B Vitamins

The eight B vitamins are coenzymes: small molecules that enzymes need in order to run the reactions of energy metabolism, DNA synthesis, and the clearing of a toxic amino acid called homocysteine. Classic deficiency — pellagra, beriberi, the anemia of B12 depletion — is rare in well-fed populations, so the modern question is not "am I deficient?" but "do I belong to a group whose intake or absorption is quietly compromised, and if so, what should I actually take?" The honest answer is that a generalised "B-complex for longevity" is mostly wasted money, while a few targeted uses are genuinely worth the trouble.

Who actually needs them

For a healthy adult eating animal products and a reasonable variety of plants, the B vitamins are abundant and the body recycles them efficiently. Four situations change that:

1. Vegetarians and especially vegans. Plant foods contain essentially no vitamin B12 — it is made by bacteria and reaches us through animal products. This group should supplement B12 without waiting for a test.
2. Long-term metformin users. Metformin interferes with the calcium-dependent step that lets the gut absorb the B12–intrinsic-factor complex, and a 16-year follow-up of the large US diabetes-prevention cohort confirmed a real, dose- and duration-dependent fall in B12 over years of use.^[1]
3. People on chronic acid suppression. Proton-pump inhibitors and H2-blockers cut the stomach acid needed to free B12 from food protein. Pharmacovigilance analysis finds the metformin-plus-acid-suppressant combination carries a compounded B12-deficiency risk beyond either drug alone.^[2]
4. Adults past about 60. Age-related atrophic gastritis becomes common, impairing the acid and intrinsic factor that dietary B12 needs; 10–30% of older adults absorb it poorly.

Outside these groups, mega-dosing a B-complex does not measurably improve energy, mood, or cognition in people who aren't deficient — a point worth holding onto, because the category is marketed almost entirely on the opposite promise.

The one cognitive result that holds up — and its catch

The headline reason people take B vitamins for the aging brain is homocysteine. This amino acid, a by-product of methionine metabolism, is toxic to neurons and blood vessels at elevated levels and is an independent risk factor for brain shrinkage and cognitive decline. Vitamins B6, B9 (folate), and B12 are the obligatory cofactors that clear it from the blood, so lowering homocysteine with B vitamins is biologically plausible.

The evidence is more conditional than the marketing. Pooled analyses are clear that in cognitively healthy adults with normal homocysteine, B-vitamin supplements do not improve memory or global cognition.^[3] The benefit, where it exists, is concentrated in older adults who already have mild cognitive impairment and elevated homocysteine — and even there it has been inconsistent across trials.^[4]

The most useful insight came from re-analysing a British trial in older adults with mild cognitive impairment (VITACOG): high-dose B6/B9/B12 slowed brain atrophy and cognitive decline only in participants who already had high blood levels of the omega-3 fatty acid DHA. In those with good omega-3 status, B vitamins cut the rate of brain shrinkage by up to roughly 40%; in those with low omega-3, the same B-vitamin dose did essentially nothing.^[5] Later metabolomic work on the same trial showed the B vitamins were reshaping homocysteine and neurotransmitter-related pathways, not acting through a single mechanism.^[6]

The practical message is a two-part one: the B-vitamin cognitive intervention is worth considering only in older adults with documented high homocysteine, and it is probably pointless without adequate omega-3 alongside it. See Omega-3 and Dementia prevention.

Vitamin B12: testing and form

Serum B12 is an insensitive test. The body defends blood levels at the expense of tissue stores, so a "normal" serum B12 can sit on top of genuine intracellular depletion. The functional markers — methylmalonic acid (MMA) and homocysteine — rise before serum B12 falls, which is why ambiguous cases should be confirmed with MMA or holotranscobalamin rather than serum B12 alone. Elevated MMA is itself associated with higher all-cause mortality in older and cardiorenal populations, making it a useful early-warning marker rather than just a deficiency test.^[7] The testing cadence for at-risk groups lives in Midlife labs.

