Nature is full of colorful and mysterious questions: Why is the sky blue? And why is pee yellow? While scientists have long provided in-depth answers to the former question, they have only recently solidified the answer to the latter (more potty-oriented) question.

Urine gets its yellow hue from a microbial enzyme known as bilirubin reductase, according to a study published last week in the Nature Microbiology journal. The findings, by researchers from the University of Maryland and National Institutes of Health, pave the way for studies into the gut microbiome’s role in adverse conditions like jaundice and inflammatory bowel disease.

Scientists identified the yellow pigment in urine, urobilin, more than 125 years ago. But the enzyme that makes urobilin remained a mystery through the years, according to the study. Scientists previously thought multiple enzymes were involved, but the study found that a single enzyme, bilirubin reductase, is responsible. 

“It’s remarkable that an everyday biological phenomenon went unexplained for so long, and our team is excited to be able to explain it,” the study’s lead author Brantley Hall said in a release. 

Why is urine yellow?

Urine is made up of water, electrolytes and waste filtered out of the blood by the kidneys. The study found that urine’s color is connected to the body’s red blood cells. When red blood cells break down after their six-month lifespan, it produces a byproduct called bilirubin, which is a bright orange pigment. 

Bilirubin is typically secreted into the gut, where it can either be excreted or partially reabsorbed. The resident microorganisms in the gut can convert bilirubin into other molecules. 

“Gut microbes encode the enzyme bilirubin reductase that converts bilirubin into a colorless byproduct called urobilinogen,” Hall said. “Urobilinogen then spontaneously degrades into a molecule called urobilin, which is responsible for the yellow color we are all familiar with.”

Why is this important?

Along with illuminating a long-standing mystery of the bathroom, the new research also shines some light on a variety of health conditions. 

Jaundice — a condition in which the skin, whites of the eyes and mucous membranes turn yellow — can be caused when an excess of bilirubin is reabsorbed into the gut and builds up in the blood.

Researchers in the latest study found that bilirubin reductase, which converts bilirubin into urobilinogen, is present in nearly all healthy adults. But, it is often missing in newborns and people with inflammatory bowel disease, and its absence may contribute to infant jaundice and the formation of pigmented gallstones.

“Now that we’ve identified this enzyme, we can start investigating how the bacteria in our gut impact circulating bilirubin levels and related health conditions like jaundice,” study co-author Xiaofang Jiang said. “This discovery lays the foundation for understanding the gut-liver axis.”