Prevalence of vitamin B12 deficiency in type-2 diabetes patients treated with metformin and its association with peripheral neuropathy

Close to one third of metformin-treated type-2 diabetes mellitus (T2DM) patients had vitamin B₁₂ deficiency and the deficiency was not associated with peripheral neuropathy.

Metformin is very common routine medication prescribed to nearly 120 million T2DM patients worldwide. Multiple studies have confirmed the correlation between long-term use of metformin and vitamin B12 deficiency. Irrespective of the evidences confirming the existence of a relationship between metformin use and vitamin B12 deficiency, the actual statistics of this association has not been quantified yet. The incidences of metformin-induced vitamin B12 deficiency ranges between 5.8% and 52%. Neuropathy might be the only clinical presentation of vitamin B12 deficiency, without haematological signs and symptoms. Vitamin B12 deficiency-associated peripheral neuropathy may interact with T2DM and thus, likely to be misdiagnosed as diabetic neuropathy. The long-term use of metformin, mediated by vitamin B12 deficiency, may contribute to increasing the substantial burden of peripheral neuropathy in T2DM patients. Several studies have tried to find the relationship between long-term metformin use and vitamin B12 deficiency-mediated peripheral neuropathy, but has come up with conflicting results.

Rationale behind research:

• The potential of the vitamin B12 deficiency to cause or worsen peripheral neuropathy in T2DM patients has been investigated with conflicting results
• The present study was conducted to investigate the prevalence of vitamin B12 deficiency in T2DM patients on metformin and also evaluated the association between vitamin B12 and peripheral neuropathy the risk factors for vitamin B12 deficiency in these patients

Objective:

The aim of this study was to investigate:

• The prevalence of vitamin B12 deficiency in T2DM patients on metformin
• The association between vitamin B12 and peripheral neuropathy
• The risk factors for vitamin B12 deficiency in these patients
  

Study outcomes

Vitamin B12 deficiency:

• Age, sex, ethnic group, date of diagnosis of T2DM, most recent HbA1c (glycated hemoglobin) level, duration of metformin use, dosage of metformin, smoking status, estimated Glomerular Filtration Rate (eGFR), Body-Mass Index (BMI), use of gastric acid suppressants or low-dose acetylsalicylic acid (both were linked to vitamin B12 deficiency, coffee consumption, presence and history of other medical conditions and alcohol consumption or any substance abuse were recorded.
• Levels of serum vitamin B12 was measured by UniCel DxI 800 instrument. Vitamin B12 deficiency was defined as levels <150 pmol/L

• Peripheral neuropathy:

NTSS-6 questionnaire was used to grade peripheral neuropathy. The investigator asked each participant about frequency and the intensity of six peripheral neuropathic symptoms, and used the answers to complete the questionnaire. The sum of scores for each patient were obtained (possible range between 0.00 and 21.96).

• Relationship between vitamin B12 and peripheral neuropathy:

The relationship between vitamin B12 and peripheral neuropathy was investigated when two variables were in the binary and continuous forms.

• Risk factors for vitamin B12 deficiency:

Multiple logistic regression analysis was used to determine the risk factors for vitamin B12 deficiency.

Outcomes:

• Vitamin B12 deficency: Thirty four participants, representing 28.1% of the sample, were found to be vitamin B12-deficient
• Relationship between vitamin B12 and peripheral neuropathy: 32.3% of vitamin B12 deficient participants had neuropathy compared to 36.8% of those with normal vitamin levels. The value of Spearman’s Rank Correlation Coefficient (RHO) was 0.056 with a P value of 0.54, indicating that there was no sufficient evidence showing association between vitamin B12 levels and NTSS-6 scores.

Figure 2. Correlation of Vitamin B₁₂ and peripheral neuropathy.

• Univariate analysis revealed that vitamin B₁₂-deficient participants were significantly older than those with normal vitamin levels (62.3 vs. 57 years, P=0.012). They also had significantly longer metformin use duration and higher cumulative metformin dose. Those with deficiency, also revealed statistically significant lower HbA1c (57[7.4 %] vs. 79[9.4 %] mmol/mol, P=0.001). Duration of T2DM was higher in vitamin deficient participants with a P value that almost reached the statistical significance (12 vs. 9 years, P=0.055).
• Risk factors for vitamin B₁₂ deficiency:  The regression model has shown that the odds of having vitamin B₁₂ deficiency was significantly reduced by being from black South African descent (OR=0.34,95% CI: 0.13 to 0.92, P=0.033) and by increase in HbA1c value (mmol/mol) (OR=0.97, 95 % CI: 0.95 to 0.99, P=0.003). The model has also shown that metformin total daily dose (gram) increased the odds of having vitamin B₁₂ deficiency with an approximately significant P value (OR=1.96, 95% CI: 0.99 to 3.88, P=0.053).

The study demonstrated that the prevalence of vitamin B₁₂ deficiency, defined by levels <150 pmol/L, in metformin treated T2DM patients was as high as 28.1%. There was no association between vitamin B₁₂ and peripheral neuropathy. A novel finding was the association between black South African descent and lower odds of vitamin B₁₂ deficiency in metformin-treated patients. Regular screening for vitamin B₁₂ deficiency in patients on long-term metformin is recommended.

Vitamin B₁₂ prevalence is found to be higher in the present study as compared to previously reported estimates. It is exactly the same as that of Beulens et al. study; however, mean metformin dose and duration were higher in the present study. The study did not report any statistically significant difference in neuropathy between those with normal and deficient vitamin levels (36.8% vs. 32.3%, P=0.209). The present study findings are also in concordance with the results of the cross-sectional study of Chen et al. which revealed no significant differences between metformin users and non-users when neuropathy status was assessed by both objectives (monofilament and neurothesiometry) and relatively subjective (questionnaires) measures. Low HbA1c level was a significant risk factor for vitamin B₁₂ deficiency in the final (OR=0.97, 95% CI: 0.95 to 0.99, P=0.003) regression model. Kang et al. reported, but did not explain, the similar results. They reported stronger association between HbA1c and vitamin B₁₂ deficiency (OR=0.74, CI: 0.56 to 0.99). This study reveals the need for setting-specific evidence to tackle the subject. Further research that judiciously considers study design issues is warranted to clarify the possible impact of metformin-induced vitamin B₁₂ deficiency on peripheral neuropathy in T2DM patients.