Bay Area Medical Information
Vitamin B12

Important note:Up to 30 percent of adults over 50 have difficulty absorbing the naturally occurring form of vitamin B12 from food sources. Synthetic vitamin B12 in a supplement or in fortified foods is better absorbed and is recommended by the Nat'l Academies of Science. The Recommended dietary allowances (RDAs) of Vitamin B12 for adults is 2.4 µg/ day. Adults age 50 and over should obtain their daily requirement of Vitamin B12, and all B Vitamins, in fortified breakfast cereals or from multivitamin-mineral supplements.(10)

What is Vitamin B12?

Vitamin B-12 is also called cobalamin because it contains the metal cobalt. It is a water-soluble vitamin that is part of the vitamin B complex. Important in growth, development, metabolism and is essential in the formation of both red blood cells and DNA. Vitamin B-12 is also necessary in the maintenance of the central nervous system.

Absorption of B12 in the gastrointestinal tract requires both hydrochloric acid and intrinsic factor to be present. Hydrochloric acid in the stomach releases vitamin B12 from proteins in foods during digestion. Once released, vitamin B12 combines with a substance called intrinsic factor. This complex can then be absorbed by the intestinal tract.

The Recommended dietary allowances (RDAs) of Vitamin B12 for adults is 2.4 µg/ day.

Significant food sources of Vitamin B12 are from animal products, thus strict vegetarians usually need to take a B12 supplement daily.

What Causes a B12 Deficiency?

A vitamin B12 deficiency can occur as a result of an inability to absorb the vitamin from food, increased vitamin B12 requirements, and strict vegetarians who do not consume any foods that come from animals.

Those who have difficulty with absorption include the following:

  1. Age > 50 - Up to 30 percent of older adults have difficulty absorbing the naturally occurring form of vitamin B12 from animal sources. For this reason, it is recommended that adults age 50 and over obtain their B12 in fortified breakfast cereals or from multivitamin-mineral supplements. Synthetic B12 as a supplement or in fortified foods are better absorbed, unless the person has Pernicious Anemia.

  2. Chronic use of certain medications may decrease absorption of vitamin B12 and may result in a need for daily oral synthetic vitamin B12 supplements or fortified foods. See below

  3. Pernicious Anemia: is a form of anemia that occurs when there is an absence of intrinsic factor, a substance normally present in the stomach. Vitamin B12 binds with intrinsic factor before it is absorbed and used by the body. An absence of intrinsic factor prevents normal absorption of B12 and may result in pernicious anemia. Pernicious anemia treatment is usually lifelong supplemental vitamin B12 given either intramuscularly, intranasally, or by mouth, depending upon the individual requirement which is determined by a physician or health care professional.

  4. Gastrointestinal Disorders: Individuals with certain stomach or small intestinal disorders may be unable to absorb enough vitamin B12 from food to maintain healthy body stores. Intestinal disorders that may result in malabsorption of vitamin B12 include: Sprue, or celiac disease, and Crohn's disease. People who have undergone certain surgical procedures in the gastrointestinal tract such as surgery to remove all or part of the stomach, often result in a loss of cells that secrete hydrochloric acid and intrinsic factor. Surgical removal of the distal ileum, a section of the intestines, also can result in the inability to absorb vitamin B12. Anyone who has had either of these surgeries usually requires lifelong vitamin B12 supplements to prevent a deficiency. All of these individuals would be under the routine care of a physician or health care provider, who would periodically evaluate vitamin B12 status and recommend appropriate treatment.

Those conditions associated with increased B12 requirements include: pregnancy, thyrotoxicosis, hemolytic anemia, hemorrhage, malignancy, liver or kidney disease.

Do Younger Adults Need to Take B12 Supplements?
Older adults are at greater risk of developing a vitamin B12 deficiency than younger adults. However, one recent study suggests that the prevalence of vitamin B12 deficiency in young adults may be greater than previously thought. This study found that the percentage of subjects in three age groups (26-49 years, 50-64 years, and 65 years and older) with deficient blood levels of vitamin B12 was similar across all age groups but that symptoms of vitamin B12 deficiency were not as apparent in younger adults. Further research is needed before specific recommendations are made about the appropriateness of vitamin B12 supplements for younger adults.(8)
Symptoms of B12 Deficiency:

Characteristic signs, symptoms, and health problems associated with vitamin B12 deficiency include: anemia, fatigue, muscle weakness, muscle spasticity, incontinence, constipation, hypotension (low blood pressure), loss of appetite, weight loss, numbness and tingling in the hands and feet, difficulty in maintaining balance, ataxia (shaky movements and unsteady gait), depression, confusion, dementia, psychoses, poor memory, mood disturbances, vision problems, and soreness of the mouth or tongue.

Researchers report that the abnormal neurologic and psychiatric symptoms may occur when vitamin B12 levels are just slightly lower than normal and are considerably above the levels normally associated with anemia.(6)

Most individuals who develop a vitamin B12 deficiency have an underlying stomach or intestinal disorder that limits the absorption of vitamin B12. Sometimes the only symptom of these intestinal disorders is subtly reduced cognitive function resulting from early vitamin B12 deficiency. Anemia and dementia follow later.

Many of these symptoms are very general and can result from a variety of medical conditions other than vitamin B12 deficiency. It is important to have a physician evaluate these symptoms so that appropriate medical care can be given.

Signs of vitamin B12 deficiency in infants include failure to thrive, movement disorders, delayed development, and megaloblastic anemia. (1)

How is B12 Deficiency Diagnosed?

