As a gastroenterologist I spend a lot of time in people’s colons, and I’ve noticed over the years that there’s tremendous variation in not just what the colon looks like, but in what it smells like too. Inflammatory conditions like Crohn’s disease and ulcerative colitis cause redness and ulceration that’s easily identifiable, but they also have a characteristic odor.

When it comes to what stool smells like, there’s no real hierarchy of good versus bad. What comes out one end is mostly a reflection of what goes in the other – dedicated carnivores have different smelling stool from those who are plant based, and if you eat a lot of sulfurous foods like eggs and beans, that’s whiffable too. But inflammation has it’s own particular scent – a slightly sour odor that smells very different from a healthy colon.

The difference is the result of changes in the bacteria present in the gut, and we can actually measure both the vapors those bacteria produce (VOCs or volatile organic compounds), as well as the bacteria themselves. Crohn’s and ulcerative colitis have specific microbial signatures – lower diversity of bacterial species in general, and an increase in adherent-invasive E. coli (AIEC) and other pathogenic bacteria. Infectious diarrhea will usually cause a dramatic change in odor also, as “good” bacteria are replaced by more virulent species that rapidly replicate and alter the composition of the gut flora.

So when we think about diagnosing conditions in the colon, it’s not just what the eyes see, it’s what the nose knows too that’s important.

By: Dr. Robynne Chutkan