The Prevalence of Vitamin B₁₂ Deficiency in Patients with Type 2 Diabetes: A Cross-Sectional Study

Methods

Results

A total of 203 patients enrolled in the study. Eight patients were excluded: 2 had ileum resections, 2 were younger than 45 years of age, 2 were taking supplemental B₁₂ >500 mcg/day, one had a gastrectomy, and one had Crohn's disease. The remaining 195 patients were included in the final analysis (Table 1).

Serum B₁₂ levels ranged from 91 to 2818 pg/mL. Only one individual had a B₁₂ level <100 pg/mL; 79 had intermediate levels between 100 and 350 pg/mL and required confirmatory testing to assess for deficiency. Forty-three of these patients (54%) with intermediate B₁₂ levels had elevations of either methylmalonic acid or homocysteine, resulting in a total of 22% (n = 44) of patients diagnosed with metabolic B₁₂ deficiency.

Table 1 shows the participants’ demographic data. The majority of participants took metformin and approximately half were on a dose of ≥2000 mg daily at the time of enrollment. Approximately one-third of patients took either an H₂ blocker or a proton pump inhibitor, whereas just over half took a multivitamin daily (Table 2).

Table 2 shows key variable comparisons between patients with and without B₁₂ deficiency. Metformin use, including both current use and history of metformin use, was not associated with B₁₂ deficiency. On average, patients with a B₁₂ deficiency suffered from diabetes for a significantly longer time than patients without a B₁₂ deficiency (P = .04). In addition, the percentage of patients identified as having a B₁₂ deficiency was significantly lower among patients using a multivitamin compared with those patients who did not take a multivitamin supplement (P < .001). Only 4 patients reported taking B-complex vitamins, none of which had B₁₂ deficiency.

When a second χ² was performed defining at risk for B₁₂ deficiency as any serum B₁₂ level <350 pg/mL, the results indicated that patients using metformin were statistically at higher risk for B₁₂ deficiency (P = .04). A series of further analyses were subsequently performed to determine whether metformin dose was associated with B₁₂ deficiency. In this study population, there was no statistically significant association between the average prescribed metformin dosage for patients with B₁₂ deficiency (mean, 1776.32; SD, 578.06) and patients without B₁₂ deficiency (mean, 1602.52; SD, 554.48), as shown by t(155) = 1.67 (P = .09).

A t test was conducted to determine whether any difference existed in the B₁₂ levels of individuals currently using metformin versus those who were not. Patients taking metformin had statistically significant lower levels of B₁₂ (425.99 pg/mL vs 527.49 pg/mL; P = .01).

Multivariate logistic regression analysis showed that in this study sample, use of a multivitamin significantly reduced odds of B₁₂ deficiency (odds ratio, 0.31; 95% CI, 0.15–0.63). Use of H₂ blockers or proton pump inhibitors, age, and metformin use were not significantly associated with B₁₂ deficiency.

Discussion

This is the first cross-sectional study we are aware of that was specifically designed to define the prevalence of B₁₂ deficiency in patients with type 2 diabetes. In our cohort, we identified 22% of type 2 diabetic patients with B₁₂ deficiency. Although it is possible to deduce the prevalence of B₁₂ deficiency in diabetic patients from previous studies,⁷ we feel our broad inclusion criteria provided an estimate that is more generalizable to the entire type 2 diabetic population. With a prevalence of 22%, physicians will need to consider the comorbid effects of B₁₂ deficiency in a population already predisposed to neuropathic complications. More investigations are required to chart the full clinical impact of deficiency in these patients.

Despite that the clinical significance of a 22% prevalence of B₁₂ deficiency in the diabetic population is unknown, the impact B₁₂ deficiency may have as a cause of peripheral neuropathy in this population should be explored. Sixty percent to 70% of diabetic patients have mild to severe forms of nervous system damage,¹³ the most common being peripheral neuropathy. The relatively high prevalence of B₁₂ deficiency found in this study makes it likely that at least a portion of peripheral neuropathy cases in diabetic patients may be attributed to B₁₂ deficiency. Previous studies have demonstrated that supplemental vitamin B₁₂ improved somatic and autonomic symptoms of diabetic neuropathy.^23,24 Testing for, and treating, B₁₂ deficiency in those patients with neuropathy may lead to improved clinical outcomes. Clinical trials are needed to further evaluate this link.

To improve the diagnosis of B₁₂ deficiency, we measured methylmalonic acid and homocysteine blood levels. These markers of B₁₂ deficiency have been shown to improve the sensitivity and specificity for detecting B₁₂ deficiency. More importantly, these markers may discover tissue B₁₂ deficiency in an early, asymptomatic, and reversible state. Other studies have used methylmalonic acid and homocysteine levels to define the prevalence of B₁₂ deficiency in the elderly, but not in the diabetic population. These studies demonstrate a prevalence of B₁₂ deficiency in the elderly that ranges from 12% to 23%.^9–11 Although the prevalence of B₁₂ deficiency in our diabetic patients was in line with these results, it is important to note that the average age of our population was approximately 15 years younger than the average age of the elderly volunteers enrolled in these other studies. In addition, both bivariate and multivariate analyses demonstrated that age was not significantly associated with B₁₂ deficiency. This suggests that type 2 diabetes, not age, may account for the 22% prevalence of B₁₂ deficiency. Unfortunately, the ability to compare our prevalence results to the general population is limited because there are no published studies that examine the prevalence of B₁₂ deficiency in the general population using methylmalonic acid and homocysteine levels for diagnosis.

