Golden news for a new year: Scientists now know why urine is yellow.

It's only taken 100 years, but researchers say they've pinpointed the enzyme in urine behind its buttery hue.

“This enzyme discovery finally unravels the mystery behind urine’s yellow color,” said study lead author Brantley Hall. He's 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," he said in a university news release.

It all starts with the degradation of the body's trillions of red blood cells once they end their average six-month life span.

Scientists have long understood that bilirubin, a bright orange pigment, is a byproduct of this cellular change. Bilirubin is secreted into the gut, where it lies ready for excretion.

While bilirubin waits in this digestive way station, the gut's microbes get busy breaking it down into other molecules.

“Gut microbes encode the enzyme bilirubin reductase that converts bilirubin into a colorless byproduct called urobilinogen,” Hall explained. “Urobilinogen then spontaneously degrades into a molecule called urobilin."

It is this final molecule, urobilin, "which is responsible for the yellow color we are all familiar with," Hall's team said. They published their findings Jan. 3 in the journal Nature Microbiology.

Besides answering a longstanding physiological puzzle, the identification of bilirubin reductase and urobilinogen could further the understanding of various illnesses.

For example, Hall's team found that bilirubin reductase is actually missing from newborns, as well as people with inflammatory bowel disease. So, bilirubin reductase deficiency might contribute to infant jaundice and in the formation of gallstones, they hypothesized.

“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.”