Reviews/Vitamin B12/Vegans risk vitamin B12 deficiency
Deficiency And Status · VITB12-STATUS-01 · v2
Borderline: within 4 points of the Established/Likely boundary, so a re-grade could land either side.
The evidence in this corpus consistently points one way: when animal foods are removed from the diet, vitamin B12 intake and blood markers of B12 fall. A month-long controlled trial in healthy people found a B12 transport marker dropped on a vegan diet [12], a whole-diet trial found B12 status declined as the plant share rose [15], and an intake analysis singled out B12 as the one nutrient a plant-based diet failed to supply [17]. In a group of long-term vegetarians, most already had low blood B12 [13]. Two other trials were run specifically in vegans or vegetarians who were already low, which fits the idea that this group starts at higher risk [14],[18]. The main reason the panel did not go higher is that most of this is blood-marker change over weeks rather than diagnosed clinical deficiency, and some trials deliberately enrolled people who were already deficient.
Removing or reducing animal-source foods lowers B12 intake and biomarker status across independent randomised trials: holotranscobalamin fell on a 4-week vegan diet in healthy adults [12], B12 intake and status declined as the plant-protein fraction rose [15], and a vegetarian diet gave lower B12 than a Mediterranean diet by 32.32 pg/mL (P<0.01) [2]. An intake analysis found all nutrients met except vitamin B12 in the plant-based group [17]. In long-term (≥6 year) vegetarians, 35 of 50 subjects (70%) had serum B12 <150 pmol/L [13]. Two supplementation trials were conducted in vegans/vegetarians already carrying a marginal deficiency [14],[18]. The contested point: The division is over endpoint and population, not direction. The strongest causal trials measure B12 status/biomarkers over 4–12 weeks rather than clinically manifest deficiency — [12] found MMA and tHcy unchanged at 4 weeks — and two supporting trials [14],[18] pre-selected already-deficient subjects, so they show a deficient sub-population exists rather than quantifying population-level risk.
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Two of the supporting trials [14], [18] pre-selected subjects with marginal/existing deficiency, so they demonstrate a deficient sub-population exists rather than quantifying population-level risk.
The strongest causal RCTs [12], [2] measure biomarkers (holo-TC, serum B12) over 4–12 weeks — surrogate status, not clinically manifest deficiency; [12] found MMA and tHcy unchanged at 4 weeks.
Where it remains uncertain. Much of the direct causal evidence is short-term biomarker change [12],[2] rather than clinical deficiency, and two supporting trials pre-selected marginally deficient subjects [14],[18].
Aleatory
Hepatic B12 reserves can mask intake reduction for long periods, so short trials capture status change without predicting who becomes clinically deficient.
Epistemic
Corpus reports biomarker/status change [12],[15],[2],[17] but no trial measures diagnosed deficiency (anaemia, neuropathy) as an outcome of a vegetarian/vegan diet.
Inferential
[14] and [18] enrolled already-marginal vegans/vegetarians, so inferring population-level elevated risk from them extrapolates beyond their design.
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.
RCT crossover showing a significant B12 deficit on the vegetarian arm versus Mediterranean.
“significant differences between the 2 interventions were obtained for low-density lipoprotein cholesterol, triglycerides, and vitamin B12 levels.”
Panel auditInternal validity: medium · scope of inference: Short-term B12 level change in overweight omnivores adopting a vegetarian vs Mediterranean diet.Key limitations: direction of B12 difference unstated · short duration · B12 level not deficiency
Randomised causal-direction evidence that a vegan diet lowers B12 status within weeks (biomarker, not clinical, endpoint).
“Holo-TC decreased significantly in the VD compared to the MD group after four weeks of intervention, whereas metabolites MMA and tHcy were unaffected.”
Panel auditInternal validity: medium · scope of inference: Short-term change in B12 status biomarkers when healthy omnivores adopt a vegan diet.Key limitations: short duration · biomarker not clinical deficiency · small n
Direct prevalence evidence: a majority of long-term vegetarians had subnormal serum B12.
“Thirty-five subjects (70%) had serum B-12 levels <150 pmol/l.”
Panel auditInternal validity: medium · scope of inference: B12 status of long-term community-dwelling vegetarians (≥6 years).Key limitations: no omnivore comparator · selected sample · prevalence not risk ratio
Marginal deficiency was an enrollment criterion, not a measured comparison, so the abstract does not itself establish elevated risk versus a non-vegetarian group.
Panel auditInternal validity: medium · scope of inference: Restoration of B12 status by sublingual dosing in already marginally deficient vegans/vegetarians.Key limitations: deficiency is inclusion criterion · no omnivore comparator · supplementation design
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.
Directionally unanimous primary evidence plus a strong prevalence datum and clear biological mechanism support the claim, tempered by surrogate endpoints and some selected populations.
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.
Clinical deficiency, not just biomarkers. Every direct causal trial here reports status/biomarkers over weeks [12],[2],[15]; [12] found MMA and tHcy unchanged at 4 weeks, so the link to clinical deficiency is unmeasured. The panel rates this the highest-importance open gap for the claim, and names the study design that would close it: A long-duration prospective cohort or trial following unsupplemented vegetarians/vegans to hard endpoints (megaloblastic anaemia, neuropathy) with functional markers.
Open question: Does the short-term fall in B12 biomarkers on animal-food removal progress to clinically manifest deficiency over years? Why it is still open: Corpus trials measure status over 4–12 weeks [12],[15]; [12] found MMA and tHcy unchanged at 4 weeks, and no included study reports clinical deficiency outcomes.
Priors is a sceptical first reader, not peer review, and not medical advice.
Release 2026-q4 · graded 18 Sep 2026 · protocol v0.19