European Journal of Cardiovascular Medicine

Metformin has been a cornerstone of type 2 diabetes mellitus (T2DM) management for decades, prized for its efficacy, safety profile, and cardiovascular benefits.[1] However, a growing body of evidence highlights a significant adverse effect: the malabsorption of vitamin B12 (cobalamin), leading to biochemical deficiency.[2] Vitamin B12 is a crucial coenzyme for DNA synthesis and neurological function. Its deficiency can manifest insidiously, progressing from subclinical states to severe and sometimes irreversible conditions, including megaloblastic anemia and peripheral or central neuropathy.[3]

The mechanism of metformin-induced B12 malabsorption is thought to involve interference with the calcium-dependent binding of the intrinsic factor-B12 complex in the terminal ileum.[4] While the association is well-established, there is variability in reported prevalence rates, and a clearer understanding of the most significant predictors is needed to guide clinical practice. Furthermore, the clinical consequences, particularly the exacerbation of diabetic peripheral neuropathy, represent a major concern.[5]

This study aimed to determine how common vitamin B12 deficiency is among long-term metformin users, identify its predictors, and examine its link with peripheral neuropathy and macrocytic anemia. The goal was to better understand and prevent complications related to prolonged metformin therapy.

A retrospective cohort study was performed by analyzing electronic health records (EHR) from January 2015 to December 2024. We identified all patients with a T2DM diagnosis who had been prescribed metformin for a cumulative duration of at least two years. Inclusion criteria required at least one serum vitamin B12 measurement during the study period. Patients receiving B12 supplementation prior to their index B12 measurement or with a history of gastric surgery or pernicious anemia were excluded. A final cohort of 850 patients was included for analysis. Data extracted from the EHR included: patient demographics (age, gender), metformin treatment details (average daily dose, total duration in years), relevant laboratory results (serum B12, HbA1c, hemoglobin, Mean Corpuscular Volume [MCV]), and concurrent prescriptions for proton pump inhibitors (PPIs). • Vitamin B12 Status: Defined as Deficient (<150 pmol/L), Borderline (150-220 pmol/L), and Sufficient (>220 pmol/L). • Macrocytic Anemia: Defined as hemoglobin <13 g/dL for men or <12 g/dL for women, with an MCV > 100 fL. • Peripheral Neuropathy: Identified by the presence of a corresponding ICD-10 diagnosis code in the patient's record. Descriptive statistics were used to summarize the cohort's characteristics. Differences between B12-deficient and sufficient groups were assessed using Student’s t-tests for continuous variables and chi-square tests for categorical variables. A multivariate logistic regression model was constructed to identify independent predictors of vitamin B12 deficiency, with results reported as Odds Ratios (OR) and 95% Confidence Intervals (CI). A p-value < 0.05 was considered statistically significant.

The baseline characteristics of the 850 patients are shown in Table 1. The average age was 64.3 years, with a mean duration of metformin use of 6.8 years.

The overall prevalence of vitamin B12 deficiency was 22.4% (n=190), with an additional 31.5% (n=268) of patients having borderline levels (Table 2).

A comparison between B12-deficient and sufficient patients (Table 3) revealed that the deficient group was significantly older, had been on metformin longer, used a higher daily dose, and had higher rates of concurrent PPI use.

The multivariate logistic regression analysis (Table 4) confirmed that metformin duration, daily dose, older age, and PPI use were all significant and independent predictors of B12 deficiency. For every 1000 mg/day increase in metformin dose, the odds of deficiency increased by 88%.

Finally, the clinical outcomes analysis (Table 5) showed that patients with vitamin B12 deficiency had a significantly higher prevalence of both peripheral neuropathy and macrocytic anemia compared to their B12-sufficient counterparts.

In this large retrospective study, we found that over one-fifth of long-term metformin users had vitamin B12 deficiency, with nearly half the cohort having levels below the optimal range. Our findings strongly reinforce the dose- and duration-dependent nature of this adverse effect, a conclusion supported by several other large-scale studies and meta-analyses.[6,7] The identification of older age and concurrent PPI use as independent risk factors provides clinicians with additional criteria for risk stratification. PPIs, which reduce gastric acid, are known to impair B12 absorption independently, and their combined effect with metformin appears to be synergistic.[8]

Perhaps the most crucial finding is the stark difference in clinical outcomes. The prevalence of peripheral neuropathy was more than double in the B12-deficient group. This is particularly concerning in a diabetic population where distal symmetric polyneuropathy is already a common and debilitating complication. B12 deficiency can cause a similar neuropathy, and it is plausible that it either mimics, co-exists with, or exacerbates diabetic neuropathy.[5] Differentiating between the two etiologies based on clinical symptoms alone is challenging, which underscores the importance of biochemical testing, as B12 replacement can lead to significant neurological improvement.[9]

The clinical significance of our findings aligns with current recommendations from major clinical bodies. For instance, the American Diabetes Association (ADA) now recommends periodic testing of vitamin B12 levels in metformin-treated patients, especially those with anemia or peripheral neuropathy.[10] Our data, demonstrating a high prevalence and clear association with adverse outcomes, provides strong support for the implementation of such screening protocols as a standard of care. The debate should no longer be if we should screen, but rather how often and by what method.

