Reviews/Vitamin B12/Vitamin B12 lowers homocysteine
Homocysteine Cardiovascular · VITB12-CV-01 · v2
Borderline: within 4 points of the Established/Likely boundary, so a re-grade could land either side.
Across roughly twenty randomised trials in very different groups of patients, every trial that gave a supplement containing vitamin B12 saw plasma homocysteine fall, usually by about a quarter to a third. This happened even in trials whose main clinical goal (fewer heart attacks, less cognitive decline) was not met, which is why the biochemical drop itself is treated as well established. The catch is that in almost every trial B12 was given together with folic acid, the strongest known homocysteine-lowering agent, so the trials show that B12-containing regimens lower homocysteine rather than proving B12 does it on its own. The effect also held in a trial of people with normal starting levels [4]. The panel therefore treats the lowering as a firmly demonstrated fact, held just short of the top because B12's independent contribution is never cleanly separated.
Every trial in this corpus that administered a B12-containing supplement reported a statistically significant fall in plasma homocysteine: 27 percent after myocardial infarction [1], 25.8% (6.3 micromol/L; P < .001) in advanced kidney disease [6], 28% (3.8 micromol/L) in MI survivors [9], and 4.36 micromol per liter lower (P<0.001) in healthy older people [8]. The drop appeared even where baseline levels were normal ([4], P < .001) and where clinical endpoints were null [1][9][10]. B12-forward regimens combining methylcobalamin with folic acid also lowered homocysteine [14][18]. The signal is consistent across post-MI, renal, transplant, diabetic and elderly populations. The contested point: The division is not over whether homocysteine falls but over what causes it: essentially every active arm pairs B12 with folic acid, so the corpus cannot attribute the fall to B12 specifically [1][9]. Even the folic-acid-plus-B12 trial [9] and the B12-forward trials [14][18] leave folic acid as an uncontrolled co-intervention.
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Attribution to B12 is not isolated: essentially every active arm combines B12 with folic acid, the dominant homocysteine-lowering agent. No trial administers B12 monotherapy versus placebo, so the corpus demonstrates that B12-CONTAINING regimens lower homocysteine, not B12's independent effect.
Even the trial that drops B6 ([9] SEARCH: folic acid + B12) cannot separate B12 from folic acid, and the B12-forward trials ([14][18][19]) still co-administer folic acid, leaving folic acid as an uncontrolled co-intervention throughout.
Where it remains uncertain. Because B12 is co-administered with folic acid in essentially every arm [1][9][14][18], the effect cannot be attributed to B12 in isolation.
Aleatory
Reported reductions vary across trials and populations (e.g. 27% [1] vs 25.8% [6] vs 28% [9]), reflecting differing baselines and biological variability rather than measurement error.
Epistemic
All trials use high/pharmacological doses [3][4][6]; whether ordinary supplemental B12 doses lower homocysteine is untested in the corpus.
Inferential
Because B12 and folic acid are co-administered in essentially every arm [1][9][14][18], the fall cannot be inferred to B12 specifically.
The load-bearing papers from a corpus of 20, each classified by what it does to the claim. Every DOI is taken from the corpus the claim was graded on.
Large factorial RCT; folic acid + B12 arm lowered Hcy 27%.
“The mean total homocysteine level was lowered by 27 percent among patients given folic acid plus vitamin B12”
Panel auditInternal validity: high · scope of inference: Post-MI adults; homocysteine response to combined folic acid plus B12.Key limitations: B12 not given alone · no p value for the homocysteine change · post-MI population
Demonstrates Hcy lowering even in patients with normal baseline homocysteine.
“the vitamin supplement regimen was effective in reducing homocysteine levels (mean [SD], -2.42 [3.35] in active treatment group vs -0.86 [2.59] in placebo group; P < .001)”
Panel auditInternal validity: high · scope of inference: Mild-to-moderate Alzheimer patients with normal baseline B12/homocysteine.Key limitations: B12 not isolated · AD population with normal baseline homocysteine
CKD/ESRD RCT; ~26% Hcy reduction on B12-containing regimen vs placebo.
“It was lowered 6.3 micromol/L (25.8%; P < .001) in the vitamin group and 0.4 micromol/L (1.7%; P = .14) in the placebo group at 3 months”
Panel auditInternal validity: high · scope of inference: Advanced CKD / ESRD patients with high baseline homocysteine.Key limitations: B12 not isolated · renal population may differ metabolically
Homocysteine fell in the active (B12-containing) group and rose in placebo.
Panel auditInternal validity: high · scope of inference: Adults 55+ with vascular disease or diabetes; combined B-vitamin effect on homocysteine.Key limitations: B12 not isolated · no p value reported for homocysteine change
4 of 20 papers shown. The full review lists all of them, with the panel’s arbitration →
Claims are graded at each quarterly release, and re-graded between releases only when a pivotal trial lands. Every change is a new version of the claim.
The corpus unanimously and mechanistically supports homocysteine lowering by B12-containing supplementation, but confidence is held below ceiling because no trial isolates B12 from co-administered folic acid.
Release 2026-q4 re-graded every claim from its corpus under protocol v0.19. An earlier release's grade is shown for the record; it came from an earlier protocol and is not a movement.
Assumptions that carry weight but are not themselves established.
The panel records the gap that would most move the claim, and the study that would close it.
B12 monotherapy versus placebo. Every active arm in the corpus combines B12 with folic acid [1][9][14][18], so the isolated effect of B12 on homocysteine is unmeasured. The panel rates this the highest-importance open gap for the claim, and names the study design that would close it: A randomised trial with a B12-only arm, a folic-acid-only arm, a combination arm and placebo, with plasma homocysteine as the primary endpoint.
Open question: Does vitamin B12 lower plasma homocysteine independently of folic acid? Why it is still open: No trial in the corpus administers B12 without folic acid; even the folic-acid-plus-B12 trial [9] and the B12-forward trials [14][18] cannot separate the two.
Priors is a sceptical first reader, not peer review, and not medical advice.
Release 2026-q4 · graded 18 Sep 2026 · protocol v0.19