Can a multivitamin prevent vitamin B-12 deficiency caused by metformin?

This study aimed to determine the relationship between metformin (Glucophage) use in older adults with diabetes and vitamin B-12 concentrations, and assess whether B-12 supplementation can alter this. Metformin use was shown to be linked to lower B-12 levels, and multivitamin use may be protective against this. 

Metformin is a common first therapy option for patient with type 2 diabetes (T2D). It lowers blood glucose levels by increasing sensitivity of cells to insulin (hormone needed to break down glucose taken in from food) and lowers the amount of glucose produced in the liver. Long-term metformin use, however, has been linked to vitamin B-12 deficiency. Vitamin B-12 is important for production of red blood cells, DNA and for brain and nervous system function. It is unknown whether B-12 supplementation can reverse B-12 deficiency in older adults taking metformin for T2D. 

2,510 adults over 50 years old were assessed in this study. Participants were divided into those with diabetes taking metformin (212 participants), those with diabetes not taking metformin (331 participants) and those without diabetes (1967 participants). 1431 of the total participants reported multivitamin use.

Participants taking a multivitamin had significantly higher B-12 levels. Multivitamin users were less likely to have diabetes or be taking metformin. 59% of participants without diabetes were taking a multivitamin compared to 52% of non-metformin using diabetics and 47% of diabetes patients using metformin.

Diabetic metformin users had significantly lower B-12 levels compared with diabetic participants not taking metformin and compared to participants without diabetes.

Increased vitamin B-12 levels were seen in all groups with addition of a B-12 containing multivitamin. The largest increase in B-12 concentrations (50%) was seen in the diabetic metformin group. A 41% increase in B-12 was seen with multivitamin use in diabetic patients not taking metformin. A 34% increase was seen in those without diabetes.

Participants taking metformin were 86% less likely to have low or borderline B-12 levels if they were also taking a B-12 containing multivitamin.

This study concluded that metformin use was associated with lower B-12 levels. The authors suggested that B-12 containing multivitamin use can protect against B-12 deficiency in metformin users with diabetes. 

Measuring B-12 is challenging and may result in over- or under-diagnosis of B-12 deficiency.