Metformin-Induced Vitamin B12 Deficiency in Patients with Type 2 Diabetes: A Narrative Review with a Practical Approach for Screening, Diagnosing, and Managing Vitamin B12 Deficiency

Meriem Yazidi; Elyes Kammoun; Ibtissem Oueslati; Melika Chihaoui

doi:10.4082/kjfm.24.0090

Korean J Fam Med > Volume 45(4); 2024 > Article

Yazidi, Kammoun, Oueslati, and Chihaoui: Metformin-Induced Vitamin B12 Deficiency in Patients with Type 2 Diabetes: A Narrative Review with a Practical Approach for Screening, Diagnosing, and Managing Vitamin B12 Deficiency

Review Article

Korean Journal of Family Medicine 2024;45(4):189-198.

Published online: July 18, 2024

DOI: https://doi.org/10.4082/kjfm.24.0090

Metformin-Induced Vitamin B12 Deficiency in Patients with Type 2 Diabetes: A Narrative Review with a Practical Approach for Screening, Diagnosing, and Managing Vitamin B12 Deficiency

Department of Endocrinology, La Rabta Hospital, Faculty of Medicine of Tunis, University of Tunis El Manar, Tunis, Tunisia

^*Corresponding Author: Meriem Yazidi
Tel: +216-98266061, E-mail: meriem.yazidi@fmt.utm.tn

Received April 24, 2024 Revised May 27, 2024 Accepted June 12, 2024

This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Metformin is the most widely used antihyperglycemic drug in patients with type 2 diabetes (T2D). Over the past 2 decades, several studies have highlighted a substantial increase in the risk of vitamin B12 deficiency in patients with T2D on metformin therapy. This can lead to several complications and induce or exacerbate peripheral neuropathy. Despite these data, there are no definite guidelines for screening, diagnosing, and treating vitamin B12 deficiency in patients with T2D on metformin therapy. Therefore, in this narrative review, we aimed to suggest a practical diagnostic and therapeutic strategy to address vitamin B12 deficiency in patients with T2D receiving metformin treatment. Clinical evidence supporting an increased risk of vitamin B12 deficiency in patients with T2D on metformin therapy and its risk factors and potential complications are also discussed.

Keywords: Type 2 Diabetes Mellitus; Metformin; Vitamin B12 Deficiency; Screening; Disease Management

INTRODUCTION

Over the past two decades, several studies have highlighted a considerable increase in the risk of vitamin B12 deficiency in patients with type 2 diabetes (T2D) using metformin [1-5]. Many risk factors have been identified, and notably, a neurological impact has been demonstrated by numerous authors [2,6-8]. Despite these data, it was only in 2017 that the American Diabetes Association (ADA) recommended considering periodic vitamin B12 testing in metformin-treated patients [9]. No clear guidelines have been published regarding screening, diagnosing, and managing these deficiencies. The best cutoff value for diagnosing vitamin B12 deficiency and the usefulness of vitamin B12 biomarkers, particularly in asymptomatic patients, remain controversial.

The worldwide prevalence of T2D is steadily increasing, and the International Diabetes Federation predicts that the prevalence of diabetes, 10.5% in 2021, will rise to 11.3% by 2030 [10]. Metformin is the firstline glucose-lowering drug recommended for treating and preventing T2D [11]. This medication is widely prescribed, either alone or in combination with other antidiabetic agents, because of its effectiveness, low cost, beneficial effects on weight, and ability to reduce cardiovascular morbidity and mortality [12,13].

Considering the absence of a consensus on diagnostic methods, screening criteria, and treatment for vitamin B12 deficiency in patients with T2D receiving metformin, this narrative review had the following aims: (1) Discuss the diagnostic criteria for vitamin B12 deficiency and the utility of vitamin B12 biomarkers. (2) Suggest a practical diagnostic and therapeutic strategy for addressing vitamin B12 deficiency in individuals with T2D receiving metformin treatment.

Additionally, we examined pertinent topics within this field, including clinical evidence demonstrating an increased risk of vitamin B12 deficiency, potential mechanisms elucidating this elevated risk, and the risk factors and consequences of vitamin B12 deficiency in patients with T2D receiving metformin therapy.

ASSESSMENT OF VITAMIN B12 STATUS: USEFULNESS AND LIMITATIONS OF BIOMARKERS

The total serum vitamin B12 (cobalamin) level is frequently insufficient as a standalone marker to accurately evaluate vitamin B12 status. This is because serum vitamin B12 quantifies biologically active and inactive forms, which serve to establish liver stores.

Holotranscobalamin is the biologically active form bound to transcobalamin II, accounting for 20% to 25% of the total measured vitamin B12. The inactive form, bound to transcobalamins I and III, is holohaptocorrine, represents 75%–80% of the total measured vitamin B12 [14,15]. Thus, serum vitamin B12 levels can be falsely normal in cases of liver damage, alcoholism, or myeloproliferative syndrome and falsely decreased in other situations, such as pregnancy. Measuring holotranscobalamin, the active form of vitamin B12, offers better sensitivity and specificity; however, it is not widely available and requires validation in patients with hepatic, renal, and hematological comorbidities. Furthermore, measuring circulating vitamin B12 levels does not reflect cellular function. Normal or elevated vitamin B12 levels can be observed in individuals with functional abnormalities of vitamin B12. Therefore, some scientific societies have recommended measuring cellular vitamin B12 biomarkers, such as homocysteine and methylmalonic acid (MMA), to uncover intracellular vitamin B12 deficiency [14-16]. Vitamin B12 acts as a cofactor for the enzymes involved in the metabolism of homocysteine and MMA. Elevated levels of these biomarkers would, therefore, reflect tissue-level vitamin B12 deficiency [15-17].

Folate deficiency is also associated with increased levels of these biomarkers, especially homocysteine. Therefore, it is important to first rule out folate deficiency in patients with elevated biomarker levels. MMA levels should be interpreted with caution in individuals with renal failure because this situation is associated with elevated levels of this biomarker [14].

