B-Complex, explained properly.

What B-complex actually is

B-complex is a group of 8 water-soluble vitamins that act as cofactors (helper molecules) in 100+ enzyme systems throughout the body. Unlike fat-soluble vitamins (A, D, E, K), B-vitamins don't store in the body — excess is excreted in urine within hours. This makes them very low-risk but also means you need regular intake.

The 8 B-vitamins are: B1 (thiamine), B2 (riboflavin), B3 (niacin), B5 (pantothenic acid), B6 (pyridoxine), B7 (biotin), B9 (folate), and B12 (cobalamin). B-complex supplements typically contain all 8, though ratios vary widely. A "B-complex" is not a single molecule — it's a category of 8 different molecules with different roles, doses, and toxicities.

How each B-vitamin works in the body

Each B-vitamin has a specific enzymatic role. Here's what each one does and what deficiency looks like:

1. B1 Thiamine. Required for glucose metabolism and nerve function. Deficiency causes beriberi (cardiovascular and neurological symptoms) and Wernicke-Korsakoff syndrome in alcohol users. Found in whole grains, pork, legumes. Heavy alcohol use is the #1 cause of B1 deficiency in developed countries.
2. B2 Riboflavin. Required for energy metabolism and glutathione (antioxidant) regeneration. Deficiency causes mouth sores, skin cracks, and light sensitivity. Found in dairy, eggs, meat, almonds. The "neon yellow urine" after a B-complex is harmless excess B2.
3. B3 Niacin (as nicotinic acid or niacinamide). Required for NAD/NADH cofactors in 400+ enzyme reactions. Nicotinic acid form at 50+ mg causes flushing (the "niacin flush"). Niacinamide form does not flush. Found in meat, fish, fortified cereals. Deficiency causes pellagra (diarrhea, dermatitis, dementia — the 3 D's).
4. B5 Pantothenic acid. Required for coenzyme A synthesis and adrenal hormone production. Deficiency is rare (found in nearly all foods). Found in mushrooms, avocados, sweet potatoes. Marketed for adrenal support though evidence is weak.
5. B6 Pyridoxine (or P5P form). Required for amino acid metabolism, neurotransmitter synthesis (serotonin, dopamine, GABA), and red blood cell formation. Deficiency causes anemia, peripheral neuropathy, and depression. Found in chicken, fish, chickpeas, bananas. The only B-vitamin with a strict UL (100 mg/day) due to neuropathy risk.
6. B7 Biotin. Required for fatty acid synthesis and keratin production. Deficiency causes hair loss and skin rashes. Found in eggs, nuts, salmon. Marketed heavily for hair/skin/nails though evidence is weak outside deficiency. High-dose biotin (5,000-10,000 mcg) can interfere with lab tests (troponin, thyroid).
7. B9 Folate (as folic acid or 5-MTHF). Required for DNA synthesis, cell division, and homocysteine metabolism. Deficiency causes megaloblastic anemia and neural tube defects in pregnancy. Found in leafy greens, legumes, fortified cereals. The synthetic folic acid must be converted to 5-MTHF by MTHFR enzyme (40% of population has reduced activity).
8. B12 Cobalamin (as cyanocobalamin, methylcobalamin, or hydroxocobalamin). Required for nerve function, DNA synthesis, and red blood cell formation. Deficiency causes megaloblastic anemia, peripheral neuropathy, and cognitive decline. Found in animal products (meat, fish, dairy, eggs). Vegans are at high risk — strict vegans need B12 supplementation.

Forms of B-complex supplements, compared

The form matters for two reasons: B12 (cyanocobalamin vs methylcobalamin) and folate (folic acid vs 5-MTHF). The other 6 B-vitamins are mostly the same molecule across products.

Who is actually deficient

B-vitamin deficiency is under-diagnosed because the standard serum tests are unreliable (B12 serum, for example, doesn't drop below normal until severe deficiency). Functional deficiency — when B-vitamin-dependent processes fail despite "normal" serum levels — is much more common. The at-risk groups per NIH ODS and Kennedy 2016:

• Older adults (60+) — reduced stomach acid impairs B12 absorption. B12 deficiency affects 6-20% of older adults per NIH ODS. Often misdiagnosed as dementia or depression.
• Vegans and strict vegetarians — B12 is found almost exclusively in animal products. B12 supplementation is mandatory for strict vegans. Lacto-ovo vegetarians get some B12 from dairy/eggs.
• Heavy alcohol users — alcohol impairs B1 (thiamine) absorption. Wernicke-Korsakoff syndrome is the classic B1-deficiency presentation in alcohol users. ER treatment is IV thiamine.
• Pregnant and lactating women — increased folate demand for fetal neural tube development. All prenatal vitamins contain folic acid. Folate deficiency is the leading cause of preventable birth defects globally.
• Long-term PPI users (omeprazole, esomeprazole) — reduced stomach acid impairs B12 absorption. Long-term metformin users also at risk of B12 depletion.
• MTHFR mutation carriers (~40% of population) — reduced conversion of folic acid to 5-MTHF. Methylated B-complex is genuinely better for this group.
• People with celiac disease, Crohn's, IBD — chronic malabsorption of B12, folate, and other B-vitamins.
• Chronic stress / burnout — B-vitamins (especially B5, B6, B12) are rapidly depleted during prolonged stress. Kennedy 2010 found high-dose B-complex improves mood and reduces perceived stress in healthy adults.
• Heavy exercisers / athletes — B-vitamins are consumed faster during high-intensity training. B1, B2, B6 are particularly relevant for energy metabolism.

How much B-vitamins you need (RDA + UL)

Per the US Institute of Medicine (IOM 1998, updated 2006) and the Australian NHMRC. B-vitamins are water-soluble — excess is excreted in urine — but B6 has a hard UL (100 mg/day for adults) due to peripheral neuropathy risk.

Source: NIH ODS B-Vitamin Fact Sheets · UL = tolerable upper intake level for supplements only. B6 is the only B-vitamin with a strict UL — chronic doses >100 mg/day cause peripheral neuropathy.

B-complex by life stage and condition

The MTHFR question: who needs methylated B-complex

MTHFR (methylenetetrahydrofolate reductase) is an enzyme that converts synthetic folic acid into the active form 5-MTHF (5-methyltetrahydrofolate). About 40% of the population carries at least one MTHFR variant (C677T or A1298C) that reduces enzyme activity. For these people, folic acid accumulates without being converted, and 5-MTHF levels stay low.

The clinical relevance:

• Elevated homocysteine (cardiovascular risk marker) — MTHFR variants increase risk; methylated folate reduces it
• Depression and anxiety — linked to low 5-MTHF per Kelly 2016 systematic review
• Pregnancy complications — MTHFR variants increase preeclampsia and recurrent miscarriage risk
• Neural tube defects — MTHFR carriers respond better to 5-MTHF than folic acid per Greenberg 2011

The "everyone should take methylated B-complex" trend is partly overhyped. For people without MTHFR variants, methylated forms offer no meaningful advantage over standard (and cost more). The best approach is genetic testing (23andMe, AncestryDNA) plus a serum homocysteine test. If homocysteine is elevated despite standard B-complex, switch to methylated.

What to pair / what to avoid

Frequently asked questions