On the form, the marketing runs ahead of the evidence. B12 is sold cheaply as cyanocobalamin and at a premium as methylcobalamin (the active coenzyme form). For correcting deficiency and raising blood B12, systematic reviews find the two essentially equivalent — high-dose oral or sublingual cyanocobalamin works fine for most people.^[8] Methylcobalamin's theoretical advantage (it skips a conversion step and acts as a direct methyl donor) is real biochemistry but has not translated into a meaningful clinical edge for routine repletion. If you want it, the cost is modest; just don't expect the form alone to do anything dramatic.

Dose: 25–250 µg/day for prevention in an at-risk group; 1000 µg/day for documented deficiency. High oral doses work even when absorption is impaired by drugs or atrophic gastritis, because 1–2% crosses the gut by passive diffusion independent of intrinsic factor. B12 is water-soluble with no established upper limit — oversupplementation is not a meaningful toxicity concern.

Folate: methylfolate versus folic acid

This is the form debate that actually matters. Natural food folate, the synthetic folic acid in fortified foods and cheap supplements, and 5-methyltetrahydrofolate (5-MTHF, the active circulating form) are not interchangeable.

Folic acid is biologically inert until the body reduces it, a multi-step conversion whose first enzyme (dihydrofolate reductase) has low and variable capacity in humans. High intakes can outrun it, leaving unmetabolised folic acid circulating in the blood — a state of uncertain long-term significance that can also mask the anemia which normally signals a hidden B12 deficiency, delaying its diagnosis. A large share of people also carry common variants in the MTHFR gene that impair the conversion of folate to its active form. Direct supplementation with 5-MTHF sidesteps the MTHFR bottleneck, produces a more durable drop in homocysteine, and avoids unmetabolised folic acid altogether.^[9] For these reasons 5-MTHF is the better default when folate supplementation is actually indicated.^[10]

One firm exception holds in the other direction: women who could become pregnant should ensure adequate folate for neural-tube-defect prevention, and folic acid is the form with the deepest evidence base in that specific context. The 5-MTHF case is about optimisation in adults, not about overturning prenatal folic-acid guidance.

Vitamin B6: the one with a real toxicity ceiling

B6 is the B vitamin most worth being careful with, because more is not safer. Chronically exceeding the intake limit over months to years can cause a reversible sensory peripheral neuropathy — tingling and numbness in the hands and feet. The European Food Safety Authority sets the tolerable upper limit for adults at 12 mg/day, far below the doses packed into many "energy" and "stress" B-complex formulas.^[11] If you take a B-complex, check the B6 content against that ceiling. The active form, pyridoxal-5-phosphate (P5P), is sometimes preferred but does not exempt you from the dose caution.

Niacin, NAD+, and the longevity-molecule question

Vitamin B3 is the precursor for NAD+, the coenzyme central to energy metabolism, DNA repair, and the sirtuin enzymes that feature heavily in aging research. This is the biochemistry behind the NMN and nicotinamide riboside (NR) supplements sold as anti-aging compounds — and it is where the gap between mechanism and proven human benefit is widest. These precursors reliably raise blood NAD+, but the functional results in people have been small and inconsistent, and the longevity claims rest on animal data and surrogate markers rather than hard outcomes. The full, skeptical treatment of NMN and NR is in Geroprotectors; the short version is that the hype substantially outruns the evidence.

The older, cheaper end of B3 has a longer track record. Plain niacin (nicotinic acid) raises NAD+ severalfold and has decades of cardiovascular data, but its use is limited by an uncomfortable prostaglandin-mediated flush, and its EFSA upper limit is correspondingly low at 10 mg/day for supplemental nicotinic acid. The other B3 form, nicotinamide, carries a much higher limit (900 mg/day). Neither is a longevity intervention for a healthy adult on current evidence.