DIAGNOSTIC CRITERIA FOR VITAMIN B12 DEFICIENCY

In situations where vitamin B12 levels are not markedly low, and there are no apparent symptoms, such as macrocytic anemia, it remains difficult to diagnose vitamin B12 deficiency based solely on the measurement of vitamin B12. A level below 148 pmol/L (200 pg/mL) is the cutoff value usually applied to define low vitamin B12 status [14-18]. A level below this cutoff, coupled with clinical signs of vitamin B12 deficiency, establishes a diagnosis of vitamin B12 deficiency [14]. A level ranging from 148 to 221 pmol/L (range, 200 to 300 pg/mL) is considered “borderline,” and the measurement of one of the vitamin B12 biomarkers, such as holotranscobalamin, homocysteine, or MMA, has been suggested [14-17]. In the absence of suggestive symptoms, the British Society of Hematology suggests a lower cutoff, less than 110 pmol/L, to consider the diagnosis of vitamin B12 deficiency. If the vitamin B12 level is between 110 pmol/L and 148 pmol/L without suggestive symptoms, the measurement of anti-intrinsic factor antibodies is recommended [14]. If these antibodies are negative, the assessment of one of the vitamin B12 biomarkers (holotranscobalamin, MMA, or homocysteine) is required to confirm biochemical deficiency [14].

DIAGNOSTIC CRITERIA FOR VITAMIN B12 DEFICIENCY USED IN STUDIES ASSESSING THE VITAMIN B12 STATUS IN PATIENTS WITH T2D ON METFORMIN THERAPY

Despite the diagnostic specificities of vitamin B12 deficiency and the necessity to measure biomarkers without suggestive symptoms, most studies assessing the risk of vitamin B12 deficiency in metformintreated patients rely solely on total vitamin B12 levels to confirm the diagnosis. The cutoff value was usually 150 pmol/L (203 pg/mL). Numerous studies have examined homocysteine levels to determine whether vitamin B12 deficiency occurs at the tissue level [2,3,19-21]. Some studies have concurrently assessed folate and vitamin B12 levels [21-23]. Holotranscobalamin, the biologically active form of vitamin B12, and MMA have been jointly analyzed with respect to vitamin B12 levels in some studies [20-22,24-26]. However, the size of the study populations in the latter studies was smaller than in those relying solely on vitamin B12 levels to define vitamin B12 deficiency. Only the study by Davis et al. [24], which considered holotranscobalamin levels, included a sample of more than 1,000 patients. Numerous studies have additionally examined the prevalence of a “borderline” vitamin B12 level, often defined within the range of 148 to 221 pmol/L (range, 200 to 300 pg/mL) [2-5,19,27].

INCREASED PREVALENCE OF VITAMIN B12 DEFICIENCY IN PATIENTS WITH T2D ON METFORMIN THERAPY: CLINICAL EVIDENCE

Determining the prevalence of vitamin B12 deficiency in patients with T2D on metformin therapy is difficult because of the different cutoff values used to define vitamin B12 deficiency, the use or lack of biomarkers in diagnostic criteria, and the heterogeneity of studied populations concerning age, geographical origin, dietary patterns, and duration of metformin use. This prevalence ranged from 6% to 50% in the literature [17,28-31].

Table 1 summarizes the findings of the main studies that investigated vitamin B12 deficiency in patients with T2D using metformin over the past decade. We analyzed only comparative studies that included a control group and involved more than 300 patients. Most of these studies reported a higher frequency of vitamin B12 deficiency in the metformin-treated group, irrespective of the definition adopted. The study published by Aroda et al. [2] in 2016 was the main interventional study on which the ADA has relied since 2017 to recommend regular monitoring of vitamin B12 levels in individuals treated with metformin, particularly in the presence of neuropathy or anemia [9]. In this study, the authors found a higher frequency of vitamin B12 deficiency (≤203 pg/mL) in the “metformin” group compared with the “placebo” group after 5 years of follow-up. Moreover, nearly half of patients taking metformin with low or borderline vitamin B12 levels exhibited elevated homocysteine levels, suggesting a true tissue deficiency in vitamin B12 [2].

In addition to the cohort studies, four meta-analyses confirmed a higher frequency of vitamin B12 deficiency in patients with T2D taking metformin [28-31]. The likelihood of experiencing a deficiency in these patients was significantly increased, with a 2.45-fold risk according to a meta-analysis conducted by Niafar et al. [29] and a 2.09-fold risk reported in a meta-analysis by Yang et al. [30]

Borderline vitamin B12 levels are also more common in patients treated with metformin [2,4,5,19,32].

MECHANISMS OF VITAMIN B12 DEFICIENCY IN PATIENTS TREATED WITH METFORMIN

The underlying mechanisms implicated in reduced vitamin B12 levels secondary to metformin use remain unclear. Several mechanisms have been suggested [5,17,33,34]: (1) reduced intrinsic factor secretion, (2) decreased vitamin B12 release from protein complexes, (3) microbial overgrowth due to slowed intestinal transit, and (4) decreased vitamin B12-intrinsic factor complex receptors.

Presently, the most recognized mechanism involves disrupted transmembrane absorption of the intrinsic factor-vitamin B12 complex. Metformin blocks calcium channels necessary for vitamin B12 absorption at the ileal level. Thus, in the study by Bauman et al. [35], a decreased holotranscobalamin II (the active complex of vitamin B12) level was observed after 4 months of metformin treatment. After introducing 1.2 g per day of calcium supplementation for 3 months, a significant increase in the holotranscobalamin II levels was observed. Regardless of the mechanism involved, an etiological investigation of vitamin B12 deficiency is necessary before attributing it to metformin [14].

RISK FACTORS FOR VITAMIN B12 DEFICIENCY IN PATIENTS WITH T2D ON METFORMIN THERAPY

1. Metformin Treatment Duration

Numerous studies have highlighted a positive correlation between the duration of metformin treatment and the risk of vitamin B12 deficiency. In the study by Ko et al. [23], the risk of developing vitamin B12 deficiency was significantly correlated with the duration of metformin intake, with a ninefold increased risk beyond 10 years and a 4.6-fold increased risk for a treatment duration between 4 years and 10 years. In a meta-analysis by Yang et al. [30], the risk of developing vitamin B12 deficiency increased 2.9 times after 3 years of treatment. Below this duration, no significant difference was observed in the risk of vitamin B12 deficiency. In a retrospective cohort study by Martin et al. [31], the mean duration between the initiation of metformin treatment and the onset of vitamin B12 deficiency was 5.3 years. Finally, in a recent cohort study conducted by Hurley-Kim et al. [5], metformin was associated with a significantly increased risk of vitamin B12 deficiency starting in the fourth year of treatment. Interestingly, vitamin B12 deficiency resulting from insufficient alimentary intake typically takes approximately 2 to 5 years to develop, considering the important liver stores [36]. This may explain the delay between the initiation of metformin treatment and the onset of vitamin B12 deficiency.