Furthermore, while serum B12 is a widely available test, its limitations are well-documented. A significant portion of circulating B12 is bound to haptocorrin and is biologically inactive. Therefore, measuring holotranscobalamin (active B12) or metabolic markers like methylmalonic acid (MMA) and homocysteine may offer superior diagnostic accuracy, particularly in detecting subclinical or borderline deficiency.[11,12] Adopting these more sensitive markers could help identify at-risk patients even earlier, before the onset of irreversible neurological damage.

Beyond neuropathy and anemia, chronic subclinical B12 deficiency in this aging diabetic population may have other consequences, including an increased risk of cognitive decline and depression.[13] While our study did not assess cognitive outcomes, it is an essential area for future research. Finally, the treatment for this condition is simple, effective, and inexpensive. Studies have shown that both oral and parenteral B12 supplementation are effective at repleting stores and improving hematological parameters in metformin users, making the argument for proactive screening even more compelling from a public health perspective.[14]

Limitations         

The retrospective nature of our study means we can only establish association, not causation. The diagnosis of peripheral neuropathy was based on ICD codes, which may lack uniformity. Furthermore, we did not have data on dietary habits, such as vegetarianism, which could also influence B12 levels.

Vitamin B12 deficiency is a common and clinically significant consequence of long-term metformin therapy. The risk is highest in older patients, those on higher doses for longer durations, and those concurrently using PPIs. Given the strong association with peripheral neuropathy and anemia, our findings advocate for the implementation of routine B12 screening protocols for high-risk patients on metformin. Early detection and supplementation can prevent the progression to serious and potentially irreversible complications.

[1] American Diabetes Association. 9. Pharmacologic Approaches to Glycemic Treatment: Standards of Medical Care in Diabetes-2022. Diabetes Care 2022;45(Suppl 1):S125-43.

[2] Aroda VR, Edelstein SL, Goldberg RB, et al. Long-term metformin use and vitamin B12 deficiency in the diabetes prevention program outcomes study. The Journal of Clinical Endocrinology & Metabolism 2016;101(4):1754-61.

[3] Langan RC, Goodbred AJ. Vitamin B12 deficiency: recognition and management. American Family Physician 2017;96(6):384-9.

[4] Bauman WA, Shaw S, Jayatilleke E, et al. Increased intake of calcium reverses vitamin B12 malabsorption induced by metformin. Diabetes Care 2000;23(9):1227-31.

[5] Singh AK, Kumar A, Singh SK. Metformin-induced vitamin B12 deficiency: a review of existing knowledge. Diabetes & Metabolic Syndrome: Clinical Research & Reviews 2018;12(4):519-23.

[6] Kozyra M, Zglobi K. Metformin-induced vitamin B12 deficiency. Journal of Education, Health and Sport 2016;6(8):527-32.

[7] de Jager J, Kooy A, Lehert P, et al. Long term treatment with metformin in patients with type 2 diabetes and risk of vitamin B-12 deficiency: randomised placebo controlled trial. BMJ 2010;340:c2181.

[8] Lam JR, Schneider JL, Zhao W, et al. Proton pump inhibitor and histamine 2 receptor antagonist use and vitamin B12 deficiency. JAMA 2013;310(22):2435-42.

[9] Kuwabara S, Nakazawa R. Diagnosis and treatment of vitamin B12 deficiency-related neurological disorders. Clinical Neurology and Neurosurgery 2019;185:105474.

[10] ElSayed NA, Aleppo G, Aroda VR, et al. 4. Comprehensive Medical Evaluation and Assessment of Comorbidities: Standards of Care in Diabetes-2023. Diabetes Care 2023;46(Suppl 1):S49-67.

[11] Obeid R, Herrmann W. Holotranscobalamin in laboratory diagnosis of vitamin B12 deficiency. Seminars in Thrombosis and Hemostasis 2007;33(7):736-42.

[12] Miller JW, Garrod MG, Rockwood AL, et al. Measurement of total vitamin B12 and holotranscobalamin, singly and in combination, in screening for metabolic vitamin B12 deficiency. Clinical Chemistry 2006;52(2):278-85.

[13] Moore E, Mander A, Ames D, et al. Cognitive impairment and vitamin B12: a review. International Psychogeriatrics 2012;24(4):541-56.

[14] Butler CC, Vidal-Alaball J, Cannings-John R, et al. Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency: a systematic review of randomized controlled trials. Family Practice 2006;23(3):279-85.