I must admit that this is not a question that I had been wondering about, mainly because I assumed that the reasons would have been known some time ago, given our current ability to do microanalysis of pretty much anything.
So I was surprised to learn that researchers have only just found the reason, that it is due to a particular microbial enzyme.
The enzyme is called bilirubin reductase, and it’s a result of the degradation of red blood cells. Once they break down, a bright orange pigment called bilirubin is produced. Typically, bilirubin is secreted into the gut where it has to be discharged. It can also be reabsorbed, which in excess can cause jaundice, which is when a person’s skin and eyes become yellow.
“Gut microbes encode the enzyme bilirubin reductase that converts bilirubin into a colorless byproduct called urobilinogen,” lead author Brantley Hall, an assistant professor in the University of Maryland’s Department of Cell Biology and Molecular Genetics, said in a media statement. “Urobilinogen then spontaneously degrades into a molecule called urobilin, which is responsible for the yellow color we are all familiar with.”
Now at any parties social gatherings, whenever the conversation turns to the topic of urine, you can impress your friends this little factoid.
You can read the paper that was published in Nature Microbiology here.