Why Pee Is Yellow, According to a New Study
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Paying attention to the color of your urine is one way to gauge if you’re dehydrated. But have you ever stopped to wonder why pee is yellow in the first place?
For the first time, researchers have identified the enzyme responsible for urine’s golden hue, in a new study published on January 3 in Nature Microbiology.
The discovery of this enzyme, called bilirubin reductase, finally solves the mystery behind why urine is yellow, says lead author Brantley Hall, PhD, assistant professor in the department of cell biology and molecular genetics at the University of Maryland in College Park.
The study results may do more than answer an age-old question. “Although more research is needed, this finding could lead to improved treatments for jaundice and inflammatory bowel disease,” says Dr. Hall.
Research Into Jaundice Leads to Discovery
Hall and his team didn’t actually set out to find out why urine is yellow. “Our lab studies gut microbes; we’re interested in understanding the functions these microbes perform and how that relates to human health,” Hall says.
In this study, investigators were studying how the gut microbiome — the trillions of microorganisms that populate our digestive tracts — might be related to jaundice in babies.
Jaundice is the yellow color seen on the skin of many newborns, caused by a buildup in the infant’s blood of the yellow compound bilirubin. This happens because babies’ livers aren’t developed enough to get rid of the bilirubin.
Bilirubin Reductase Is the Enzyme That Causes Pee to Be Yellow
For over a century, scientists have known that the body turns bilirubin in the blood into molecules in urine called urobilin, which impart a yellow color. But they didn’t know which enzyme or collection of enzymes in the gut was responsible for this transformation.
Hall and his team discovered that gut microbes encode (make) bilirubin reductase, which then converts bilirubin into a colorless byproduct called urobilinogen. “Urobilinogen then spontaneously degrades [breaks down] into urobilin, which is responsible for the yellow color,” Hall says.
Bilirubin Often Absent in Babies With Jaundice and Some People With IBD
The researchers hope that this finding could have important health implications. Although bilirubin reductase is present in almost all healthy adults, the scientists found that it’s often missing from newborns.
“This absence may contribute to infant jaundice and the formation of pigmented gallstones,” says Hall. If this hypothesis is confirmed, one approach to treating infant jaundice might be to add back the bacteria through a probiotic strategy, he says.
The enzyme is often lacking or missing in people with inflammatory bowel diseases such as Crohn’s disease or ulcerative colitis, according to researchers. “The absence of bilirubin microbes may also contribute to the formation of gallstones, which has been observed in some people with inflammatory bowel disease,” says Hall.
More research is needed to confirm these associations before any conclusions can be made, but the findings highlight yet another reason why our gut microbiome is so vital to human health, he adds.
It’s Unlikely That People Who Are Dehydrated Produce More Bilirubin
The color of urine is often used to gauge hydration levels, with darker yellow signaling potential dehydration. How does bilirubin fit in with that?
“People ask me if there is more bilirubin produced when you’re dehydrated, and we don’t think that’s the answer. We think there’s basically a constant amount of bilirubin produced by gut microbes, and the color that you see is a factor of hydration,” says Hall.
What Color Yellow Should Your Urine Be?
It’s normal for the color of urine to vary from day to day, but it should stay within a certain range of yellow, according to the Cleveland Clinic. Pee that is pale straw or amber is ideal and signals that you’re properly hydrated, according to experts.
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