It is well known that the risks of both type 2 diabetes and B12 deficiency increase with age and that metformin has been reported to cause a decrease in serum B12, however there are no formal recommendations on the use of vitamin B12 supplements among those with type 2 diabetes that are being treated with metformin.
To describe the prevalence of biochemical B12 deficiency in adults with type 2 diabetes taking metformin compared with those not taking metformin and those without diabetes, and explore whether this relationship is modified by vitamin B12 supplements, researchers used data from the US National Health and Nutrition Examination Survey (NHANES) 1999–2006 population.
The analysis included adults over 50 years of age with (n = 1,621) or without type 2 diabetes (n = 6,867). Type 2 diabetes was defined as clinical diagnosis after age 30 without initiation of insulin therapy within one year. Those with diabetes were classified according to their current metformin use.
The primary outcome was vitamin B12 deficiency, defined as serum B12 concentrations ≤148 pmol/L, and borderline deficiency, defined as >148 to ≤221 pmol/L.
The following results were reported:
• Biochemical B12 deficiency was present in 5.8% of those with diabetes using metformin compared with 2.4% of those not using metformin (P = 0.0026) and 3.3% of those without diabetes (P = 0.0002).
• Among those with diabetes, metformin use was associated with biochemical B12 deficiency (adjusted odds ratio 2.92; 95% CI 1.26–6.78).
• Consumption of any supplement containing B12 was not associated with a reduction in the prevalence of biochemical B12 deficiency among those with diabetes, whereas consumption of any supplement containing B12 was associated with a two-thirds reduction among those without diabetes.
The authors concluded that metformin therapy is associated with a higher prevalence of biochemical B12 deficiency. The amount of vitamin B12 recommended by the Institute of Medicine (2.4 micrograms/day) and the amount available in general multivitamins (6 micrograms) may not be enough to correct this deficiency among those with diabetes.