Why is pee yellow? The color of urine and the role of urobilin

By Cristiano Antonino On Dec 12, 2022

Urobilin and yellow pee: urine is composed mostly of water (95%), urea and mineral salts, as well as substances in lower concentrations

It is Urobilin that gives urine its yellow color

This molecule is derived from the breakdown of hemoglobin, a protein highly concentrated in red blood cells and responsible for oxygen transport in the blood.

Erythrocytes, at the end of their life cycle of about 120 days, are degraded.

Hemoglobin is broken down into a protein part (globins), from which amino acids are recovered, and a non-protein part, the heme group, from which Bilirubin is obtained.

How to get to urobilin

From it, through several intermediate steps, stercobilin is obtained, excreted via the feces, and urobilin, which is instead partly excreted with urine and partly reused by the liver in the production of bile.

The shade of yellow, more or less intense, of urine depends mainly on how hydrated we are: the more water we drink, the more the pigment will be diluted, giving a more “transparent” appearance to urine.

Blue urine: truth or legend?

The color of urine is a very valuable data to monitor our health status.

The “straw yellow” color of urine is an indicator of good health while too much yellow indicates that we should drink more.

Far more alarming is the case of red urine, which may indicate the presence of blood due to inflammation or infection of the urinary tract[1].

Should you show this symptom, don’t wait to tell your primary care physician about it!

It is hard to believe, but in rare cases the urine may have a tending to blue coloration.

This unusual color can be attributed to the use of a drug used to treat cases of Methemoglobinemia.

Called “Methylene Blue,” it is, indeed, dark blue in color.

For several years, this drug also had an application as an antimalarial[2].

There are many substances that alter urine coloration.

Asparagus, for example, can impart a green hue (as well as an unpleasant odor); excess beets, blackberries, or rhubarb can result in a reddish tint to urine; pumpkin and carrots give an orange tone.

The same applies to food dyes, which excreted with urine alter its color.