Pee is yellow: Scientists unravel the mystery

 Scientists have finally unraveled the mystery behind why urine is yellow

The Week, January 06, 2024 : Scientists have finally unraveled the mystery behind why urine is yellow. For years, this question has puzzled researchers, but a recent study has shed light on the exact reason behind the colour of pee .

The colour of urine is primarily due to an enzyme called bilirubin reductase, which is produced by bacteria in the gut. When red blood cells reach the end of their life cycle, they are broken down in the liver, resulting in the production of bilirubin. This bright orange substance is then secreted into the gut and converted by gut bacteria into a colourless substance called urobilinogen. Urobilinogen further degrades into urobilin, a yellow pigment that gives urine its characteristic colour.

Until now, scientists were unable to identify the specific bacterial enzyme responsible for converting bilirubin to urobilinogen. However, researchers from the University of Maryland and the National Institutes of Health have successfully identified the gene that encodes bilirubin reductase, the enzyme responsible for this conversion .

The discovery of this enzyme has potential health implications, as it could enhance our understanding of the role of the gut microbiome in conditions such as jaundice and inflammatory bowel disease (IBD). Jaundice occurs due to a buildup of bilirubin in the blood, while individuals with IBD have been observed to have lower levels of urobilin compared to those without the condition.

The study involved comparing the genomes of human gut bacteria species that can convert bilirubin into urobilinogen with those that lack this capacity. Through this comparison, the researchers identified the gene responsible for bilirubin reductase. They then tested the enzyme's ability to facilitate the conversion in the model organism Escherichia coli (E. coli).

The researchers also conducted genetic screening of the gut microbiomes of 1,801 healthy adults, searching for the gene associated with the colour of urine. They found that 99.9% of the individuals had gut bacteria carrying the gene for bilirubin reductase. In contrast, the gene was less prevalent in individuals with IBD and in infants under three months old, who are at a higher risk of jaundice.

Further research is needed to fully understand the implications of the absence of bilirubin reductase and its potential contribution to jaundice and IBD. However, this discovery opens up new avenues for investigating the impact of gut bacteria on bilirubin levels and related health conditions.

"This enzyme discovery finally unravels the mystery behind urine's yellow colour," said Brantley Hall, the lead study author and an assistant professor at the University of Maryland.