Vitamin B12: Understanding Deficiency and Supplementation

Vitamin B12 deficiency is a common condition that can lead to serious health issues like anemia and neurological problems.

However, by consuming various B12 sources you can achieve the recommended daily intake without trouble.

This article explains the benefits of vitamin B12 and how to reach your daily needs.

What is Vitamin B12?

What is Vitamin B12?

Vitamin B12 is also known as cyanocobalamin or methylcobalamin.

It’s an essential, water-soluble nutrient we must get from food.

It’s found in various animal-based products, fortified foods like breakfast cereals and nutritional supplements.

This vitamin primarily acts as an enzyme cofactor, helping with DNA and energy production. B12 is also necessary for proper red blood cell (RBC) formation, neurological and brain function and protein and fat metabolism.

Summary: Vitamin B12 is an essential, water-soluble vitamin that helps produce DNA, energy and RBCs. It’s also necessary for brain health and metabolism.

Vitamin B12 Deficiency

Vitamin B12 Deficiency

Low B12 levels cause a range of health problems.

B12 deficiency symptoms include (2, 3):

  • Megaloblastic anemia
  • Fatigue and weakness
  • Constipation
  • Loss of appetite and weight loss.
  • Numbness and tingling in hands and feet
  • Poor balance
  • Poor memory and dementia
  • Mouth or tongue soreness
  • Depression

Deficiency is primarily due to issues of malabsorption, poor dietary intake and pernicious anemia. Rates of deficiency increase with age.


Malabsorption of B12 is prevalent in people with gastrointestinal (GI) disorders and GI surgeries.

GI disorders like celiac disease and inflammatory bowel disease (IBD) reduce the body’s ability to absorb and maintain healthy B12 stores. Those who undergo GI surgery, like removal of intestines or gastric bypass, may also have difficulty with B12 absorption (4, 5).

Atrophic gastritis, a condition that depletes stomach acid, also reduces B12 absorption. It’s estimated that 10 to 30% of older adults have this condition (6).

Poor Dietary Intake

Vegetarians, particularly those restricting milk and eggs, are at higher risk of B12 deficiency because there are limited plant-based food sources containing B12.

In this case, it’s important to incorporate fortified foods or supplements to meet your daily B12 needs. It’s even more vital for pregnant or lactating women who are vegetarian/vegan to consume enough B12, as infants are at higher risk for B12 deficiency (7).

Pernicious Anemia

Pernicious anemia, an autoimmune disease that destroys stomach cells, reduces the body’s ability to produce intrinsic factor.

Intrinsic factor is a glycoprotein produced in the stomach. It’s required for proper absorption of B12. A lack of B12 leads to reduced red blood cells.

Intramuscular B12 injections, rather than oral B12 supplements, help correct the deficiency.

Summary: B12 deficiency may be caused by malabsorption, poor dietary intake and/or pernicious anemia. Symptoms include loss of appetite, tingling in hands and feet, poor memory and depression.

Vitamin B12 Testing

Vitamin B12 Testing

Several lab markers assess Vitamin B12 status.

Typically, plasma or serum B12 levels are measured, with the ideal range for adults between 170 and 250 pg/mL.

Unfortunately, this measure doesn’t show the level of B12 within cells and has a low predictive ability (8).

Levels of homocysteine, an amino acid that can cause inflammation of blood vessels when elevated, helps predict B12 status as well. Homocysteine also shows folate and vitamin B6 levels.

Therefore, its overall value is used in conjunction with plasma levels to assess B12 status.

Urinary or plasma methylmalonic acid (MMA), a substance produced in the body that’s dependent on B12, is also helpful for detecting early stages of B12 deficiency. However, MMA levels can be elevated in those with thyroid and kidney conditions, which limits the test’s specificity (9, 10).

Holotranscobalamin (holoTC) reflects B12 levels in the body too. However, like MMA, false positive results may occur if one has kidney issues. This test should be used in conjunction with serum B12 and MMA and not as a sole measurement (11, 12).

Summary: Various tests measure B12 status, including plasma B12, homocysteine, MMA and holoTC. Because these tests don’t always show the full picture, it’s best to measure at least two parameters before diagnosing a B12 deficiency.

Benefits of Additional Vitamin B12

Benefits of Additional Vitamin B12

The body requires adequate B12 levels, but additional supplementation may not live up to the hype.

B12, along with B6 and folate, help metabolize homocysteine levels. Still, it’s not clear whether B12 supplementation can reduce the risk of heart disease directly (13).

Along those lines, many assume that B12 improves brain and cognitive function due to its ability to reduce homocysteine levels. However, three major reviews didn’t show sufficient evidence to support this claim (14, 15, 16).

Many people use supplemental B12 to improve energy and physical performance. Once again, the research doesn’t show any additional benefits in these areas either (17).

