The over 150-year mystery of why urine is yellow has finally been cracked: study

Scientists have some whiz-dom to share with those who have wondered what gives urine its distinct color.

For over 150 years, it has been a mystery why urine from the human body comes out as yellow.

But a new study published Wednesday in the journal Nature Microbiology has cracked the yellow code.

Researchers from the University of Maryland and the National Institute of Health say they have identified bilirubin (BilR) as the key enzyme that makes one’s liquid gold, well… gold.

Urine constitutes a blend of water, electrolytes, and waste that’s filtered out by the kidneys.

Scientists had identified urobilin as the culprit behind the yellow pigmentation in urine in 1868, but what caused the color had baffled researchers until recently.

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

The process occurs when red blood cells reach the end of their life cycle at six months and degrade into the bright orange pigment bilirubin.

Typically, the pigment begins seeping into the gut, where it may either be excreted or partially reabsorbed.

Upon reaching the gut, the study found that microorganisms in the intestines can transform bilirubin into various other molecules.

“Gut microbes encode the enzyme bilirubin reductase that converts bilirubin into a colorless byproduct called urobilinogen,” Hall explained.

“Urobilinogen then spontaneously degrades into a molecule called urobilin, which is responsible for the yellow color we are all familiar with.”

The discovery of what makes urine — which plays a crucial role for doctors in helping diagnose an extensive range of illnesses and disorders in the human body — the color yellow is being hailed as a “remarkable” breakthrough that has solved a critical piece of the “puzzle” about understanding more about the human body.

“We’re definitely standing on the shoulders of giants. If some of these older scientists had the technology we had today, they probably would’ve found it,” Hall said.

Hall, the study’s lead author, explained the bathroom business discovery could usher in other medical breakthroughs to help people combat things like inflammatory bowel disease and jaundice — a condition that causes a patient to have a yellowish tinge to their skin, mucous membranes, and the whites of the eyes.

“One of the major findings of our studies is that this gene was often absent in newborn babies,” the researcher added.

The plan is to take the next step in human studies, specifically for premature infants with high jaundice rates.

“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,” study co-author and National Institutes of Health investigator Xiaofang Jiang said.

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

In addition to jaundice and inflammatory bowel disease, the gut microbiome has been linked to various diseases and conditions, from allergies to arthritis. 

The scientific breakthrough brings researchers one step closer to understanding the role of gut microbiomes in human health.