B-Complex Vitamins: Complete Guide to B Vitamins

The Eight, and What Each Does

• B1, thiamine. Cofactor for pyruvate dehydrogenase and transketolase, central to carbohydrate metabolism. Low status is associated with heavy alcohol use, and thiamine repletion is standard clinical practice in that context.
• B2, riboflavin. Precursor to FAD and FMN, cofactors throughout energy metabolism. Notably has trial evidence at 400 mg per day for migraine frequency reduction, which is a genuine and under-known use.
• B3, niacin. Precursor to NAD+, discussed at length in the NAD+ articles. Nicotinic acid flushes, nicotinamide does not, and at high doses nicotinamide inhibits sirtuins.
• B5, pantothenic acid. Precursor to coenzyme A, required for fatty acid metabolism. Inadequate intake is rare.
• B6, pyridoxine. Cofactor for over a hundred enzymes including transaminases and the enzymes making serotonin, dopamine and GABA. The one B vitamin with a meaningful upper limit.
• B7, biotin. Cofactor for carboxylases. Rarely inadequate, heavily marketed for hair and nails on weak evidence, and it interferes with several laboratory immunoassays at high doses, which matters more than its benefits.
• B9, folate. One-carbon metabolism, DNA synthesis, homocysteine handling. The most important in pregnancy, where supplementation prevents neural tube defects.
• B12, cobalamin. One-carbon metabolism alongside folate, myelin maintenance, red cell production. The one with the clearest and most consequential insufficiency pattern.

Where Inadequate Intake Genuinely Occurs

Most B vitamins are widely distributed in food and rarely short in a varied diet. Four situations account for most genuine cases.

B12 in plant-based diets. B12 is produced by bacteria and reaches humans through animal foods. Vegans without supplementation develop low status reliably, and the consequences, including irreversible neurological effects if prolonged, make this the single most important supplementation case in the whole B group.

B12 with age and with medication. Absorption requires stomach acid and intrinsic factor, both of which decline with age. Long-term proton pump inhibitor use and long-term metformin use both lower B12 status measurably. Periodic testing in these groups is standard and frequently skipped.

Folate in pregnancy. Requirements rise sharply and the window for neural tube protection is early, often before pregnancy is recognised, which is why supplementation is recommended before conception rather than after.

Thiamine with heavy alcohol use. Alcohol impairs absorption and increases requirement, and the neurological consequences of severe insufficiency are serious and partly irreversible.

Beyond these, marginal status can occur with restricted diets, malabsorption conditions, bariatric surgery and some medications, which is a reason for testing rather than blanket supplementation.

The Methylation Question

Methylfolate and methylcobalamin are sold at a premium on the argument that people carrying common variants in the MTHFR gene convert folic acid to its active form less efficiently and should therefore take the pre-converted form.

What is true: MTHFR variants are common, the homozygous form reduces enzyme activity substantially, and it is associated with modestly higher homocysteine. What is not established: that people with these variants derive meaningfully better outcomes from methylfolate than from folic acid or from food folate at ordinary intakes. Trials comparing the forms in unselected populations show both raise folate status effectively.

The nuance worth knowing is different from the marketing. Very high folic acid intake can lead to unmetabolised folic acid appearing in circulation, whose significance is debated, and folic acid supplementation can mask the haematological signs of low B12 while neurological damage continues. That second point is the practically important one: B12 status should be assessed rather than assumed whenever folate is supplemented at higher doses.

Methylated forms are reasonable and not magic. For someone with elevated homocysteine, the intervention with trial support is the combination of folate, B12 and B6, in whichever form, alongside establishing why homocysteine is elevated.

Doses, Upper Limits and the B6 Problem

Most B vitamins have no established upper limit because excess is excreted. Two exceptions matter.

B6. Chronic intake above roughly 100 mg per day, and in some case series considerably less, is associated with sensory peripheral neuropathy, which usually but not always resolves on discontinuation. Many B-complex products contain 25 to 100 mg of B6, which is fifty times the requirement and inside the range where case reports exist. This is the single most important safety point in the B group and it is routinely ignored.

Niacin. Nicotinic acid at gram-level doses is a pharmacological intervention requiring monitoring of liver markers and glucose handling. High-dose nicotinamide inhibits sirtuins.

Typical supplemental amounts worth knowing: B12 at 250 to 1,000 mcg daily for vegans and older adults, since absorption at high doses proceeds by passive diffusion when the active pathway is saturated; folate at 400 mcg for general adequacy and 400 to 800 mcg preconception; riboflavin at 400 mg for migraine prophylaxis specifically; thiamine at 50 to 100 mg where indicated.

Testing

Testing is worthwhile for exactly two of them in most people.

B12. Serum B12 is widely available and imperfect: it measures total B12 including the inactive fraction, and values in the lower part of the reference range can accompany functional insufficiency. Methylmalonic acid is the better functional marker, rising when B12 is inadequate at the cellular level, and homocysteine adds information. For anyone vegan, over sixty, or on long-term metformin or acid suppression, this is a test worth having.

Folate. Red cell folate reflects status over months better than serum folate, which reflects recent intake.

Testing the rest is rarely informative outside specific clinical suspicion. Whole-blood thiamine and functional enzyme assays exist and belong in clinical contexts.

One laboratory caution: high-dose biotin, common in hair and nail products, interferes with many immunoassays including thyroid and cardiac troponin tests, producing falsely high or low results. Stopping biotin for several days before blood work is standard advice.

A Sensible Position

The B group is where supplement marketing and nutritional reality diverge most cleanly. Energy claims are the obvious example: B vitamins are coenzymes in energy metabolism, which does not mean that supplementing them in someone with adequate status produces energy. It does not.

The defensible approach: target the specific vitamins where a specific case exists. B12 for vegans, older adults and long-term metformin or acid suppression users, at 250 to 1,000 mcg daily with periodic testing. Folate preconception and in pregnancy. Riboflavin at 400 mg if migraine prophylaxis is the objective. Thiamine where alcohol intake or malabsorption warrants it.

Where a general B-complex is used for convenience, prefer one with B6 at or near the requirement rather than fifty times it, and audit it against any NAD+ precursor in the stack, since the niacin content overlaps.