2. Metformin Dose

In line with the duration of metformin treatment, multiple studies and meta-analyses have consistently demonstrated a positive correlation between the daily dosage of metformin and the risk of vitamin B12 deficiency. In the meta-analysis by Yang et al. [30], a daily metformin dosage exceeding 2,000 mg was associated with a more pronounced decrease in vitamin B12 levels. In the study by Alharbi et al. [37], the risk of vitamin B12 deficiency was multiplied by 21.6 in patients taking more than 2,000 mg/d of metformin. According to a case-control study conducted in Hong Kong, each 1 g/d metformin dose increment conferred a 2.88 increased risk of developing vitamin B12 deficiency [38]. More recently, in 2020, Shivaprasad et al. [4] defined a new parameter to quantify metformin exposure by combining daily dose and duration of metformin treatment. This parameter, referred to as the “Metformin Usage Index” (MUI), is determined by the formula: (daily metformin dose in milligrams×duration of metformin intake in years)/1,000. The risk of developing a vitamin B12 deficiency significantly increased when the MUI exceeded 5, with odds ratios of 6.74, 5.12, and 3.56 for MUI values exceeding 15, between 10 and 15, and between 5 and 10, respectively. Hence, the MUI can be a reliable tool for identifying individuals with an elevated risk of vitamin B12 deficiency among patients with T2D receiving metformin treatment. Finally, in 2023, Wee et al. [39] showed that vitamin B12 deficiency was present in one out of three patients receiving 1,500 mg/d of metformin. This prevalence rose to two out of three patients taking the maximum metformin dose of 3,000 mg/d.

3. Other Risk Factors

Several studies have demonstrated an increased risk of vitamin B12 deficiency among older patients using metformin [17,24,31,40,41]. Nervo et al. [40] observed a negative correlation between age and vitamin B12 levels. Notably, advanced age is a risk factor for vitamin B12 deficiency, regardless of metformin intake [42,43]. Vitamin B12 stores are depleted more rapidly in older individuals, making them more susceptible to vitamin B12 deficiency, even after a brief period of metformin usage [43]. Leung et al. [41] provided evidence of a significant decrease in total vitamin B12 and haptocorrin levels after only 3 months of metformin use in older adults.

The literature further indicates that the simultaneous intake of acidsuppressing medications (such as proton pump inhibitors [PPIs] and histamine 2 receptor antagonists [H2RAs]) with metformin increases the risk of vitamin B12 deficiency in individuals with T2D [44-48]. PPIs block the H+/K+ ATPase pump within parietal cells, consequently diminishing the transportation of H+ ions into the gastric lumen. H2RAs impede the interaction between histamine and gastric parietal cells, whose role is to stimulate the H+/K+ ATPase pump [45]. These two mechanisms reduce gastric acidity, potentially decreasing vitamin B12 levels. This reduction may occur through the decreased release of protein-bound vitamin B12 or by creating an environment with elevated pH that fosters intestinal bacterial overgrowth, subsequently diminishing vitamin B12 absorption [45,49]. In the meta-analysis conducted by Jung et al. [49], which included 4,254 patients using PPIs or H2RAs and 19,228 controls, a significant association was found between the chronic use of acid-suppressing medications (>2 years) and vitamin B12 deficiency (odds ratio, 1.83; P<0.001).

Metformin is associated with digestive disorders in 10% to 20% of cases [50], and nearly 40% of individuals with diabetes experience symptomatic gastroesophageal reflux disease [47,50]. This implies that a significant proportion of patients with diabetes who regularly use metformin take acid-suppressing medications, potentially increasing the risk of developing a vitamin B12 deficiency [17,50].

A significant correlation has been observed among diabetes duration, diabetes control, and vitamin B12 deficiency in some studies. Thus, in studies by Khan et al. [51] and Pfilpsen et al. [52], the risk of vitamin B12 deficiency increased with the duration of diabetes. However, the duration of metformin intake was not adjusted in these two studies. Similarly, Ahmed et al. [53] and Yazidi et al. [19] reported that better-controlled diabetes and lower hemoglobin A1c levels were associated with a higher frequency of vitamin B12 deficiency. This association was explained by better adherence to metformin in patients with well-controlled disease.

COMPLICATIONS ASSOCIATED WITH VITAMIN B12 DEFICIENCY IN PATIENTS WITH T2D ON METFORMIN THERAPY

1. Vitamin B12 Deficiency and Diabetic Neuropathy Risk in Patients with T2D on Metformin Therapy

Vitamin B12 is essential in axonal myelination by producing methionine in the central and peripheral nervous systems [54]. Several large studies and meta-analyses have highlighted an association between vitamin B12 status in patients with T2D and metformin treatment and the presence or exacerbation of peripheral diabetic neuropathy [2,3,6-8,26,55-58]. In the meta-analysis by Wang et al. [59], vitamin B12 levels were significantly lower in patients with T2D and peripheral neuropathy than in those without peripheral neuropathy. However, this metaanalysis did not consider the potential link with metformin use. Singh et al. [55] observed a negative correlation between vitamin B12 levels and the Toronto Clinical Scoring System score used for diagnosing peripheral neuropathy. In a recent cohort study, the risk of hospitalization for painful peripheral neuropathy was found to be 1.8 times higher in patients treated with metformin [8]. This risk was 1.5 times higher in patients receiving a daily metformin dose between 1 g and 2 g and 4.3 times higher in patients receiving daily metformin doses exceeding 2 g. Interestingly, in that study, patients who received vitamin B12 and metformin did not show an increased risk of hospitalization for painful peripheral neuropathy [8]. However, in the recent cohort study by Davis et al. [24], no association was observed between vitamin B12 deficiency and the frequency of peripheral diabetic neuropathy. Notably, a very low cutoff value (<80 pmol/L) was used in this study to define vitamin B12 deficiency [8]. Furthermore, according to a randomized trial conducted in a population of patients with T2D receiving metformin for at least 4 years and experiencing diabetic neuropathy, administration of 1 mg oral methylcobalamin for 1 year was associated with a significant improvement in neurophysiological parameters and scores assessing neuropathic pain and quality of life [58].