If vitamin B12 deficiency anemia is suspected, your doctor or health care provider will do a physical exam, ask questions about your symptoms, and order a blood test to help make the diagnosis. The following blood tests may be ordered by your doctor:

  • Complete blood count (CBC) The CBC is used as a broad screening test to look for such disorders as anemia, infection, and a variety of other diseases as well.

  • Vitamin B12 test to measure the level of this vitamin in the blood.

  • Homocysteine test and methylmalonic acid (MMA) test. MMA primarily is ordered, sometimes along with homocysteine, to help diagnose an early or mild B12 deficiency. It may be ordered as a follow-up to a vitamin B12 test for which the result is in the lower end of the normal range. Some researchers have suggested using MMA as a screening tool, especially among the elderly, who frequently have B12 deficiencies and may have few recognizable symptoms. However, the use of MMA is still controversial in the medical community and not used frequently for this purpose.
How is B12 Deficiency Treated?
Administering vitamin B12 orally, intramuscularly, or intranasally is effective for preventing and treating dietary vitamin B12 deficiency. Sometimes an oral dose of 100-250 micrograms/day is adequate although patients with absorption difficulties may need 1000 micrograms/day of vitamin B12. (6) Over-the-counter multivitamins do not have this much vitamin B12. To get enough vitamin B12, you will need to take special vitamin B12 pills.

You can also get shots of vitamin B12. Usually, these shots are given every 1 to 2 days for about two weeks to raise your levels to normal. After this, a shot is given once every month. Your doctor can help you decide what is the best treatment for you.

Can B12 Injections Help Fatigue?

There is some evidence that intramuscular injections of 5mg of vitamin B12 given twice per week might improve the general well being and happiness of patients complaining of tiredness or fatigue. However, fatigue has many potential causes. Well designed clinical trials are needed before a recommendation can be made.

Do High Doses of B Vitamins Help in the Treatment of Cancer?
Some alternative medical practitioners claim that deficiencies in B vitamins weaken the immune system and make the body vulnerable to cancer. They recommend high doses of B vitamins as treatments for people with cancer. However, current scientific evidence has not found any effect of B vitamin supplements on the growth and spread of cancer.(9)
Can You Get Too Much of Vitamin B12?
Vitamin B12 has a very low potential for toxicity and there have been no adverse effects reported due to excess vitamin B12 intake from food or supplements in healthy individuals. (1)
Drugs that Might Interfere with Vitamin B12 Absorption (1)
Drug Potential Interaction

Proton pump inhibitors (PPIs) are used to treat gastroesophageal reflux disease (GERD) and peptic ulcer disease: Prilosec, Prevacid, Nexium, and Aciphex

PPI medications can interfere with vitamin B12 absorption from food by slowing the release of hydrochloric acid into the stomach. This is a concern because acid is needed to release vitamin B12 from food prior to absorption. So far, however, there is no evidence that these medications promote vitamin B12 deficiency, even after long-term use.

H2 receptor antagonists are used to treat peptic ulcer disease. Tagamet, Pepsid, and Zantac

H2 receptor antagonists can interfere with vitamin B12 absorption from food by slowing the release of hydrochloric acid into the stomach. This is a concern because acid is needed to release vitamin B12 from food prior to absorption. So far, however, there is no evidence that these medications promote vitamin B12 deficiency, even after long-term use.

Metformin is used to treat diabetes.

Metformin may interfere with calcium metabolism. This may indirectly reduce vitamin B12 absorption because vitamin B12 absorption requires calcium. Surveys suggest that from 10% to 30% of patients taking Metformin have evidence of reduced vitamin B12 absorption.
Selected food sources of vitamin B12 (1)
Food Micrograms (μg)
per serving
Daily Value
Mollusks, clam, mixed species, cooked, 3 ounces
Liver, beef, braised, 1 slice
Fortified breakfast cereals, (100%) fortified), ¾ cup
Trout, rainbow, wild, cooked, 3 ounces
Salmon, sockeye, cooked, 3 ounces
Trout, rainbow, farmed, cooked, 3 ounces
Beef, top sirloin, lean, choice, broiled, 3 ounces
Fast Food, Cheeseburger, regular, double patty & bun, 1 sandwich
Fast Food, Taco, 1 large
Fortified breakfast cereals (25% fortified), ¾ cup
Yogurt, plain, skim, with 13 grams protein per cup, 1 cup
Haddock, cooked, 3 ounces
Clams, breaded & fried, ¾ cup
Tuna, white, canned in water, drained solids, 3 ounces
Milk, 1 cup
Pork, cured, ham, lean only, canned, roasted, 3 ounces
Egg, whole, hard boiled, 1
American pasteurized cheese food, 1 ounces
Chicken, breast, meat only, roasted, ½ breast
  1. Vitamin B12: Vitamin Supplement Fact Sheet from the Office of Dietary Supplements, National Institute of Health
  2. Find a Nutrition Professional from the American Dietetic Association
  3. Dietary Supplements from the U.S. FDA
  4. Dietary Supplements from the National Agicultural Library
  5. Dietary Reference Intakes from the Institute of Medicine, Food and Nutrition Board
  6. Vitamin B12 from Medline Plus,the National Institute of Health and the National Library of Medicine
  7. Herbs & Supplements: Vitamin B12 from Medline Plus,the National Institute of Health and the National Library of Medicine
  8. Tucker et al. Plasma vitamin B12 concentrations relate to intake source in the Framingham Offspring Study. Am J Clin Nutr 2000;71:514-22.
  9. Vitamin B Complex from the American Cancer Society
  10. Institute of Medicine, National Academies of Science The Institute of Medicine serves as adviser to the nation to improve health.

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