Mystery solved: science reveals why urine is yellow

Did you know that human urine comes in all colors of the rainbow? There is red urine, which aficionados of borscht are familiar with. There's orange urine, which happens from taking laxatives. Blue or green urine can come from food coloring, or a hereditary disease. If you eat too much rhubarb, your urine could be brown. But most urine is yellow, and until now, scientists didn't know exactly why.

But a team of researchers at the University of Maryland and National Institutes of Health finally cracked the mystery. The yellow color comes from a microbial enzyme called bilirubin reductase.

"This enzyme discovery finally unravels the mystery behind urine's yellow color," said the study's lead author Brantley Hall, an assistant professor in the University of Maryland's Department of Cell Biology and Molecular Genetics. "It's remarkable that an everyday biological phenomenon went unexplained for so long, and our team is excited to be able to explain it."

From a University of Maryland press release about the discovery:

Aside from solving a scientific mystery, these findings could have important health implications. The research team found that bilirubin reductase is present in almost all healthy adults but is often missing from newborns and individuals with inflammatory bowel disease. They hypothesize 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 and NIH Investigator Xiaofang Jiang. "This discovery lays the foundation for understanding the gut-liver axis."