2. Vitamin B12 Deficiency and the Risk of Anemia in Patients with T2D on Metformin Therapy

The MASTERMIND study, published in 2020, analyzed two randomized trials (ADOPT [A Diabetes Outcome Progression Trial] and UKPDS [UK Prospective Diabetes Study]) and an observational cohort study (GoDARTS [Genetics of Diabetes Audit and Research in Tayside Scotland]) to assess the risk of anemia in patients with diabetes treated with metformin [60]. This study revealed a higher risk of anemia in patients with T2D on metformin treatment than in those on a diet or another antihyperglycemic agent (sulfonylurea or insulin). The data analysis from the GoDARTS cohort revealed that a daily intake of 1 g/d of metformin was associated with a 2% increase in the annual risk of anemia. Nevertheless, its association with B12 vitamin status has not been investigated [60]. Aroda et al. [2] revealed that anemia was higher in the metformin-treated group. It increased from 9.8% to 14.3% between years 1 and 5 of treatment and reached 21% in year 13. However, the risk of anemia did not correlate with the vitamin B12 status of patients, suggesting an alternative mechanism responsible for anemia in individuals with T2D receiving metformin therapy [2]. Similarly, Davis et al. [24] reported an increased risk of anemia in patients with T2D treated with metformin. This risk increased with the treatment dose but was not correlated with the vitamin B12 status of the patients. Moreover, the mean corpuscular volume was similar in patients treated with and without metformin [24].

These data suggest that the increased risk of anemia observed in patients with T2D treated with metformin cannot be explained by vitamin B12 deficiency.

3. Other Complications Associated with Vitamin B12 Deficiency in Patients with T2D on Metformin Therapy

Besides neuropathy and anemia, a limited number of studies have explored the potential link between vitamin B12 deficiency and other complications in patients with T2D on metformin therapy. Some studies have shown an increased risk of depression and cognitive disorders in individuals with T2D taking metformin and vitamin B12 deficiency [61,62].

The link between diabetic retinopathy and lower vitamin B12 levels has been highlighted in studies by Satyanarayana et al. [63] and Fotiou et al. [64]

Vitamin B12 deficiency is associated with increased homocysteine levels. This marker is associated in several studies with an increased risk of coronary insufficiency, ischemic stroke, and peripheral arterial occlusive disease [65-67]. Therefore, one might assume that vitamin B12 deficiency, through elevated homocysteine levels, is associated with an increased risk of cardiovascular disease. However, the relationship between cardiovascular outcomes and vitamin B12 levels in individuals with T2D treated with metformin remains unclear. Horrany et al. [68] found an association between vitamin B12 deficiency and the risk of stroke. In a cohort study involving 8,067 patients with T2D who participated in the National Health and Nutrition Examination Survey, low and high serum vitamin B12 levels were associated with a higher risk of cardiovascular disease mortality [69]. In a recent cohort study, no relationship was observed between serum vitamin B12 levels and overall or cardiac mortality in individuals with T2D, irrespective of metformin use [70]. However, high MMA levels were significantly associated with an increased risk of all-cause and cardiac mortality. The authors concluded that decreased sensitivity to vitamin B12, as indicated by high MMA levels, was associated with increased cardiac and overall mortality [70].

A PRACTICAL DIAGNOSTIC AND THERAPEUTIC APPROACH FOR VITAMIN B12 DEFICIENCY IN PATIENTS WITH T2D ON METFORMIN THERAPY

1. Screening Strategy for Vitamin B12 Deficiency in Patients with T2D on Metformin Therapy

Although the higher prevalence of vitamin B12 deficiency in patients with T2D receiving metformin therapy is well established, there are no specific guidelines regarding screening modalities for vitamin B12 deficiency in this population. The British Society of Hematology stated in its 2014 guidelines that no recommendation could be made regarding the optimal frequency of monitoring serum vitamin B12 levels in patients with T2D treated with metformin. However, it is recommended that serum vitamin B12 levels be measured in patients with high clinical suspicion of deficiency [14]. Since 2017, the ADA has recommended periodic vitamin B12 testing for individuals treated with metformin, especially in the presence of peripheral neuropathy or anemia [9]. Although updated annually, this recommendation was not detailed until 2024 [11]. In routine practice, screening for vitamin B12 deficiency in patients with T2D on metformin therapy remains suboptimal, as indicated by a recent observational study [31].

Furthermore, whether vitamin B12 biomarkers should be tested in the presence of “borderline” vitamin B12 levels when no suggestive symptoms of deficiency are present in patients taking metformin remains unclear. Finally, scientific societies do not express their positions regarding the population that should be targeted for screening. It is unclear whether screening should follow a systematic approach or selectively focus on patients with risk factors or symptoms suggestive of vitamin B12 deficiency.

In June 2022, the Medicines and Healthcare Products Regulatory Agency (MHRA) considered vitamin B12 deficiency a common side effect of metformin treatment. They advised annual monitoring of serum vitamin B12 levels in patients treated with metformin, especially those receiving high doses and those who received treatment for an extended period [71,72].

Based on recent evidence, we propose an algorithm illustrating a screening and diagnostic strategy for vitamin B12 deficiency in patients with T2D on metformin therapy (Figure 1).

2. Management of Vitamin B12 Deficiency in Patients with T2D on Metformin Therapy

Specific guidelines for treating vitamin B12 deficiency in patients with T2D receiving metformin therapy are lacking. Although most experts agree on the importance of not discontinuing metformin because of its critical role in T2D management, it is essential to supplement vitamin B12 when a deficiency is identified, even without suggestive symptoms. Supplementation is essential to prevent complications associated with vitamin B12 deficiency, particularly neurological and hematological complications. However, there are currently no explicit guidelines outlining optimal supplementation approaches. Many questions regarding vitamin B12 form, dosage, administration route, and duration of treatment require clarification.

