorption: when fat and food are not properly digested and absorbed, the excess reaching the large bowel is fermented by bacteria, producing a particularly offensive smell. The usual underlying causes are the same as for greasy stools — reduced pancreatic enzymes (as in chronic pancreatitis), small-bowel diseases such as coeliac disease, and bacterial overgrowth of the small intestine. Certain gut infections, such as giardia, are also classically associated with foul-smelling, greasy diarrhoea. So the smell alone is rarely the issue; it is the smell together with the appearance of the stools and other symptoms that guides what, if anything, needs looking into.

• Diet, supplements or antibiotics — a common, harmless and usually temporary cause.
• A gut infection (gastroenteritis, giardia) — often with diarrhoea; giardia classically causes foul, greasy stools.
• Malabsorption — pale, greasy, very smelly stools from poor fat absorption.
• Pancreatic insufficiency — as in chronic pancreatitis.
• Coeliac disease and small intestinal bacterial overgrowth.

A brief change in stool smell with diet or a passing bug needs no tests. When a very foul smell is persistent and comes with pale, greasy stools, diarrhoea or weight loss, the digestive causes of malabsorption are worth checking: fecal elastase for pancreatic enzyme output, coeliac serology (anti-tTG with total IgA), and fecal calprotectin for gut inflammation, with stool testing for infections such as giardia if suspected. Fat-soluble vitamin levels and a blood count gauge any nutritional impact. Mediora.AI can surface a low pancreatic elastase, positive coeliac markers, or inflammation from your results to point toward the cause; the assessment belongs with your doctor.