Mystery solved! Researchers crack code on why your urine is yellow

COLLEGE PARK, Md. - If you have ever been dying to know why your urine is usually yellow, some brave and bold researchers have taken a look to come up with answers. 

Researchers at the University of Maryland and National Institutes of Health said they have identified the microbial enzyme responsible for giving urine its yellow hue. 

Their findings were published in the journal Nature Microbiology this month. 

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The enzyme is named bilirubin reductase. Scientists said with their new knowledge, they can learn more about the gut microbiome’s role in ailments like jaundice and inflammatory bowel disease.

"This enzyme discovery finally unravels the mystery behind urine’s yellow color," the study’s lead author Brantley Hall,. said in a news release. "It’s remarkable that an everyday biological phenomenon went unexplained for so long, and our team is excited to be able to explain it."

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Researchers said "when red blood cells degrade after their six-month lifespan, a bright orange pigment called bilirubin is produced as a byproduct."

They said bilirubin is then released into the gut where it can be excreted, but it could also partially be reabsorbed. Too much reabsorption can lead to a buildup of bilirubin in the blood and can cause jaundice—a condition that leads to the yellowing of the skin and eyes. 

Researchers add that once inside the gut, the resident flora can convert bilirubin into other molecules.

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

Studies have shown urobilin has always been linked to urine's yellow hue, but by discovering the enzyme responsible, researchers said it helps them understand a biological process that has eluded the scientific community for more than a century. 

The recent findings also answer other health questions. Researchers found that bilirubin reductase is present in nearly all healthy adults but is often missing from newborns and individuals with inflammatory bowel disease. That's why they believe the absence of bilirubin reductase may be a factor in 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 and NIH Investigator Xiaofang Jiang explained. "This discovery lays the foundation for understanding the gut-liver axis."

This story was reported from Los Angeles.