Although malabsorption is a plausible mechanism for the vitamin B12 deficiency associated with metformin treatment, normalization of vitamin B12 levels has been observed even after oral administration [57]. Bhise et al. [73] demonstrated that a 6-month supplementation with vitamin B12 improved the symptoms of diabetic neuropathy, regardless of the mode of administration (intramuscular or oral). Another emerging administration route is the sublingual route, which avoids absorption issues in the gastrointestinal tract and bypasses the first hepatic pass. Parry-Strong et al. [74] compared the effectiveness of the sublingual route with that of the intramuscular route. They demonstrated the superiority of the sublingual route in correcting vitamin B12 deficiency in patients with T2D on metformin therapy. According to a recent literature review that included seven clinical trials assessing the efficacy of vitamin B12 supplementation in patients with T2D receiving metformin treatment, daily supplementation with methylcobalamin through oral or sublingual administration is preferred over the intramuscular route, considering its effectiveness, cost, and convenience for patients [57].

Given the lack of specific guidelines for the optimal treatment of vitamin B12 deficiency in individuals receiving metformin therapy, the MHRA [71] recommends following the National Institute for Health and Care Excellence (NICE) guidelines for managing vitamin B12 deficiency in the general population. In its most recent update, the NICE offers practitioners the option to choose between oral and intramuscular routes when malabsorption is not the cause of vitamin B12 deficiency [75]. It also mentions the possibility of offering intramuscular or oral vitamin B12 replacement to individuals with suspected or confirmed medicine-induced vitamin B12 deficiency [75]. Given the inconvenience of regular intramuscular injections and the evidence from several trials demonstrating the effectiveness of the oral route in normalizing vitamin B12 levels [57,73], we initially suggest a trial of oral replacement. Vitamin B12 levels should be checked 3 months later. If the vitamin B12 levels normalize, oral replacement may continue. Otherwise, the intramuscular route of administration should be considered. Vitamin B12 replacement should be continued as long as the patient is on metformin therapy.

Furthermore, calcium supplementation increased vitamin B12 levels in patients with T2D treated with metformin [35]. Numerous studies have demonstrated the beneficial effects of calcium supplementation on carbohydrate homeostasis [76]. Thus, calcium supplementation in patients with T2D can help prevent vitamin B12 deficiency associated with metformin intake and improve glycemic control. However, recent randomized trials and meta-analyses have shown that calcium supplementation is associated with an increased risk of cardiovascular events [77,78], making its use controversial in patients with T2D.

Regarding prevention, the British Society of Hematology states that it cannot provide recommendations on the indications for the prophylactic administration of vitamin B12 in patients with T2D treated with metformin [14]. However, such an approach is likely beneficial for patients receiving high doses of metformin over an extended period. Further studies are required to elucidate the benefits and risks of this preventive measure.

CONCLUSION

The relationship between vitamin B12 deficiency and chronic metformin use is well established, but the underlying mechanisms remain poorly understood. The risk of vitamin B12 deficiency is higher in patients with T2D receiving high metformin doses for an extended period or in the presence of other known risk factors associated with vitamin B12 deficiency, such as advanced age and regular use of acid-suppressing medications. Vitamin B12 deficiency in patients with T2D receiving metformin therapy increases the risk of developing or exacerbating peripheral neuropathy. The risk of anemia is also higher in these patients but appears to be independent of vitamin B12 status. However, its relationship with other cognitive and cardiovascular complications remains controversial. All these data highlight the importance of assessing vitamin B12 status in patients with T2D on metformin therapy, especially in those with risk factors and those presenting with neuropathy or other suggestive symptoms of vitamin B12 deficiency. Vitamin B12 biomarkers should be measured when vitamin B12 levels are borderline. Identifying vitamin B12 deficiency requires an etiological investigation similar to the general population before diagnosing metformin-induced vitamin B12 deficiency. When diagnosed, metformin-induced vitamin B12 deficiency requires supplementation through oral, sublingual, or intramuscular routes. The vitamin B12 replacement protocol is similar to vitamin B12 deficiencies related to etiologies other than metformin. Metformin treatment should not be discontinued.

Notes

CONFLICT OF INTEREST

No potential conflict of interest relevant to this article was reported.

Figure. 1.

Screening and diagnosing strategy for vitamin B12 deficiency in patients with type 2 diabetes on metformin therapy. MUI, Metformin Usage Index; AIFA, anti-intrinsic factor antibody; HTC, holotranscobalamin; Hcty, homocysteine; MMA, methylmalonic acid. *MUI=(daily metformin dose in milligrams×duration of metformin intake in years)/1,000.

Table 1.

Comparative studies investigating the vitamin B12 status in patients with type 2 diabetes on metformin therapy

Study (year)	Location	Definition of vitamin B12 deficiency	Study design	Population size	Prevalence of vitamin B12 deficiency, metformin therapy vs. controls	Vitamin B12 deficiency risk
Damião et al. [44] (2016)	Brazil	<133 pmol/L (180 pg/mL)	Cross sectional study	231 T2D patients and 231 controls	22.5% vs. 7.4%; P<0.001	NM
Aroda et al. [2] (2016)	USA	<150 pmol/L (203 pg/mL)	Interventional study	1,073 patients in metformin group and 1,082 in placebo group	4.3% vs. 2.3%; P=0.02 at 5 y	OR, 1.13 (95% CI, 1.06–1.20) per year of metformin treatment
Aroda et al. [2] (2016)	USA	<150 pmol/L (203 pg/mL)	Interventional study		7.4% vs. 5.4%; P=NS at 13 y
Alharbi et al. [37] (2018)	Saudi Arabia	<133 pmol/L (180 pg/mL)	Retrospective study	319 metformin users and 93 non- metformin users	7.7% vs. 2.2%; P=0.036	OR, 4.72 (95% CI, 1.1–20.1)
Miyan et al. [3] (2019)	Pakistan	<148 pmol/L (200 pg/mL)	Prospective observational study	645 on metformin therapy and 287 not on metformin	3.9% vs. 2.1%; P=0.002	NM
Shivaprasad et al. [4] (2020)	India	<148 pmol/L (200 pg/mL)	Prospective observational study	2,061 metformin users and 826 non-metformin users	24.5% vs. 17.3%; P<0.01	NM
Davis et al. [24] (2023)	Australia	Vitamin B12 <80 pmol/L and holotranscobalamine <23 pmol/L	Prospective observational study	1,492 participants with T2D	4.2% vs. NM; P=0.004	OR, 19.2 (95% CI, 2.6–142)
Hurley-Kim et al. [5] (2023)	USA	<148 pmol/L (200 pg/mL)	Database study	6,221 metformin users and 30,519 non-metformin users	7.46% vs. 6.29%; P<0.05	NM

T2D, type 2 diabetes; NM, not mentioned; NS, not significant; OR, odds ratio; CI, confidence interval.

