Vitamin B2 (Riboflavin)

Powers mitochondria

Riboflavin is made into two coenzymes, FMN and FAD. Both can take on and give up electrons one at a time or two at a time, which most electron carriers can't do. That flexibility puts them where electrons pass between molecules in energy metabolism:¹

• Complex I of the electron transport chain uses FMN to take electrons from NADH (made with B3) and pass them down the chain.
• Complex II, which is also the TCA cycle enzyme succinate dehydrogenase, uses FAD to feed electrons from succinate straight into the chain.
• Fat burning. The first step of each round of fatty-acid breakdown is done by FAD-dependent acyl-CoA dehydrogenases. Their electrons go to the chain through another flavoprotein (ETF).
• Pyruvate and α-ketoglutarate dehydrogenase, the same thiamine-dependent gates, each include an FAD subunit.

Riboflavin is also needed to activate other vitamins. Converting B6 to its active form (PLP) uses an FMN enzyme, and making B3 (niacin) from tryptophan uses an FAD enzyme. So low riboflavin can cause functional B6 and B3 shortfalls even when intake of those is fine.¹

Human evidence. Deficiency doesn't usually cause dramatic energy failure. The body recycles flavins efficiently, and enzymes hold onto their FAD tightly. Its clinical signs are mostly in fast-turnover tissues: cracks at the corners of the mouth, a sore magenta tongue, and scaly skin around the nose. The strongest evidence that high flavin supply helps energy-starved tissue is in migraine, which is thought to involve low mitochondrial energy reserve in the brain. In a randomised trial of 55 people, 400 mg/day of riboflavin for 3 months at least halved headache days in 59% of patients, against 15% on placebo. That is a small trial, but cheap and low-risk.²

Recharges antioxidants

Glutathione neutralises peroxides by giving up electrons, and two used molecules join into oxidised glutathione (GSSG). To reuse it, cells need glutathione reductase, an FAD enzyme that moves electrons from NADPH back onto GSSG to split it into two active glutathione molecules. Without enough FAD, oxidised glutathione builds up and the cell's main antioxidant runs down.¹

This reaction is so dependent on riboflavin that it's the standard test for riboflavin status. A lab measures red-cell glutathione reductase activity, then adds FAD. If the enzyme speeds up a lot (a high "activation coefficient," EGRAC), it was short of FAD. That test shows marginal riboflavin status is common, especially in older adults and teenagers, even in wealthy countries.¹

The MTHFR link. MTHFR, the folate enzyme behind the common 677TT gene variant, is also an FAD enzyme. The TT variant holds onto its FAD loosely. In people with that variant, high homocysteine showed up only when riboflavin status was poor. With good riboflavin status, their homocysteine was normal.³ For the roughly 10% of people who carry TT, riboflavin may matter more than extra folate. See Folate.

Intake & absorption

The RDA is 1.1 mg/day for women and 1.3 mg/day for men. Absorption in the small intestine saturates at about 25–30 mg per dose, and the body stores very little, so excess goes out in urine (which is why it turns bright yellow). It needs a regular supply. Riboflavin is destroyed by light, which is why milk used to come in opaque containers. It works as a team with B1, B3 and magnesium in the same energy pathways.

Top foods

• Beef liver: about 2.9 mg in 3 oz, more than 2 days' worth
• Yogurt and milk: about 0.4–0.6 mg per cup
• Beef: about 0.2–0.3 mg in 3 oz
• Eggs: about 0.25 mg per large egg
• Salmon: about 0.15–0.4 mg in 3 oz

Deficiency

Riboflavin deficiency (ariboflavinosis) rarely happens on its own. It usually comes with other B-vitamin shortfalls. Signs include cracks at the mouth corners, a sore magenta tongue, a sore throat, scaly skin around the nose and on the scrotum, bloodshot eyes and, because of the B6 link, anaemia.¹ People at risk include those who eat no dairy or meat, heavy drinkers, older adults, and people on some drugs (phenothiazines, tricyclics).