This is why your urine is yellow

Scientists have finally unravelled the mystery behind urine’s yellow hue, according to a new study in the journal Nature Microbiology.

The discovery of the microbial enzyme, called bilirubin reductase, could have important implications for research into related health conditions like jaundice.

When red blood cells degrade, they produce a bright orange pigment called bilirubin as a byproduct. Bilirubin is typically secreted into the gut and excreted – but it can also be partially reabsorbed into the body.

“Gut microbes encode the enzyme bilirubin reductase that converts bilirubin into a colourless byproduct called urobilinogen,” explains lead author Dr Brantley Hall, assistant professor at the University of Maryland, US.

“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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“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,” adds co-author Dr Xiaofang Jiang, from the US National Institutes of Health.

Excess reabsorption of bilirubin from the gut can lead to a buildup in the blood that can cause jaundice – a condition that leads to yellowing of the skin and eyes.

“This discovery lays the foundation for understanding the gut-liver axis,” says Jiang.

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