A note worth keeping: the long-held belief that NAD+ inevitably falls with age has been complicated by recent work showing blood NAD+ does not universally decline in healthy people, and that the depletion is tissue- and disease-specific — concentrated in conditions like neurodegeneration and certain cancers rather than tracking chronological age.^[12] That undercuts the simple "top up a universal deficit" rationale for NAD+ boosters in the well.

The rest of the complex

The remaining B vitamins are essential coenzymes but rarely the limiting factor in a mixed diet, and none has earned a stand-alone longevity case:

• B1 (thiamine) feeds carbon into the energy-producing TCA cycle and is critical for nerve function; deficiency matters most in heavy alcohol use and after bariatric surgery.
• B2 (riboflavin) is a core electron carrier in the mitochondrial respiratory chain. Cell work suggests it can suppress senescent-cell accumulation under stress, and it uniquely lowers blood pressure in people with a particular MTHFR variant — an early example of nutrition targeted to genotype, not yet a general recommendation.^[13]
• B5 (pantothenic acid) is the backbone of coenzyme A, needed for fat metabolism; dietary deficiency is almost unheard of.
• B7 (biotin) supports gluconeogenesis and participates in DNA-repair-related histone modification. Supplemental megadoses, common in hair-and-nail products, are mostly harmless but interfere with many laboratory immunoassays — including thyroid and troponin tests — and should be stopped before bloodwork.

EFSA has set no upper limit for B1, B2, B5, or B7 owing to the absence of toxicity even at high intakes.

Cardiovascular: prevention, not reversal

Higher dietary intake and blood levels of several B vitamins (B1, B2, B3, B6, B9) track with a 10–20% lower stroke risk in large US cohorts.^[14] But the most important cardiovascular fact is a negative one that recurs across the corpus: the large trials that successfully lowered homocysteine with B vitamins did not reduce heart attacks, strokes, or cardiovascular death. Homocysteine looks more like a marker of vascular risk than a lever you can pull to change outcomes, and trials aimed at acutely reversing established arterial stiffness with B vitamins have been mixed at best.^[15] B vitamins belong in the prevention-of-deficiency column, not the cardiovascular-drug column.

Practical summary

• Don't take a generic high-dose B-complex for longevity. In a non-deficient adult it does nothing measurable, and the B6 load in many formulas is a genuine downside.
• Supplement B12 if you are vegan/vegetarian, on long-term metformin or acid-suppressants, or over ~60. 25–250 µg/day for prevention, 1000 µg/day for documented deficiency.
• Confirm B12 status with MMA or holotranscobalamin, not serum B12 alone, and check homocysteine as a downstream readout of B6/B9/B12 status. Target homocysteine in the single digits (roughly below 10 µmol/L) rather than merely "in range." See Midlife labs.
• If you supplement folate, prefer 5-MTHF over folic acid — except for prenatal folic-acid guidance, which stands.
• The B-vitamin cognitive intervention (B6/B9/B12 in older adults with high homocysteine) is only worth trying alongside adequate omega-3.
• Take B vitamins in the morning with food. They are water-soluble and tied to daytime energy metabolism; evening high doses can disturb sleep in some people, and consistency matters more than timing.
• Skip the NAD+ boosters (NMN, NR) for now — see Geroprotectors.

Further reading

• Smith AD et al. Omega-3 fatty acids enhance the prevention of cognitive decline by B vitamins in mild cognitive impairment (VITACOG). Am J Clin Nutr 2016.^[16]
• Aroda VR et al. Long-term Metformin Use and Vitamin B12 Deficiency in the DPP/DPPOS. J Clin Endocrinol Metab 2016.^[17]
• Ferreira Gonçalves A et al. Active folate versus folic acid: the role of 5-MTHF in human health. PMC 2022.^[18]
• Comparative review. Vitamin B12: natural versus synthetic forms of supplementation. PMC 2025.^[19]
• EFSA. Overview of tolerable upper intake levels. EFSA 2024.^[20]
• Methylmalonic acid and all-cause mortality. Front Nutr 2025.^[21]
• B vitamins and incident stroke in >220,000 US adults. 2026.^[22]