Get to Know Vitamin B12

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Get to Know Vitamin B12

Vitamin B12 is one of the most widely known B vitamins. This vitamin is particularly important for people with diabetes to be aware of, and especially those who take the diabetes medication called metformin. Curious? Read on!

What is vitamin B12?

Vitamin B12 is also called cobalamin, and that’s because it contains the mineral cobalt. Like its cousins, vitamin B12 is water soluble, and the liver can store this vitamin for years. Any excess vitamin B12 is excreted in the urine.

This vitamin is bound to protein in food, and it must be released before it can be absorbed, says the Office of Dietary Supplements (ODS) of the National Institutes of Health (NIH). The absorption process starts in the mouth when food is mixed with saliva. Vitamin B12 then binds to a protein in the saliva, and more vitamin B12 is released from food with the help of hydrochloric acid and enzymes in the stomach. But the process doesn’t end there. Once food enters the small intestine, vitamin B12 combines with a protein called intrinsic factor, which allows this vitamin to be absorbed further down in the small intestine. Vitamin B12 that has been added to foods and dietary supplements is already in a free form and doesn’t require this complex separation process.

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What does vitamin B12 do?

Vitamin B12 plays a number of roles in the body, including:

  • Protein metabolism
  • Formation of red blood cells
  • DNA synthesis
  • Promoting the development and health of the central nervous system

Vitamin B12, along with other B vitamins, helps to break down a protein called homocysteine. High levels of homocysteine are linked with a higher risk of heart disease. However, studies have not been able to establish a link between vitamin B12 supplementation and a lower risk of heart disease.

High homocysteine levels may be linked to dementia and Alzheimer’s disease; unfortunately, studies looking at the use of B12 and folate supplements have not shown any significant benefit compared with a placebo (inactive treatment).

What foods have vitamin B12?

Vitamin B12 is found in a variety of animal foods; little of this vitamin in is found in plant foods, unless the food is fortified with this vitamin. The body absorbs vitamin B12 from animal foods much better than plant sources, according to MedlinePlus. Good food sources of vitamin B12 are:

  • Beef liver
  • Fish and shellfish
  • Red meat
  • Poultry
  • Eggs
  • Milk and yogurt
  • Cheese
  • Fortified cereals
  • Fortified nutritional yeast
  • Enriched plant-based milks

Vitamin B12 is found in multivitamin/mineral supplements, B complex supplements, and supplements that contain only vitamin B12. The form of vitamin B12 in most supplements is cyanocobalamin, but other forms include adenosylcobalamin, methylcobalamin, and hydroxycobalamin. Supplements are available in tablet, capsule, soft gel, liquid, and nasal forms. Vitamin B12 is often given as an intramuscular injection in cases of a deficiency.

How much vitamin B12 do you need?

The Recommended Daily Allowance for vitamin B12 is:

  • For men 19 years and older: 2.4 micrograms (mcg)
  • For women 19 years and older: 2.4 mcg
  • For pregnancy: 2.6 mcg
  • For lactation: 2.8 mcg

What are signs of vitamin B12 deficiency?

Most people in the U.S. get enough vitamin B12 from food sources. However, insufficient B12 intake is more common in older people, women, African Americans, and people of lower socioeconomic status. There are various causes of a vitamin B12 deficiency. These may include:

  • Difficulty absorbing vitamin B12 from food: This can result from a decrease in stomach acid, which is needed to free the vitamin from food. Older people are at risk for a deficiency for this reason.
  • Lack of intrinsic factor due to pernicious anemia: Pernicious anemia is an autoimmune condition that attacks and possibly destroys cells in the gut that make intrinsic factor.
  • Gastrointestinal disorders: People with stomach or intestinal disorders, such as Crohn’s disease and celiac disease, may not be able to absorb enough vitamin B12 from food.
  • Gastrointestinal surgery: Surgery for weight loss or to remove all or part of the stomach can cause a loss of cells that produce intrinsic factor or hydrochloric acid.
  • Long-term use of some medications: People who regularly take medications for gastroesophageal reflux disease (GERD) or for stomach ulcers, such as proton-pump inhibitors, H2 blockers, or some antacids, may have trouble absorbing vitamin B12 from food. People who take metformin, a common drug used to help manage diabetes, may be at risk for a deficiency because this drug reduces the absorption of vitamin B12. Also, taking a vitamin B12 supplement with a vitamin C supplement can reduce the amount of vitamin B12 in the body. It’s best to take the vitamin C supplement two or more hours after taking a vitamin B12 supplement.
  • Dietary deficiency: People who avoid eating animal products (e.g., vegans) may not consume enough vitamin B12 from food, since this vitamin is primarily found in animal foods.

Certain congenital conditions may also cause a severe vitamin B12 deficiency.

What are signs of vitamin B12 deficiency?

  • Megaloblastic anemia, in which red blood cells are larger than normal and smaller in amount
  • Pernicious anemia, a type of anemia caused by lack of intrinsic factor
  • Numbness and tingling in the hands and feet
  • Dementia
  • Depression
  • Seizures
  • Fatigue
  • Sore mouth or tongue
  • Palpitations
  • Pale skin
  • Weight loss
  • Infertility

Some of these side effects, particularly nervous system side effects and psychiatric problems, can be irreversible if not treated promptly.

Vitamin B12 deficiency is typically treated with intramuscular injections of the vitamin, but high doses of oral vitamin B12 may be effective, as well. Vitamin B12 injections may be given once a week for a few weeks, and then once a month or every few months. People with pernicious anemia must receive B12 injections for life.

What are signs of vitamin B12 toxicity?

There are no known toxic effects of vitamin B12, even when taken at large doses. In general, up to 1,000 mcg of vitamin B12, taken orally, is considered to be safe. However, it’s always best to check with your health care provider before taking a high-dose supplement of any type of vitamin or mineral.

Other things to know about vitamin B12

Vitamin B12 is often promoted as a way to boost energy levels and mood. If you are deficient in B12, you may feel more energetic or feel better, mentally, after the deficiency is treated. But popping a B12 tablet or capsule is not otherwise going to give you more energy or help you feel happier.

If you take metformin, a popular and effective type of diabetes medication used in the treatment of type 2 diabetes, as well as prediabetes and PCOS (polycystic ovary syndrome), keep in mind that taking this drug long term could lead to a vitamin B12 deficiency. Ask your provider to check your blood level of vitamin B12 once a year. Your provider can also check methylmalonic acid (MMA) levels in your blood or urine. High levels can indicate a B12 deficiency.

Also, if you experience any of the deficiency symptoms listed above, let your provider know. They can check for a B12 deficiency, as well as other possible causes, and then prescribe appropriate treatment. In the meantime, focus on getting enough vitamin B12 from food sources and talk with your provider or dietitian about taking a supplement if you think you’re not getting enough.

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Amy Campbell, MS, RD, LDN, CDCES

Amy Campbell, MS, RD, LDN, CDCES

Amy Campbell, MS, RD, LDN, CDCES on social media

A Registered Dietitian and Certified Diabetes Educator at Good Measures, LLC, where she is a CDE manager for a virtual diabetes program. Campbell is the author of Staying Healthy with Diabetes: Nutrition & Meal Planning, a co-author of 16 Myths of a Diabetic Diet, and has written for  publications including Diabetes Self-Management, Diabetes Spectrum, Clinical Diabetes, the Diabetes Research & Wellness Foundation’s newsletter,, and

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