REFERENCES

1. Tomkin GH, Hadden DR, Weaver JA, Montgomery DA. Vitamin-B12 status of patients on long-term metformin therapy. Br Med J 1971;2:685-7.

2. Aroda VR, Edelstein SL, Goldberg RB, Knowler WC, Marcovina SM, Orchard TJ, et al. Long-term metformin use and vitamin B12 deficiency in the diabetes prevention program outcomes study. J Clin Endocrinol Metab 2016;101:1754-61.

3. Miyan Z, Waris N, MIBD. Association of vitamin B12 deficiency in people with type 2 diabetes on metformin and without metformin: a multicenter study, Karachi, Pakistan. BMJ Open Diabetes Res Care 2020;8:e001151.

4. Shivaprasad C, Gautham K, Ramdas B, Gopaldatta KS, Nishchitha K. Metformin Usage Index and assessment of vitamin B12 deficiency among metformin and non-metformin users with type 2 diabetes mellitus. Acta Diabetol 2020;57:1073-80.

5. Hurley-Kim K, Vu CH, Dao NM, Tran LC, McBane S, Lee J, et al. Effect of metformin use on vitamin B12 deficiency over time (EMBER): a real-world evidence database study. Endocr Pract 2023;29:862-7.

6. Farooq MD, Tak FA, Ara F, Rashid S, Mir IA. Vitamin B12 deficiency and clinical neuropathy with metformin use in type 2 diabetes. J Xenobiot 2022;12:122-30.

7. Hashem MM, Esmael A, Nassar AK, El-Sherif M. The relationship between exacerbated diabetic peripheral neuropathy and metformin treatment in type 2 diabetes mellitus. Sci Rep 2021;11:1940.

8. Yang R, Yu H, Wu J, Chen H, Wang M, Wang S, et al. Metformin treatment and risk of diabetic peripheral neuropathy in patients with type 2 diabetes mellitus in Beijing, China. Front Endocrinol (Lausanne) 2023;14:1082720.

9. Marathe PH, Gao HX, Close KL. American Diabetes Association standards of medical care in diabetes 2017. J Diabetes 2017;9:320-4.

10. Magliano DJ, Boyko EJ, International Diabetes Federation. IDF Diabetes Atlas. 10th ed. Brussels: International Diabetes Federation; 2021.

11. American Diabetes Association Professional Practice Committee. 3. Prevention or delay of diabetes and associated comorbidities: standards of care in diabetes-2024. Diabetes Care 2024;47(Supplement 1):S43-51. https://doi.org/10.2337/dc24-S003

12. Holman RR, Paul SK, Bethel MA, Matthews DR, Neil HA. 10-Year follow-up of intensive glucose control in type 2 diabetes. N Engl J Med 2008;359:1577-89.

13. Maruthur NM, Tseng E, Hutfless S, Wilson LM, Suarez-Cuervo C, Berger Z, et al. Diabetes medications as monotherapy or metforminbased combination therapy for type 2 diabetes: a systematic review and meta-analysis. Ann Intern Med 2016;164:740-51.

14. Devalia V, Hamilton MS, Molloy AM, British Committee for Standards in Haematology. Guidelines for the diagnosis and treatment of cobalamin and folate disorders. Br J Haematol 2014;166:496-513.

15. Hannibal L, Lysne V, Bjorke-Monsen AL, Behringer S, Grunert SC, Spiekerkoetter U, et al. Biomarkers and algorithms for the diagnosis of vitamin B12 deficiency. Front Mol Biosci 2016;3:27.

16. Yetley EA, Pfeiffer CM, Phinney KW, Bailey RL, Blackmore S, Bock JL, et al. Biomarkers of vitamin B-12 status in NHANES: a roundtable summary. Am J Clin Nutr 2011;94:313S-321S.

17. Infante M, Leoni M, Caprio M, Fabbri A. Long-term metformin therapy and vitamin B12 deficiency: an association to bear in mind. World J Diabetes 2021;12:916-31.

18. de Benoist B. Conclusions of a WHO Technical Consultation on folate and vitamin B12 deficiencies. Food Nutr Bull 2008;29(2 Suppl):S238-44.

19. Yazidi M, Kamoun E, Hadj Taieb S, Rejeb O, Mahjoub S, Maryam S, et al. Vitamin B12 deficiency in long-term metformin treated type 2 diabetic patients: prevalence and risk factors in a Tunisian population. Chronic Illn 2023;17423953231184220.

20. Wile DJ, Toth C. Association of metformin, elevated homocysteine, and methylmalonic acid levels and clinically worsened diabetic peripheral neuropathy. Diabetes Care 2010;33:156-61.

21. de Jager J, Kooy A, Lehert P, Wulffele MG, van der Kolk J, Bets D, 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.

22. Kanti G, Anadol-Schmitz E, Bobrov P, Strassburger K, Kahl S, Zaharia OP, et al. Vitamin B12 and folate concentrations in recent-onset type 2 diabetes and the effect of metformin treatment. J Clin Endocrinol Metab 2020;105:dgaa150.

23. Ko SH, Ko SH, Ahn YB, Song KH, Han KD, Park YM, et al. Association of vitamin B12 deficiency and metformin use in patients with type 2 diabetes. J Korean Med Sci 2014;29:965-72.

24. Davis TM, Chubb SA, Peters KE, Davis WA. Serum vitamin B12, distal symmetrical polyneuropathy and anaemia in type 2 diabetes: the Fremantle Diabetes Study Phase 2. Intern Med J 2024;54:575-81.

25. Beulens JW, Hart HE, Kuijs R, Kooijman-Buiting AM, Rutten GE. Influence of duration and dose of metformin on cobalamin deficiency in type 2 diabetes patients using metformin. Acta Diabetol 2015;52:47-53.

