Scientists finally work out why urine is yellow

It’s a mystery that has remained unexplained for a long time – why is your urine yellow?

Now scientists have worked out exactly why human urine is yellow, pinpointing the enzyme responsible. Researchers at the University of Maryland and National Institutes of Health solved the puzzle – and it’s linked to both red blood cells and the pigment that causes the yellowing of the skin in jaundice. It could also help to unlock further understanding of how microbes in our gut are linked to human health, the researchers said.

Lead author Brantley Hall, an assistant professor in the University of Maryland’s Department of Cell Biology and Molecular Genetics, said: “This enzyme discovery finally unravels the mystery behind urine’s yellow colour. It’s remarkable that an everyday biological phenomenon went unexplained for so long, and our team is excited to be able to explain it.”

Why is urine yellow?

The yellow colour is related to other processes in the body, and a chemical called urobilin. Scientists have known that urobilin is responsible for urine’s yellow colour – but not how it comes into being – and it’s linked to our blood as well as to processes in the gut.

When red blood cells degrade after their six-month lifespan, a bright orange pigment called bilirubin is produced as a byproduct. Bilirubin is typically secreted into the gut, where it is excreted from the body but can also be partially reabsorbed.

Excess reabsorption can lead to a buildup of bilirubin in the blood and can cause jaundice – a condition that leads to the yellowing of the skin and eyes. Once in the gut, the resident flora can convert bilirubin into other molecules.

“Gut microbes encode the enzyme bilirubin reductase that converts bilirubin into a colourless byproduct called urobilinogen,” Hall said. “Urobilinogen then spontaneously degrades into a molecule called urobilin, which is responsible for the yellow colour we are all familiar with.”

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Why is this finding important?

Aside from solving a scientific mystery, these findings could have important health implications.

The research team found that bilirubin reductase is present in almost all healthy adults but is often missing from newborns and individuals with inflammatory bowel disease. The scientists believe that the absence of bilirubin reductase may contribute to infant jaundice and the formation of pigmented gallstones.

“Now that we’ve identified this enzyme, we can start investigating how the bacteria in our gut impact circulating bilirubin levels and related health conditions like jaundice,” said Xiaofang Jiang, study co-author and NIH investigator. “This discovery lays the foundation for understanding the gut-liver axis.”

How is the gut linked to our health?

In addition to jaundice and inflammatory bowel disease, the gut microbiome has been linked to various diseases and conditions, from allergies to arthritis to psoriasis. This latest discovery brings researchers closer to achieving a holistic understanding of the gut microbiome’s role in human health.

Hall said: “The multidisciplinary approach we were able to implement was key to solving the physiological puzzle of why our urine appears yellow… It’s the culmination of many years of work by our team and highlights yet another reason why our gut microbiome is so vital to human health.”

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