“A lie repeated enough times becomes a truth”. Most of us have heard that what makes urine yellow when we take supplements is the B vitamins, specifically the B2 or riboflavin. Is this true?

Turns out SOMETIMES. There are two molecules that contribute to the yellow color.  Riboflavin is a bright yellow-green with some florescence (generates light). When urine turns this color within a period of taking a B vitamin supplement, it does mean that the supplement was absorbed. To get to the urine, it must go through the bloodstream to the kidneys where it exits unchanged.

However, there is another yellow molecule that is paler and doesn’t glow, called urobilin. It is a complex molecule, with some features of bile, that reflect the elimination of toxins. Bile make urine yellow in a similar that it makes poop brown. People who do not make sufficient bile (certain liver and gallbladder diseases) have yellow poop, not brown, due to lack of bile-related molecules.

The two molecules can be better distinguished under the light of an UV flashlight.  The riboflavin will glow a brighter yellow-green, while the urobilin will look.   

So, urine can be either yellow or both.  

If we take riboflavin and instead of the urine turning bright green-yellow it turns a lighter yellow, then it implies that the riboflavin was utilized rather that excreting, enabling the production and excretion of urobilin.

Furthermore, the opposite argument may also be true. If the urine is not yellow enough (other than drinking lots of water), then it can suggest a need for more supplements. 

Finally, if you take supplements once a day and your urine doesn’t stay yellow, it suggests you need MORE vitamins in the 2nd half of the day (especially the water-soluble ones).