Patients on chronic metformin therapy seem to be at increased risk for B₁₂ deficiency. Its use is associated with lower serum vitamin B₁₂ levels^1–4,6 and megaloblastic anemia.¹ Several studies associate metformin use with established clinical B₁₂ deficiency.^1,5,7 In fact, higher doses and longer treatment with metformin seem to be risk factors for such deficiency.²⁵ Although we found that patients using metformin had lower B₁₂ levels, we did not find metformin use to be associated with overt B₁₂ deficiency. In addition, there were no statistically significant metformin dose-dependent relationships, despite a trend between higher dosages of metformin use and B₁₂ deficiency. Our study was not designed nor powered to find these secondary associations. As part of a post hoc analysis we did look at diabetic patients at risk for B₁₂ deficiency to determine whether there was a significant association with metformin use. Current metformin use was associated with a significantly higher risk for B₁₂ deficiency when defined as a serum B₁₂ level <350 pg/mL. Patients with B₁₂ levels <350 pg/mL may be at risk for B₁₂ deficiency becuase tissue deficiency may occur despite normal serum B₁₂ levels.^10,20–22 Identification of patients “at risk” for B₁₂ deficiency as those with serum B₁₂ <350 pg/mL may help the clinician define a level to test for B₁₂ deficiency using specific tissue markers, especially among diabetics who are using metformin.

Our multivariate analysis looked for specific associations for B₁₂ deficiency. We entered into our model known and potential risk or protective factors for B₁₂ deficiency. Multivitamin use seemed to protect diabetic patients from B₁₂ deficiency. The effect of multivitamins raising serum B₁₂ levels is documented in the literature. Randomized trials in adults taking 6 to 9 mcg of B₁₂ daily show effects of higher serum B₁₂ levels compared with placebo.^26,27 There are no known studies specifically evaluating whether a daily multivitamin prevents B₁₂ deficiency. Our findings may be noteworthy because conventional treatment of B₁₂ deficiency is with high dose oral supplementation or B₁₂ injections.²⁸ Yet, most multivitamin formulations typically contain 6 to 25 mcg of supplemental B₁₂,^29,30 which seemed to be enough to protect against B₁₂ deficiency in type 2 diabetic patients. Further research needs to be conducted before the validity of multivitamin use to prevent against B₁₂ deficiency is confirmed. Other factors known to increase risk for B₁₂ deficiency, such as advanced age and acid blocker use, were not significantly associated with B₁₂ deficiency.

There were several limitations to our study. Our cohort was a sample population from a military health care system. MAMC has been recognized as an innovator in the chronic care model of diabetes. Subsequently, most patients had well-controlled diabetes. In populations with poorer glycemic control, our study may not be generalizable because the rates of metformin use may not be similar. However, a population with poorer glycemic control is at higher risk for neuropathy and may benefit even more from early identification of concomitant B₁₂ deficiency. In addition, we only included diabetics older than 45, consistent with the age group of the majority of the type 2 diabetic population in the United States.¹³ Overall, we feel our participants represent typical primary care diabetic patients who present to an outpatient clinic for ongoing care.

The cross-sectional nature of our study limits us to describing a population. Therefore, the findings relevant to our secondary aims were limited to associations. The primary objective, however, was to define the prevalence of B₁₂ deficiency in the diabetic population, for which a cross-sectional study is appropriate. Additional studies will be needed to prove causation. In addition, defining B₁₂ deficiency based solely on biochemical markers remains controversial.^31–33 Although elevations in methylmalonic acid have been correlated with clinical manifestations of B₁₂ deficiency,^17,18 we did not evaluate for evidence of megaloblastic anemia or neuropathic disease. Therefore, the clinical significance of metabolically confirmed B₁₂ deficiency in our patient group is unknown. Finally, the B₁₂-deficient patients identified in our study were not followed for treatment effect with supplemental vitamin B₁₂ or to evaluate for normalization of B₁₂ and methylmalonic acid levels. Such follow-up would have helped confirm the diagnosis of vitamin B₁₂ deficiency.

Conclusion

Finding a prevalence of 22% raises the question of whether to screen for B₁₂ deficiency in all patients. No studies that we are aware of have looked at outcomes for treating asymptomatic B₁₂ deficiency, so the benefit of such treatment is unknown. Primary care physicians should recognize that up to one fifth of their diabetic patients may have B₁₂ deficiency; they should consider B₁₂ deficiency in the differential diagnosis when managing comorbidities of diabetes, especially neuropathy. Further studies need to be undertaken to determine whether screening and subsequent treatment can prevent peripheral neuropathy from developing in patients with type 2 diabetes before advocating for universal screening. Finally, empiric treatment with a multivitamin should also be explored as a method for reducing the incidence of B₁₂ deficiency in the type 2 diabetic population.