Why is urine's colour yellow? What does it mean?

A study published in Nature Microbiology on Wednesday identified the enzyme responsible for urine's characteristic yellow hue. This discovery sheds light on a question that has puzzled scientists for over a century, CBS News reported.

"It's remarkable that an everyday biological phenomenon went unexplained for so long, and our team is excited to be able to explain it," said Brantley Hall, an assistant professor in the University of Maryland's Department of Cell Biology and Molecular Genetics, in a statement.

Urine forms after a combination of electrolytes, and waste that a person’s kidneys filter out from their blood. More than a century back years ago, scientists found that urobilin was the yellow pigment in the urine, but were unaware of how it was produced.

Researchers said that urine’s colour is connected to the body’s red blood cells (RBCs).

Our red blood cells, like worn-out tires, eventually break down. When they do, they leave behind a bright orange stain called bilirubin. This bilirubin takes a tour of your gut, either catching a ride out with your poop or sticking around for another loop.

But guess what? Tiny bugs in your gut, like microscopic chefs, can take this orange gunk and turn it into different things, including the yellow stuff that gives your pee its sunny color. So next time you see that yellow in the bowl, remember it's all thanks to those little gut chefs working their magic!

"Gut microbes encode the enzyme bilirubin reductase that converts bilirubin into a colourless byproduct called urobilinogen," Hall, the study's lead author, said. "Urobilinogen then spontaneously degrades into a molecule called urobilin, which is responsible for the yellow colour we are all familiar with."

Before this study, scientists believed several enzymes were behind urine's colour, the researchers added.

When you're a hydrated human, your pee should be like a clear whisper (not too much water, though!), or maybe a light straw or honey shade. But if you're stingy with the water, it turns into a darker yellow or amber shout. Think of it as your body's way of saying, "Hey, refill the tank!"