What is it?
Vitamin B12 is a B vitamin that is essential to good health. A major reason that vitamin B12 is so important is that it is needed to make red blood cells. If you have too little of this vitamin, you may be unable to make enough red blood cells, a condition called Vitamin B12 deficiency anemia. And because red blood cells carry oxygen and nutrients throughout the body, anemia can make a person feel terrible.
In addition, vitamin B12 is required for normal nerve function in the brain, spinal cord, and extremities. So, deficiency of this vitamin may also cause problems with sensation, muscle strength and thinking.
Because the human body cannot produce its own vitamin B12, we must consume it via our food or supplements.
Causes of vitamin B12 deficiency
There are two main reasons that your vitamin B12 level might be low: too little intake or an inability to absorb the vitamin.
Too little intake of vitamin B12 may be due to:a diet low in animal products such as meat, poultry, eggs, or other dairy products. For example, people who are malnourished or following a strict vegan or certain vegetarian diet may be at risk.
Pregnant women who have a diet low in animal products should be particularly careful to get enough vitamin B12 since the baby needs this vitamin to develop normally.
Poor absorption of vitamin B12 may be due to:
- bowel disease, such as Crohn disease, celiac disease or a parasitic infection
- prior surgery that removed portions of the stomach or small intestine
- chronic use of medications for ulcers or heartburn such as antacids
- lack of a protein called "intrinsic factor" — This protein is needed to absorb vitamin B12 from the diet; an autoimmune condition (called pernicious anemia) develops when the immune system mistakenly attacks the cells that produce intrinsic factor.
Symptoms and signs of vitamin B12 deficiency
- lack of appetite
- problems with concentration
- changes in bowel habits (including constipation or diarrhea) — CHECK
- shortness of breath or an inability to exercise
- pallor (pale skin)
- tingling or numbness in the hands and feet
- poor balance
- confusion or dementia.
However, if the deficiency developed recently or is mild, you may have no symptoms at all. In such cases, the diagnosis may be suspected when a blood test is done for other reasons and a certain type of anemia (called megaloblastic anemia) is identified.
How is vitamin B12 deficiency diagnosed?
The first steps include a thorough review of your symptoms and a physical examination looking for the symptoms and signs described above. A particularly important part of the examination is checking the function of the nervous system; for example, your doctor will check reflexes, strength, and sensation in the extremities as well as memory and thinking ability.
The diagnosis can be confirmed with blood tests, including a blood count to identify the typical type of anemia and a Vitamin B12 level. A blood methylmalonic acid (MMA) level may also be measured because it tends to rise as B12 levels become low. Depending on the situation, additional tests of the stomach or intestines may be performed. For example, your doctor may recommend an esophago-gastroduodenoscopy (EGD) or colonoscopy to see whether there is evidence of intestinal disease contributing to the problem.
Vitamin B12 deficiency treatment
Although vitamin B12 deficiency treatment varies depending on the cause, the goal is to increase the blood level of vitamin B12. This can be done by changing the diet, discontinuing medications (with your doctor's approval) that can impair B12 absorption, moderating alcohol use, eliminating gluten from the diet (if you have celiac disease), receiving treatment for Crohn disease, adding oral supplements or having regular injections of vitamin B12. Many people require more than one of these approaches.
While the anemia can be readily and reliably treated by increasing the vitamin B12 level, nerve damage may be permanent.
Most healthy people who have a well-balanced diet do not need to worry much about vitamin B12 deficiency. However, for those at risk (such as those with celiac disease), measures of blood vitamin B12 levels and, in some cases, preventive therapy with vitamin B12 may be recommended. Permanent nerve damage can be prevented with early diagnosis and treatment. Checking labels of foods for vitamin B12 levels can be helpful if you've had low or borderline levels in the past.
The bottom line
Before the discovery and medicinal use of vitamin B12 in the 1930s and 1940s, deficiency could be fatal. Fortunately, as our understanding of this remarkable vitamin has increased, debilitating or deadly vitamin B12 deficiency is relatively rare today. If you have symptoms of vitamin B12 deficiency, see your doctor. It could turn out be something else, but it's worth getting checked out, especially since delayed diagnosis can lead to permanent complications. You can find a list of foods rich in vitamin B12 here.
Medications that can interfere with B12 absorption
Average daily recommendation
Birth to 6 months
Infants 7–12 months
Children 1–3 years
Children 4–8 years
Children 9–13 years
Teens 14–18 years
Breastfeeding teens and women
Robert H. Shmerling, MD, is associate professor of medicine at Harvard Medical School and Clinical Chief of Rheumatology at Beth Israel Deaconess Medical Center in Boston where he teaches in the Internal Medicine Residency Program. He is also the program director of the Rheumatology Fellowship. He has been a practicing rheumatologist for over 25 years.
To learn more about the evidence behind the benefits and safety profiles of various vitamins and minerals, buy the Harvard Special Health Report Making Sense of Vitamins and Minerals: Choosing the foods and nutrients you need to stay healthy.
About the Author
Robert H. Shmerling, MD, Senior Faculty Editor, Harvard Health Publishing; Editorial Advisory Board Member, Harvard Health Publishing
As a service to our readers, Harvard Health Publishing provides access to our library of archived content. Please note the date of last review or update on all articles.
No content on this site, regardless of date, should ever be used as a substitute for direct medical advice from your doctor or other qualified clinician.