Since you were old enough to be potty trained, you’ve known that your urine was yellow. But it’s taken until 2024 for scientists to determine how that yellow colour was being produced.

New research in the journal Nature Microbiology found that the secret ingredient to yellow urine is an enzyme called bilirubin reductase (BilR).

When our red blood cells break down, they produce a byproduct called bilirubin which we excrete through our gut. If that process isn’t working properly, our bodies can reabsorb the bilirubin leading to jaundice - where the skin and eyes turn yellow.

Scientists have known that somehow our body turns bilirubin into the yellow coloured urobilin, but until now they haven’t known what was involved in that process.

The reason why it’s been so hard to study, is that many of our digestive system processes involve bacteria that live in very low-oxygen environments. Growing and studying these bacteria in a science lab is difficult as they don’t survive when oxygen is present so researchers have struggled to prove which bacteria are involved in this process.

Rather than try to grow and identify the specific bacterial species that was metabolising bilirubin into urobilin, the scientists used genome sequencing to unravel the genetic codes used in the process instead.

They found that gut microbes encode the enzyme bilirubin reductase and this converts bilirubin into a colourless byproduct called urobilinogen. Urobilinogen then spontaneously breaks down into the yellow molecule urobilin.

Researchers have found that bilirubin reductase is present in nearly all healthy adults, but not in those with inflammatory bowel disease or newborn babies. This can lead to gallstones in adults and jaundice in newborns.

These results could be used to help study the links between the gut microbiome and health conditions such as jaundice and inflammatory bowel disease.

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