Poop (or feces) isn’t always brown, but it often is. That’s because of pigments. Pigments are molecules that give things color. If you’re wondering why our bodies would make a pigment just to waste it, good question! The answer is we don’t. But some pigments do wind up as waste products.

This story really starts in our blood. One of the big jobs of blood is to move oxygen from the lungs to our cells. Our red blood cells are filled with a molecule called hemoglobin, which carries the oxygen. Hemoglobin is made of iron and protein.

Our red blood cells are the only cells in our bodies without nuclei (the plural of nucleus). This allows them to carry more hemoglobin. It also means that the red blood cells don’t have instructions for repairing themselves, so they don’t last long, only about 4 months. Our bodies are good at recycling, so when blood cells are old, they break down the hemoglobin and re-use parts of it. The protein is broken down and used to make new proteins. Some of the iron is recycled directly to make new hemoglobin. Some of the parts with iron are changed to something called bilirubin (sounds like “Billy Rueben”).

The next stop for bilirubin could be the kidneys, where it is put into urine. It makes urine yellow. Another path it could take is to be turned into bile by the liver. The liver helps to clean our blood of toxic stuff. The bile that it makes is stored in the gall bladder. When we eat fatty foods, the gall bladder squirts bile into the small intestine to help break up the fat. Some of the bile and the bilirubin it contains keeps going through the small intestine, then the large intestine, and leaves in the feces, giving them the brownish color. Some of the bile might get absorbed into the blood vessels in the walls of the intestines. When this blood goes to the liver for cleaning, the bilirubin is put back into bile for another trip.

What might a doctor learn by knowing about a person’s feces?

If questions like this interest you, you might be interested in studying biology or medicine.

Poop is basically the stuff your body can't digest or absorb after you eat it. There is also waste from your liver - old red blood cells that have an orange-brown substance called billirubin mixed with yellowish bile. Most of the time, mixing all of this together just results in "brown" - kind of like when your watercolor paints run over into each other and you get "brown."

As you know, if you eat, you poop! Some foods can turn your poop a distinct color. Eating beets tends to make the poop a little purple and eating lost of green leafy veggies like spinach and kale turn it a bit green.