It seems like an answer that the scientific community should have already had, but the exact reason as to why urine is yellow has been elusive until now.
Researchers at the University of Maryland have cracked the code, identifying an enzyme that’s responsible for giving urine its yellow hue.
“We’re elated,” said Brantley Hall, a professor in the department of cell biology and molecular genetics at the University of Maryland in College Park.
Hall is the lead author of the study, which was published Wednesday in the journal Nature Microbiology.
“It’s a really important fundamental discovery, and we’re so happy that people can engage with this because it’s something they’re familiar with,” said Hall.
For decades, researchers had identified the molecule “urobilin” as the thing that makes urine yellow. But Hall’s research went further, finding out where urobilin actually comes from and how the body produces it.
When red blood cells degrade after their six-month life span, a bright orange pigment called “bilirubin” is produced as a byproduct.
Bilirubin is typically secreted into the gut, where it is destined for excretion.
Gut microbes encode the enzyme “bilirubin reductase” that converts bilirubin into a colorless byproduct called urobilinogen.
Urobilinogen then degrades into urobilin.
“We’re building on the shoulders of a lot of people who have studied this phenomenon before,” Hall said. “It’s remarkable that an everyday biological phenomenon went unexplained for so long.”
Aside from solving a scientific mystery, the study could have other health implications.
Hall’s team found that bilirubin reductase is present in almost all healthy adults but is often missing from newborns and people who have inflammatory bowel disease.
Their theory is that the absence of bilirubin reductase 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,” said study co-author Xiaofang Jiang. “This discovery lays the foundation for understanding the gut-liver axis.”
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