26. Out M, Kooy A, Lehert P, Schalkwijk CA, Stehouwer CD. Long-term treatment with metformin in type 2 diabetes and methylmalonic acid: post hoc analysis of a randomized controlled 4.3 year trial. J Diabetes Complications 2018;32:171-8.

27. Akinlade KS, Agbebaku SO, Rahamon SK, Balogun WO. Vitamin B12 levels in patients with type 2 diabetes mellitus on metformin. Ann Ib Postgrad Med 2015;13:79-83.

28. Chapman LE, Darling AL, Brown JE. Association between metformin and vitamin B12 deficiency in patients with type 2 diabetes: a systematic review and meta-analysis. Diabetes Metab 2016;42:316-27.

29. Niafar M, Hai F, Porhomayon J, Nader ND. The role of metformin on vitamin B12 deficiency: a meta-analysis review. Intern Emerg Med 2015;10:93-102.

30. Yang W, Cai X, Wu H, Ji L. Associations between metformin use and vitamin B12 levels, anemia, and neuropathy in patients with diabetes: a meta-analysis. J Diabetes 2019;11:729-43.

31. Martin D, Thaker J, Shreve M, Lamerato L, Budzynska K. Assessment of vitamin B12 deficiency and B12 screening trends for patients on metformin: a retrospective cohort case review. BMJ Nutr Prev Health 2021;4:30-5.

32. Reinstatler L, Qi YP, Williamson RS, Garn JV, Oakley GP Jr. Association of biochemical B12 deficiency with metformin therapy and vitamin B12 supplements: the National Health and Nutrition Examination Survey, 1999-2006. Diabetes Care 2012;35:327-33.

33. Adams JF, Clark JS, Ireland JT, Kesson CM, Watson WS. Malabsorption of vitamin B12 and intrinsic factor secretion during biguanide therapy. Diabetologia 1983;24:16-8.

34. Scarpello JH, Hodgson E, Howlett HC. Effect of metformin on bile salt circulation and intestinal motility in type 2 diabetes mellitus. Diabet Med 1998;15:651-6.

35. Bauman WA, Shaw S, Jayatilleke E, Spungen AM, Herbert V. Increased intake of calcium reverses vitamin B12 malabsorption induced by metformin. Diabetes Care 2000;23:1227-31.

36. Oberley MJ, Yang DT. Laboratory testing for cobalamin deficiency in megaloblastic anemia. Am J Hematol 2013;88:522-6.

37. Alharbi TJ, Tourkmani AM, Abdelhay O, Alkhashan HI, Al-Asmari AK, Bin Rsheed AM, et al. The association of metformin use with vitamin B12 deficiency and peripheral neuropathy in Saudi individuals with type 2 diabetes mellitus. PLoS One 2018;13:e0204420.

38. Ting RZ, Szeto CC, Chan MH, Ma KK, Chow KM. Risk factors of vitamin B(12) deficiency in patients receiving metformin. Arch Intern Med 2006;166:1975-9.

39. Wee AK, Sultana R. Determinants of vitamin B12 deficiency in patients with type-2 diabetes mellitus: a primary-care retrospective cohort study. BMC Prim Care 2023;24:102.

40. Nervo M, Lubini A, Raimundo FV, Faulhaber GA, Leite C, Fischer LM, et al. Vitamin B12 in metformin-treated diabetic patients: a cross-sectional study in Brazil. Rev Assoc Med Bras (1992) 2011;57:46-9.

41. Leung S, Mattman A, Snyder F, Kassam R, Meneilly G, Nexo E. Metformin induces reductions in plasma cobalamin and haptocorrin bound cobalamin levels in elderly diabetic patients. Clin Biochem 2010;43:759-60.

42. Hunt A, Harrington D, Robinson S. Vitamin B12 deficiency. BMJ 2014;349:g5226.

43. Stover PJ. Vitamin B12 and older adults. Curr Opin Clin Nutr Metab Care 2010;13:24-7.

44. Damiao CP, Rodrigues AO, Pinheiro MF, Cruz RA Filho, Cardoso GP, Taboada GF, et al. Prevalence of vitamin B12 deficiency in type 2 diabetic patients using metformin: a cross-sectional study. Sao Paulo Med J 2016;134:473-9.

45. Miller JW. Proton pump inhibitors, H2-receptor antagonists, metformin, and vitamin B-12 deficiency: clinical implications. Adv Nutr 2018;9:511S-518S.

46. Long AN, Atwell CL, Yoo W, Solomon SS. Vitamin B(12) deficiency associated with concomitant metformin and proton pump inhibitor use. Diabetes Care 2012;35:e84.

47. Wakeman M, Archer DT. Metformin and micronutrient status in type 2 diabetes: does polypharmacy involving acid-suppressing medications affect vitamin B12 levels? Diabetes Metab Syndr Obes 2020;13:2093-108.

48. Dinesh D, Lee JS, Scott TM, Tucker KL, Palacios N. Association between acid-lowering agents, metformin, and vitamin B12 among Boston-area Puerto Ricans. J Nutr 2023;153:2380-8.

49. Jung SB, Nagaraja V, Kapur A, Eslick GD. Association between vitamin B12 deficiency and long-term use of acid-lowering agents: a systematic review and meta-analysis. Intern Med J 2015;45:409-16.

50. Bonnet F, Scheen A. Understanding and overcoming metformin gastrointestinal intolerance. Diabetes Obes Metab 2017;19:473-81.

51. Khan A, Shafiq I, Hassan Shah M. Prevalence of vitamin B12 deficiency in patients with type II diabetes mellitus on metformin: a study from Khyber Pakhtunkhwa. Cureus 2017;9:e1577.

52. Pflipsen MC, Oh RC, Saguil A, Seehusen DA, Seaquist D, Topolski R. The prevalence of vitamin B(12) deficiency in patients with type 2 diabetes: a cross-sectional study. J Am Board Fam Med 2009;22:528-34.

53. Ahmed MA, Muntingh G, Rheeder P. Vitamin B12 deficiency in metformin-treated type-2 diabetes patients, prevalence and association with peripheral neuropathy. BMC Pharmacol Toxicol 2016;17:44.

