Why pee is yellow and how that keeps us healthy
Science has long known why urine from healthy, well-hydrated humans is a sunny yellow. The color comes from urobilinogen, a by-product the body creates as it disposes of 5 million dead red blood cells every second. The mystery was which of the 1,000 or so microbes in our intestinal tracts is responsible for turning potential toxins into what makes our pee yellow while keeping our blood chemistry mellow. On January 3, 2024, the peer-reviewed journal Nature Microbiology published a study that finally identifies the enzyme responsible, as well as the microbes that produce it. The discovery could lead to disease prevention and better medical treatments.
Why pee is yellow
In the liver, red blood cells at the end of their usefulness break down, creating bilirubin in the process. A little bilirubin is okay, but too much can lead to serious disease and death. The recent research paper that explains how bilirubin is kept in check also describes the risk when it’s not:
In moderate concentrations, bilirubin serves as an important antioxidant with potential health benefits. However, elevated serum bilirubin concentrations can become toxic, leading to jaundice and, in extreme cases, kernicterus, a type of bilirubin-induced neurological damage.
The body’s answer to the excess bilirubin problem is to turn most of it into something less harmful. The body transforms bilirubin into urobilinogen using the newly identified enzyme, bilirubin reductase. According to the researchers, a key gene found in a class of gut-dwelling bacteria known as Firmicutes produce bilirubin reductase.
Yellow pee is a sign of good health
The study also discovered that people whose gut flora lacks the gene for making pee yellow are more prone to certain digestive diseases.
When analyzing human gut metagenomes, we found that bilirubin reductase was nearly universally present in healthy adults. The prevalence of the gene was much lower in patients with inflammatory bowel disease (IBD) and in infants, especially during the first few months of life when infants are most susceptible to developing jaundice.
Figuring out which intestinal microbes are responsible for making bilirubin reductase wasn’t an easy task. Brantley Hall, assistant professor in the University of Maryland’s Department of Cell Biology and Molecular Genetics and the study’s lead author, described the difficult hunt for Healthline.com:
The gut is a low-oxygen environment, and many of the bacteria in our guts can’t survive if too much oxygen is present, making them difficult to grow and perform experiments on in labs. This ultimately meant that only a handful of bacterial species had ever been identified as being able to metabolize bilirubin, limiting the amount of data that was available.
Opening the door to new medical treatments
Because elevated bilirubin in the blood is linked to jaundice, bowel disease and kidney dysfunction, figuring out how healthy gut bacteria keep bilirubin in check could lead to new medical treatments. Hill told Healthline that the next step is studying how gut microbes regulate bilirubin levels:
We hope to conduct observational human studies to better understand how bilirubin reduction by gut microbes influences the concentration of bilirubin in circulation. We are especially interested in looking at premature infants where jaundice rates are high, and the prevalence of bilirubin-reducing microbes is low.
Bottom line: A recent study revealed why human urine is yellow and why it’s a sign of good health.