What gives urine its yellow color? The answer lies in the intestine.

A year ago, a study from the University of Maryland (USA), led by Professor Brantley Hall, revealed the discovery of the enzyme bilirubin reductase , which is responsible for the yellowish color of urine. The research, published in Nature Microbiology, solved a mystery that had intrigued scientists for centuries.

"The discovery of this enzyme finally unravels the mystery of urine's yellow color. It's incredible that an everyday biological phenomenon went unexplained for so long, and our team is thrilled to be able to explain it," Hall said.

The University of Maryland team explained that when red blood cells break down after about six months of existence, they produce a yellow-orange bile pigment called bilirubin.

In the gut, microbes create the enzyme bilirubin reductase to convert bilirubin into a colorless compound called urobilinogen, which in turn spontaneously degrades into a molecule called urobilin.

"Urobilin is ultimately responsible for the yellow color we're all familiar with," Hall added. But beyond answering a curiosity that has intrigued generations, the discovery of bilirubin reductase may also facilitate research into the role of the gut microbiome in diseases such as jaundice and intestinal inflammation.

When bilirubin, which is secreted in the intestine for later excretion, is absorbed in excess into the bloodstream, it can cause jaundice, a condition that causes the eyes and skin to appear yellowish.

The research team also discovered that although bilirubin reductase is present in almost all healthy adults, newborns and individuals with inflammatory bowel disease often lack this enzyme .

Another of the study's authors, researcher Xiaofang Jiang, stated that by identifying the bilirubin reductase enzyme, we can begin to investigate how gut bacteria affect bilirubin levels and diseases such as jaundice.

Furthermore, the gut microbiome has also been linked to other diseases such as allergies, arthritis, and psoriasis (a chronic inflammatory skin disease), so the impact of discovering the origin of the yellow color of urine may go beyond the simple anecdote.