Foods high in vitamin B2 (riboflavin) and what it does

It becomes the two coenzymes FAD and FMN, which take part in almost every oxidation-reduction reaction of energy metabolism. Beyond that, the enzyme that converts vitamin B6 into its active pyridoxal phosphate form, methylenetetrahydrofolate reductase in folate metabolism, and the pathway that turns tryptophan into niacin all use riboflavin. Glutathione reductase needs it too, linking riboflavin to antioxidant defence. It is also involved in moving and using iron, so anaemia corrects poorly when riboflavin is short.

The US recommended intake for adults is 1.3 mg a day for men and 1.1 mg for women. No upper limit is set. Milk and dairy, eggs, liver, lean meat, almonds and enriched grains are the main sources. Intake tends to run low on diets with little dairy, so eggs, nuts, fortified grains and mushrooms are worth watching there.

It is relatively heat-stable, so cooking does not cost much. Light is the problem — riboflavin breaks down in milk left in a clear bottle in bright light, one of the reasons milk is packaged in opaque containers. It dissolves in water, so draining cooking water takes some with it. Absorption happens in the small intestine, and there is a ceiling on how much is taken up from a single large dose.

No toxicity from excess intake has been reported. High-dose supplements turn urine bright or fluorescent yellow; that is surplus riboflavin passing straight through and it is harmless. It is also a sign that much of the dose was not absorbed. Riboflavin deficiency on its own is rare, and usually comes alongside shortages of other B vitamins.