So THAT's Why Pee Is Yellow

Recent research reveals it's linked to our gut health.
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In 2016 or so, my Instagram feed was filled with quack theories that if your urine isn’t completely transparent, you’re not drinking enough water. That isn’t true (clear pee might actually mean you’re overhydrated) ― but why is wee yellow to begin with?

It turns out that scientists only recently discovered the answer. Researchers at the University of Maryland and National Institutes of Health sought to find the cause behind the excreta’s hue ― and it could help us to learn more about our gut health.

In a news statement, Brantley Hall, assistant professor at the University of Maryland’s Department of Cell Biology and Molecular Genetics said that “It’s remarkable that an everyday biological phenomenon went unexplained for so long, and our team is excited to be able to explain it.“

So what’s going on?

Well, we already knew that pee gets its yellow tinge from a substance called urobilin. We just didn’t know where the colour-enhancing acid came from ― until now.

It turns out that the substance is made when red blood cells degrade (which takes about six months), the researchers found. That’s because, after they’ve broken down, red blood cells create an orange substance called bilirubin.

This is stored in your gut and can sometimes be re-absorbed. If too much of it goes back into your body, you might get jaundice. But if it stays in your gut, your body can convert it into multiple substances ― including urobilin.

Your gut’s microbiome will determine the production of the urine-colouring molecule, scientists found.

“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.”

Until this research, scientists thought that this process involved multiple enzymes. But the scientists found that a single enzyme ― bilirubin reductase enzyme ― is responsible for the transformation.

Why should I care?

Well, scientists have found that some newborns and some adults with IBS are missing bilirubin reductase ― and its absence is also linked to gallstones and jaundice.

We now also have evidence which suggests your gut health is linked to your liver’s wellbeing.

“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,” Xiaofang Jiang, study co-author, said. “This discovery lays the foundation for understanding the gut-liver axis.”

“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,” Hall added.