Metformin, also known as Glucophage, is a drug doctors prescribe to help control blood sugars in diabetics. "The Physicians' Desk Reference" states that metformin works differently from other classes of diabetes medications. Metformin interferes with the production of glucose in your liver, improves the sensitivity of your cells to insulin and reduces the absorption of glucose from your intestine. Metformin also appears to interfere with the absorption of vitamin B-12 from your intestine.
Vitamin B-12 Absorption
Vitamin B-12 is separated from foods by the action of hydrochloric acid and digestive enzymes in your stomach. Once liberated, B-12 binds to gastric "R proteins" that protect the vitamin until it leaves the stomach's acidic environment. As it passes into the small intestine, B-12 is again freed when pancreatic secretions separate it from the R proteins. However, B-12 is immediately bound again to another gastric protein called intrinsic factor. The intrinsic factor-B-12 complex, or IF-B-12, is finally absorbed in the lower end of your small intestine. According to the Linus Pauling Institute at Oregon State University, your body requires calcium for IF-B-12 absorption. Metformin appears to reduce intestinal IF-B-12 absorption.
B-12 Deficiency
Vitamin B-12 is required for the synthesis of hemoglobin, which is the oxygen-carrying protein in your red blood cells. It also plays an essential role in the formation and repair of myelin, which helps to insulate and protect your nerve cells. B-12 deficiency leads to anemia and nerve damage, which manifests as numbness in your extremities, loss of balance, memory loss, disorientation and paranoia. "The Merck Manual of Diagnosis and Therapy" reports that the nerve damage due to B-12 deficiency can become irreversible if treatment is delayed.
Metformin Blocks B-12 Absorption
The PDR states that about 7 percent of patients taking metformin develop vitamin B-12 deficiency. However, a June 1971 "British Medical Journal" article reports that up to 30 percent of patients taking metformin for more than two years become vitamin B-12 deficient. Because your body stores and recycles vitamin B-12, it can take several years for a deficiency to develop after you stop absorbing it. Researchers believe that metformin interferes with the calcium-dependent absorption of the IF-B-12 complex in your small intestine.
Considerations
Metformin possesses several properties that make it useful in the treatment of diabetes. For example, it is less likely to cause hypoglycemia than other medications, and patients taking metformin are less likely to gain weight than those on other diabetes medications. However, individuals who take metformin for several years run a substantial risk of becoming vitamin B-12 deficient. If you have been taking metformin for longer than two years, ask your doctor if you should have your B-12 level checked or if you should take supplemental vitamin B-12.
References
- "Physicians' Desk Reference, 60th Edition: Fortamet"; Lori Murray, Senior Editor; 2006
- Linus Pauling Institute at Oregon State University: Vitamin B12
- "The Merck Manual of Diagnosis and Therapy, 18th Edition: Vitamin B12 Deficiency"; Mark H. Beers, M.D., Editor-in-Chief; 2006
- "British Medical Journal"; Vitamin-B12 Status of Patients on Long-term Metformin Therapy; G.H. Tomkin, et al.; June 1971
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