Why Is Pee Yellow?
A stream of research reveals why urine isn’t (normally!) pink or green
It’s incredible that an everyday phenomenon went unexplained for so long. But now researchers know precisely why urine is yellow, and they’re not holding it in. The final link was discovered in, of all places, the gut.
It all flows from bilirubin, a breakdown product of hemoglobin, the protein in red blood cells that enables oxygen to travel around the body. After red blood cells circulate for six months, they degrade into a bright orange pigment, bilirubin, which is secreted into the gut for excretion.
So why isn’t pee pink? The key that eluded scientists for over a century was that microbes living in the gut encode an enzyme, bilirubin reductase, that converts bilirubin into a colorless byproduct called urobilinogen.
A portion of urobilinogen gets reabsorbed from the gut back into the bloodstream, is filtered by the kidneys into the bladder, and spontaneously degrades into urobilin, which imparts the yellow color. Voilà, conundrum solved.
“This enzyme discovery finally unravels the mystery behind urine’s yellow color,” Brantley Hall, PhD, an assistant professor in the University of Maryland’s Department of Cell Biology and Molecular Genetics, the study’s lead author, said in a statement.
