Why vitamin B2 turns urine bright yellow, and who actually makes it
Animals cannot make riboflavin; bacteria, fungi and plants do. Our bodies do not store it either, so whatever we absorb beyond need is flushed out by the kidneys, tinting urine a vivid yellow called flavinuria. Industry now brews thousands of tonnes a year using fungi and genetically modified bacteria.
Riboflavin, or vitamin B2, is a single chemical compound, unlike vitamins such as B6 that come in several related forms. In pure form it is a bitter, yellow-orange crystalline powder that dissolves in water, stays stable when heated unless exposed to light, and doubles as a food colouring labelled E101 in Europe. Its real work begins once cells convert it into two coenzymes, FMN and FAD.
Those coenzymes shuttle electrons. They support roughly 70 to 80 human enzymes that switch between oxidised, half-reduced and fully reduced states, and FAD alone serves 84 percent of the flavoproteins our genes encode. The knock-on effects are wide. FAD helps turn the amino acid tryptophan into niacin, FMN is needed to activate vitamin B6, and a folate enzyme relies on FAD to convert homocysteine into methionine, so a shortage of B2 can drag other vitamins down with it. Deficiency also seems to hamper iron metabolism, and correcting it can make iron treatment for anaemia work better.
Deficiency is rare and usually comes with other nutritional gaps. Meat, fish, eggs, dairy, green vegetables, mushrooms and almonds supply it, and because milling leaves white flour with only 67 percent of the original amount, some countries enrich flour and cereals. US agencies found no human evidence of toxicity at high intakes and set no upper limit; absorption simply becomes less efficient as the dose rises.
Medicine has found specific uses. In keratoconus, a progressive thinning of the cornea, a riboflavin solution is applied to the eye and activated with ultraviolet A light to stiffen the tissue. The American Academy of Neurology judged high-dose riboflavin probably effective for preventing migraines in adults in 2012, while evidence in children remains inconclusive. Richard Kuhn's group achieved the first total synthesis, but by 2012 fermentation, notably with engineered Bacillus subtilis, was producing over 4,000 tonnes annually.
Source: Riboflavin
