Science Question from a Toddler: Why is poop brown?

By at 6:00 am Wednesday, Dec 30

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Two Science Questions from a Toddler in the same month? It's a Festivus miracle! Or, you know, the unexpected byproduct of trying to write weekly blog posts during a month where damn-near all the sources you need to talk to are on vacation. But I'm a glass is half-full kind of woman.

Speaking of byproducts, BB reader Tammy says there's a small person in her life who wants to know, "Why is poop brown?"

First off, the fact that this kid's poop is brown is a really good sign. See, your stool can come in several different colors. Brown just happens to be the color of good health.

"Bile comes from your gall bladder and helps your body digest food," said Anish Sheth, M.D., assistant professor at Yale Medical School and author of the book What's Your Poo Telling You? "It's metabolized by the bacteria in your large intestine, leaving behind a byproduct called stercobilin—and it's that stercobilin that gives stool a brown pigment."

Without stercobilin, your poo would actually be a sort of pale, off-grey color, like white clay. This really does happen from time to time, Dr. Sheth said, when something is blocking a patient's bile duct, so that bile can't get from the gall bladder into the intestinal tract. The cause could be as simple as a gall stone, or as ominous as pancreatic cancer.

In fact, the color of poop can offer some surprising insights into what's going on with the human body. In the days before fancy medical technology, doctors looked at the color and texture of poop to help diagnose gastro-intestinal illness. Today, changes in stool are still frequently the first sign that something is wrong. There's three main "wrong colors" your poop can be:

  • Red
    Means: Internal bleeding, or that you've recently eaten beets

    If it only happens once, it's probably the beets. But ongoing red poop likely means you're bleeding somewhere along your G-I plumbing. If the problem is near the bottom, in the intestines, Dr. Sheth said, the poop will be bright red. But if you're bleeding from the throat or stomach, then the blood will get digested along with whatever you've eaten—leaving you with black, tar-like poop. "It has a very distinctive smell, too. I'm not really sure how to describe it. But once you've smelled it, you don't forget it," Dr. Sheth said.
  • Yellow
    Means: There's fat in your poop

    The yellow color is the least disgusting part of this problem. Fatty poop also smells way worse than normal and it tends to float. "Like an oil slick," Dr. Sheth said. That's not a good thing, to put it mildly. In general, your body ought to be able to digest and absorb fat. If it's not, there's probably something wrong with your digestive system.
  • Green
    Means: You've probably got a bacterial infection

    A particular kind of bacteria that can infect your colon can also turn your poop green—the color is a direct result of the presence of bacteria in the poop.

While I had Dr. Sheth on the line, I decided that I had to ask him the ultimate "Why does my poop look like that" question. Oh yes, I asked about corn.

"There are a lot of things that we can't fully digest. I call it 'Deja Poo'. Corn is just the most common example," Dr. Sheth said.

The key is fiber. There's two kinds: Soluble and insoluble. If what you eat has a lot of insoluble fiber, it'll come out your other end mostly intact, because your body can't digest it. The foods that contain insoluble fiber foods are almost all plants, Dr. Sheth said, because humans haven't evolved the enzymes necessary to break down some plant cell walls. In the case of corn, some of the plant is soluble fiber and some isn't.

"There's two parts of the corn—the exterior kernel that we pass through and the germ inside of it. The germ is actually extracted. The whole kernel is immersed in digestive enzymes and your body pulls out what it can use," Dr. Sheth said. "What it can't use just passes on."

Ask Dr. Sheth your poop-related questions at his Web site, Dr. Stool

Image courtesy Flickr user GregtheBusker, via CC

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Maggie Koerth-Baker

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