Reduced serum vitamin B-12 in patients taking metformin

BMJ 2010; 340 doi: http://dx.doi.org/10.1136/bmj.c2198 (Published 20 May 2010) Cite this as: BMJ 2010;340:c2198
  1. Josep Vidal-Alaball, honorary research fellow,
  2. Christopher C Butler, professor of primary care medicine
  1. 1Department of Primary Care and Public Health, Heath Park, Cardiff CF14 4YS
  1. vidal-alaballj{at}cf.ac.uk

    Until the clinical importance is clear, simple dietary advice could solve the problem

    As many as 22% of people with type 2 diabetes could have vitamin B-12 deficiency.1 The cause of vitamin B-12 deficiency in these patients remains controversial,2 but it can have important consequences and should be considered in the differential diagnosis of diabetic neuropathy. In small short term studies, treatment with metformin has been associated with reduced vitamin B-12 concentrations and an increased risk of vitamin B-12 deficiency.3 4

    Kooy and colleagues conducted a trial of the long term effects of metformin treatment on metabolic and microvascular and macrovascular complications in patients with type 2 diabetes who were already taking insulin.5 Three hundred and ninety patients were randomised to receive insulin plus metformin or insulin plus placebo. After a follow-up of 4.3 years, the study found fewer macrovascular events in the metformin group, which was largely accounted for by the beneficial effect of metformin on weight.

    In the linked study (doi:10.1136/bmj.c2181) this same group of authors, but with de Jager as first author, perform a further analysis of their earlier study and report important differences in serum vitamin B-12 in participants. In the placebo group, vitamin B-12 increased by 0.2 pmol/l (0%, 95% confidence interval −3% to 4%), whereas in the group with the addition of metformin vitamin B-12 was reduced by 89.8 pmol/l (−19%, −22% to −15%). At baseline, three participants in the metformin group and four in the placebo group had vitamin B-12 concentrations below 150 pmol/l. At follow-up after 4.3 years this had increased to 19 and five participants, respectively. The reduction in concentrations of vitamin B-12 in the metformin group persisted and become more apparent over time. The number needed to harm per 4.3 years was 13.8.6

    Although the absolute number of affected participants was small the differences in vitamin B-12 concentrations are convincing. A strength of this study is the long period of follow-up.

    The authors recommend that regular measurement of vitamin B-12 concentrations during long term metformin treatment should be considered. Sadly, though, they do not report on quality of life, neurological status, or measures of fatigue. Neither do they report whether participants received any dietary advice regarding vitamin B-12. As their introduction makes clear, we have no robust research underpinning the assumption that people with vitamin B-12 concentrations of 150 pmol/l or less are indeed vitamin B-12 deficient. Although they may be at higher risk for vitamin B-12 deficiency related effects, these risks are not clearly quantified or always directly related to vitamin B-12 concentrations. An opportunity may have been missed to elucidate whether reduced vitamin B-12 concentrations resulted in meaningful symptoms. Were the participants with lower concentrations of vitamin B-12 more likely to feel worse and have adverse effects?

    Other uncertainties include whether or not these findings apply to most patients managed in primary care, who are not usually treated with insulin—should these patients also be monitored for vitamin B-12 concentrations? The conversion of patients with type 2 diabetes to insulin treatment is controversial,7 and it is the minority of patients whose diabetes is hardest to control who typically start insulin treatment. If patients are monitored, how should serum vitamin B-12 be measured?8 Furthermore, it is not clear what form the intervention should take. The options include dietary advice; increase intake of foods rich in calcium and vitamin B-12; and vitamin B-12 supplements, which could be taken orally or injected9 10—most patients will not have problems absorbing vitamin B-12 from the gut because this form of vitamin B-12 deficiency is unlikely to be related to intrinsic factor or bowel disease.

    There are several other questions related to vitamin B-12 replacement. Given the long time it takes to deplete vitamin B-12 stores, how often—if at all—would intramuscular injections need to be repeated? Should concentrations be monitored once treatment is started? Can treatment be stopped after dietary improvement? Would we miss important morbidity by proceeding on the basis of symptoms and heightened clinical suspicion rather than on systematic screening in all patients taking metformin?

    The case that de Jager and colleagues make for routine assessment of serum vitamin B-12 concentrations is based on findings from patients taking insulin and with no evidence that monitoring will benefit patients. The mechanisms are unclear, but it could be something as simple as the dietary change associated with metformin. We first need to determine whether simple dietary counselling when metformin is started and at medication/annual reviews will solve the problem. If it does not, a trial of screening for vitamin B-12 deficiency in patients taking metformin would be needed. Patients taking metformin (not just those also treated with insulin) should be randomised to systematic serum vitamin B-12 screening or routine care, with patient oriented outcomes and costs included in the outcomes. Otherwise, we risk increasing the burden on patients and the costs of care by treating biochemical outcomes rather than outcomes that matter to patients.


    Cite this as: BMJ 2010;340:c2198


    • Research, doi:10.1136/bmj.c2181
    • Competing interests: All authors have completed the Unified Competing Interest form at www.icmje.org/coi_disclosure.pdf (available on request from the corresponding author) and declare: (1) No financial support for the submitted work from anyone other than their employer; (2) No financial relationships with commercial entities that might have an interest in the submitted work; (3) No spouses, partners, or children with relationships with commercial entities that might have an interest in the submitted work; (4) No non-financial interests that may be relevant to the submitted work.”

    • Provenance and peer review: Commissioned; not externally peer reviewed.