Scientists have finally answered an age-old question: Why is our urine yellow? The latest research has found that an enzyme produced by gut bacteria plays an important role in the process of turning urine a mild yellow color. The discovery not only satisfies curiosity, but may also help scientists better understand the gut microbiota and how they affect certain health conditions, such as neonatal jaundice.
Urine is the end product of our body's natural excretory system. It is made up of excess water and waste products that are filtered from the blood through the kidneys. Some of these waste products are produced at the end of our cell lifespan, including red blood cells that use hemoglobin to deliver oxygen. An important compound produced by red blood cells is heme, which is a precursor to hemoglobin, and the eventual degradation of red blood cells and their heme leads to a series of events that give urine a yellow color.
Scientists already know that the chemical urobiliin is mainly responsible for the yellow color of urine. But according to Brantley Hall, an assistant professor in the Department of Cell Biology and Molecular Genetics at the University of Maryland, we don't know much about the key step in the appearance of urobiliin in urine — the role that gut bacteria play in the degradation products of heme.
In a study published in the journal Nature Microbiology, Hall and his team detailed the missing piece of the puzzle.
"When red blood cells degrade at the end of their six-month lifespan, they produce a bright orange pigment called bilirubin as a byproduct," Hall said. Once in the gut, the gut flora can convert bilirubin into a molecule that turns yellow when exposed to oxygen. This molecule, urobiliin, is the main reason why urine is yellow. In this study, we identified the enzyme responsible for this reaction. ”
The team discovered a new enzyme they call bilirubin reductase (BILR). They found the genes that produce this enzyme in the genes of certain intestinal bacteria in the large intestine, in particular a group of bacteria called Firmicutes, and confirmed its role in the degradation of bilirubin (more specifically, bilr converts bilirubin into urobilinogen and then into urobililin in the air). They also confirmed that bilr is readily found in the healthy** gut microbiome, further confirming its important role.
As for why it took so long to figure out how urine turns yellow, Hall noted that "the gut is a low-oxygen environment, and many gut bacteria can't survive when there is too much oxygen, making it difficult for them to grow and experiment in the lab," he said. "Our search for BILR relied on a combination of experimental screening and genomic analysis, which was only possible with the isolation of more gut bacterial species and advances in genome sequencing technology. ”
They noted that in newborns and people with inflammatory bowel disease, there were fewer findings of BILR. Moreover, the inability of the gut microbiome to produce bilr may lead to neonatal jaundice and pigmented gallstones – both of which are closely linked to excess bilirubin in the body.
Hall said more research will need to confirm the link between gut microbes, bilr and these conditions, which the team already plans to do. They next hope to conduct observational studies in humans to better understand how gut microbes affect bilirubin concentrations in our blood by reducing bilirubin. They were particularly interested in studying preterm infants because they had a higher risk of jaundice and lower bilr levels.