On the other hand, B12 has been shown to slow down the replication of the hepatitis C virus. When patients with hepatitis C were given standard therapy or standard therapy with a B12 injection of 5,000mcg, every 4 weeks for 48 weeks, those with the additional B12 responded better to treatment (18).

B12 has also been proven to reduce the risk of acute lymphoblastic leukemia in children when pregnant mothers supplement during the last six months of pregnancy (19).

Summary: Vitamin B12 has many alleged health benefits but the studies do not provide enough evidence for these claims. However, B12 supplementation does support those with hepatitis C and children of pregnant women.

Foods High in Vitamin B12

Naturally occurring food sources of B12 are animal-based, such as seafood, meat, poultry, eggs and dairy.

There are also foods fortified (supplemented) with B12 including breakfast cereals, nutritional yeast and infant formulas.

Below is a list of foods containing B12:

Foods High in Vitamin B12

* DV = daily value. The DV for vitamin B12 is 6.0 mcg. Image source

Adequate stomach acid and enzymes are required to free B12 from the proteins it is bound to in food.

In cases of compromised stomach functioning, B12-fortified foods and/or B12 supplements allow for better absorption of this nutrient.

Summary: Vitamin B12 is naturally occurring in a variety of animal-based foods. With gastrointestinal impairment, consuming B12-fortified foods and/or supplements may be ideal.

Vitamin B12 Supplements

Vitamin B12 Supplements

It’s vital to treat a B12 deficiency to avoid permanent nerve damage, among other problems.

B12 supplements are typically present in the cyanocobalamin form, which must be converted in the body to active-B12, methylcobalamin and 5-deoxyadenosylcobalamin. They come in multivitamin formulations, vitamin B-complexes, and also as a lone nutrient.

Supplementing with B12 alone is typically best when correcting a deficiency. This is because most foods contain a variety of B-vitamins (i.e. B1, B2, B3, B5, B6 and B7), while B12 is less common.

Most products provide 500 to 1000mcg of B12, despite the Recommended Daily Allowance (RDA) being significantly lower. This is due to wide variations in absorption rates.

Vitamin B12 Supplements 2

Image source

B12 absorption varies from person to person. This primarily stems from internal differences, like the amount of intrinsic factor released in the stomach or genetic defects (much like an MTHFR mutation).

Even changing the oral supplement to a sublingual tablet or lozenge has not been found to drastically change absorption rates (20, 21).

Intramuscular B12 injections can also be used. Some don’t prefer this route as it requires a medical professional to administer. However, in cases of severe nutrient malabsorption and pernicious anemia, injections are more efficient at correcting the deficiency.

There are also gel formulations of B12, available as a prescription medication, that is applied to the inside of the nose. While this is an effective method, it hasn’t been carefully studied in clinical trials (22).

Also note that high intake of folic acid can mask B12 deficiency and exacerbate anemia. Therefore you should limit folic acid intake from foods and supplements to 1000 mcg per day.

Many opt to take supplemental B12 alongside a folate supplement to avoid this risk (23, 24).

Summary: B12 is available in a variety of forms including oral supplements, sublingual tablets, injectable shots and gel formulas. In cases of severe deficiency or malabsorption, it’s more effective to use a B12 injection.

Can Too Much Vitamin B12 Cause Problems?

Can Too Much Vitamin B12 Cause Problems?

A high level of serum B12, known as hypercobalaminemia (>950pg/mL), doesn’t directly cause side effects, but it can indicate more serious conditions.

Chronic myelogenous leukemia, promyelocytic leukemia, polycythemia vera and hypereosinophilic syndrome have all been associated with high B12 levels (25).

Currently there is no established upper limit for B12 due to the low potential for toxicity.

Summarizing Vitamin B12

Vitamin B12 is an essential nutrient that is commonly deficient.

A B12 deficiency is typically caused by malabsorption, poor dietary intake and pernicious anemia.

Intramuscular B12 injections are ideal for those with pernicious anemia or severe malabsorption due to GI disease or surgery. Otherwise deficiency can be treated with vitamin B12 supplements, B12-fortified foods and natural B12 food sources.

And while adequate B12 levels are essential for good health, research has shown that additional intake will not provide additional health benefits for those who are otherwise healthy.

Overall, eating a balanced diet and supplementing when necessary will keep your B12 levels in check.

Vitamin B12 Understanding Deficiency and Supplementation 1


About Erin Peisach (Registered Dietitian Nutritionist)

Erin Peisach, RDN, CLT attended the University of Maryland, College Park for her Bachelor's degree in Dietetics. She is now the owner of Nutrition by Erin, a San Diego-based virtual nutrition private practice specializing in gastrointestinal disorders. 

Learn more about her on the About page