Researchers have finally figured out the reason behind urine’s yellow colour, answering an age-old question that has stumped scientists for years.
In the study published in the journal Nature Microbiology, Brantley Hall, an assistant professor in the University of Maryland’s Department of Cell Biology and Molecular Genetics, and his team explain the chain of events that cause urine to be yellow and why it took so long to figure out.
Urine, the last product of our body’s natural drainage system, consists of excess water and waste filtered out of blood via the kidneys. These waste products include dead cells that have reached the end of their life cycle, like the RBCs (red blood cells) that carry oxygen throughout the body using haemoglobin.
Red blood cells produce a compound called heme. Eventually, the red blood cells degrade, and their heme results in a chain of events that makes the urine yellow.
Scientists have known for some time that urobilin is the chemical primarily responsible for the yellow colour of urine. However, they were unsure about the steps in the process leading to the presence of urobilin in urine.
In the study, Hall explained the role of gut bacteria in breaking down the byproducts of heme, which causes the urine to be yellow.
“When red blood cells degrade after their six-month lifespan, a bright orange pigment called bilirubin is produced as a byproduct. Once in the gut, the resident flora can convert bilirubin into a molecule which can turn yellow if exposed to oxygen. This molecule, urobilin, is a major reason behind urine’s yellow colour,” Hall told Gizmodo.
The study talks about the enzyme responsible for this reaction. Hall’s team has coined the newly discovered enzyme bilirubin reductase or BilR. This enzyme breaks down into urobilinogen with the help of Firmicutes and other gut bacteria in the large intestine. Urobilinogen then turns into urobilin in the presence of air.
The study also showed that the gut microbiomes of healthy adults readily contain BilR, highlighting its crucial role.
Hall noted that studying gut microbiomes is a difficult task. That’s why it took so long to find how urine turns yellow.
“Unfortunately, gut microbes can be challenging to study. The gut is a low-oxygen environment, and many of the bacteria in our guts can’t survive if too much oxygen is present, making them difficult to grow and perform experiments on in labs,” Hall said.
(With inputs from agencies)
