It's a question children ask as often as they ask why the sky is blue. Why is pee yellow? Any answer outside of "I don't know" was most likely more entertaining than the truth, which scientists have revealed to us. A previously unknown enzyme called bilirubin reductase is behind it. Bilirubin reductase is not yellow, but it factors into the process.
Our bodies are always producing new red blood cells and eliminating old ones. Old red blood cells release an orange pigment called bilirubin as they die. Several species of gut microbes in our bodies use the enzyme bilirubin reductase to break down the bilirubin into urobilinogen, which turns yellow as it degrades. Biochemists who've been studying the question for more than a century (not the same scientists) knew about bilirubin and urobilinogen, but the step in between them is the discovery that makes it all work. Those microbes producing bilirubin reductase are doing us a great favor, because too much bilirubin causes jaundice. You can read up on the experiments that revealed the new enzyme at Ars Technica.
But what if your urine isn't yellow? You might need to see a doctor.
(Image credit: Turbotorque)