54. Stabler SP. Clinical practice: vitamin B12 deficiency. N Engl J Med 2013;368:149-60.

55. Singh AK, Kumar A, Karmakar D, Jha RK. Association of B12 deficiency and clinical neuropathy with metformin use in type 2 diabetes patients. J Postgrad Med 2013;59:253-7.

56. Jin HY, Lee KA, Kim YJ, Gwak IS, Park TS, Yeom SW, et al. Bidirectional association between diabetic peripheral neuropathy and vitamin B12 deficiency: two longitudinal 9-year follow-up studies using a national sample cohort. Prim Care Diabetes 2023;17:436-43.

57. Pratama S, Lauren BC, Wisnu W. The efficacy of vitamin B12 supplementation for treating vitamin B12 deficiency and peripheral neuropathy in metformin-treated type 2 diabetes mellitus patients: a systematic review. Diabetes Metab Syndr 2022;16:102634.

58. Didangelos T, Karlafti E, Kotzakioulafi E, Margariti E, Giannoulaki P, Batanis G, et al. Vitamin B12 supplementation in diabetic neuropathy: a 1-year, randomized, double-blind, placebo-controlled trial. Nutrients 2021;13:395.

59. Wang D, Zhai JX, Liu DW. Serum folate, vitamin B12 levels and diabetic peripheral neuropathy in type 2 diabetes: a meta-analysis. Mol Cell Endocrinol 2017;443:72-9.

60. Donnelly LA, Dennis JM, Coleman RL, Sattar N, Hattersley AT, Holman RR, et al. Risk of anemia with metformin use in type 2 diabetes: a MASTERMIND study. Diabetes Care 2020;43:2493-9.

61. Biemans E, Hart HE, Rutten GE, Cuellar Renteria VG, Kooijman-Buiting AM, Beulens JW. Cobalamin status and its relation with depression, cognition and neuropathy in patients with type 2 diabetes mellitus using metformin. Acta Diabetol 2015;52:383-93.

62. Moore EM, Mander AG, Ames D, Kotowicz MA, Carne RP, Brodaty H, et al. Increased risk of cognitive impairment in patients with diabetes is associated with metformin. Diabetes Care 2013;36:2981-7.

63. Satyanarayana A, Balakrishna N, Pitla S, Reddy PY, Mudili S, Lopamudra P, et al. Status of B-vitamins and homocysteine in diabetic retinopathy: association with vitamin-B12 deficiency and hyperhomocysteinemia. PLoS One 2011;6:e26747.

64. Fotiou P, Raptis A, Apergis G, Dimitriadis G, Vergados I, Theodossiadis P. Vitamin status as a determinant of serum homocysteine concentration in type 2 diabetic retinopathy. J Diabetes Res 2014;2014:807209.

65. Humphrey LL, Fu R, Rogers K, Freeman M, Helfand M. Homocysteine level and coronary heart disease incidence: a systematic review and meta-analysis. Mayo Clin Proc 2008;83:1203-12.

66. Khandanpour N, Loke YK, Meyer FJ, Jennings B, Armon MP. Homocysteine and peripheral arterial disease: systematic review and metaanalysis. Eur J Vasc Endovasc Surg 2009;38:316-22.

67. He Y, Li Y, Chen Y, Feng L, Nie Z. Homocysteine level and risk of different stroke types: a meta-analysis of prospective observational studies. Nutr Metab Cardiovasc Dis 2014;24:1158-65.

68. Horrany N, Abu Dahoud W, Moallem Y, Hajouj T, Zreik M, Blum A. The effect of metformin on vitamin B12 deficiency and stroke. Isr Med Assoc J 2023;25:122-5.

69. Liu Y, Geng T, Wan Z, Lu Q, Zhang X, Qiu Z, et al. Associations of serum folate and vitamin B12 levels with cardiovascular disease mortality among patients with type 2 diabetes. JAMA Netw Open 2022;5:e2146124.

70. Wang S, Wang Y, Wan X, Guo J, Zhang Y, Tian M, et al. Cobalamin intake and related biomarkers: examining associations with mortality risk among adults with type 2 diabetes in NHANES. Diabetes Care 2022;45:276-84.

71. Medicines and Healthcare Products Regulatory Agency. Metformin and reduced vitamin B12 levels: new advice for monitoring patients at risk [Internet]. London: Medicines and Healthcare Products Regulatory Agency; 2022 [cited 2024 May 22]. Available from: https://bit.ly/3BlGkhJ

72. Davies S. Metformin use and vitamin B12 deficiency: new MHRA guidance. J Diabetes Nurs 2022;[cited 2024 May 20]. 26:1-4. Available from: https://diabetesonthenet.com/wp-content/uploads/3.-Davies.pdf

73. Bhise SB, Kadam YD, Ismaeel GL. Effect of vitamin B12 supplement in metformin treated diabetic patients and it’s correlation to peripheral neuropathy. Int J Pharma Res Health Sci 2018;6:2394-400.

74. Parry-Strong A, Langdana F, Haeusler S, Weatherall M, Krebs J. Sublingual vitamin B12 compared to intramuscular injection in patients with type 2 diabetes treated with metformin: a randomised trial. N Z Med J 2016;129:67-75.

75. National Institute for Health and Care Excellence. Vitamin B12 deficiency in over 16s: diagnosis and management (NICE Guideline NG239) [Internet]. Manchester: National Institute for Health and Care Excellence; 2024 [cited 2024 May 22]. Available from: https://www.nice.org.uk/guidance/ng239

76. Hajhashemy Z, Rouhani P, Saneei P. Dietary calcium intake in relation to type-2 diabetes and hyperglycemia in adults: a systematic review and dose-response meta-analysis of epidemiologic studies. Sci Rep 2022;12:1050.

77. Myung SK, Kim HB, Lee YJ, Choi YJ, Oh SW. Calcium supplements and risk of cardiovascular disease: a meta-analysis of clinical trials. Nutrients 2021;13:368.

78. Park JM, Lee B, Kim YS, Hong KW, Park YC, Shin DH, et al. Calcium supplementation, risk of cardiovascular diseases, and mortality: a realworld study of the Korean National Health Insurance Service data. Nutrients 2022;14:2538.