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Vitamin B12 101

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Vitamin B12 101

Vitamin B12 is an essential B vitamin for human health, although 3-43% of the adult population is deficient in it. This article will discuss vitamin B12's role in the body, deficiency symptoms, how to test B12 levels, and sources of B12.


What is Vitamin B12?

Cobalamin, the other name for vitamin B12, is a water-soluble vitamin required for the human body. There are many types of cobalamin, with methylcobalamin and 5-deoxyadenosylcobalamin being the active forms. Hydroxocobalamin and cyanocobalamin are other non-active forms that must convert into methylcobalamin or 5-deoxyadenosylcobalamin in order to be utilized.

Vitamin B12’s Role in The Body

B12 digestion begins with digestive enzymes in the mouth. It's further broken down in the stomach, and then it moves to the small intestine, where it will pair with a protein produced in the stomach, called intrinsic factor, before being absorbed.

B12 is a necessary vitamin for the formation of DNA and red blood cells. It is a critical vitamin for the central nervous system (CNS), as it can affect the development of the CNS as well as the myelination of the nerves. Myelin is a sheath wrapped around nerve cells, allowing them to communicate with other cells at a rapid, and thus efficient, rate.

B12 is required for two enzymes, methionine synthase and L-methylmalonyl-CoA mutase. Methionine synthase activation converts homocysteine into methionine. Homocysteine, an amino acid, can cause blood clots, stroke, and heart attacks when elevated. Methionine is required to create S-adenosylmethionine (SAM-e), which is required for over 100 compounds, including DNA, RNA, proteins, lipids, and neurotransmitters.

Vitamin B12 Deficiency Symptoms

B12 deficiency can be due to congenital conditions, including hereditary intrinsic factor defects and Imerslund-Gräsbeck disease, characterized by congenital B12 malabsorption. Other causes of B12 deficiency include surgery in the gastrointestinal tract, difficulty absorbing B12 from food, pernicious anemia, atrophic gastritis, Helicobacter pylori infection, Celiac disease, Crohn’s disease, and chronic use of certain medications, including the diabetic medication metformin, and proton pump inhibitors which are medications used in conditions such as gastroesophageal reflux disease (GERD). People with little or no animal product consumption are at an increased risk for B12 deficiency, too.

B12 deficiency often results in B12 deficiency anemia, also called megaloblastic anemia. Symptoms of B12 deficiency anemia include:

  • Cold hands and feet
  • Fatigue
  • Headaches
  • Pale skin
  • Muscle weakness
  • Shortness of breath
  • Decreased exercise tolerance
  • Irregular heartbeat
  • Dizziness
  • Atrophic glossitis (painful, smooth, glossy red tongue)

Other signs and symptoms of having a B12 deficiency include infertility, weight loss, dementia, and pale skin.

B12 deficiency during pregnancy can result in neural tube defects, failure to thrive, developmental delays, and anemia.

How to Test Vitamin B12 Levels

B12 levels can be directly detected in the blood. Other markers, including methylmalonic acid (MMA) and homocysteine, may also be used to gauge the levels of B12 in the body.

B12 Test

B12 can be checked in the blood, such as the vitamin B12 test by Access Medical Labs. B12 levels should be 160 to 950 picograms per milliliter (pg/mL).


Methylmalonic acid (MMA) is a B12-associated metabolite. High levels of MMA above 0.271 micromoles per liter (mcmol/L), as indicated on tests such as the MMA test by Vibrant America, indicate a B12 deficiency.


Levels of homocysteine can be measured in the blood, such as through the Homocysteine test by Great Plains Laboratory. As discussed above, B12 is required for the conversion of homocysteine into methionine. Thus, high levels of homocysteine may indicate a B12 deficiency. Homocysteine levels should be between 4-14 μmol/L.


What Foods Have Vitamin B12?

The Food and Nutrition Board, created by the NASEM (National Academies of Sciences, Engineering, & Medicine), created Dietary Reference Intakes (DRIs) for recommending vitamins and minerals. One DRI is the Recommended Dietary Allowance or RDA. It's defined as the average amount of a vitamin or mineral required to reach nutritional adequacy in most healthy people.

Pregnant and lactating women have different nutritional needs; thus, RDAs are often different from non-pregnant or lactating women. The RDA for pregnant women 14+ years is 2.6 mcg. The RDA for lactating women 14+ years is 2.8 mcg.

When there is not enough evidence to formulate an RDA, an Adequate Intake (AI) level is designated. AI is the level assumed to be adequate to reach nutritional requirements. The AI for birth to six-month-old males and females is 0.4 mcg, and for seven to 12-month-old males and females is 0.5 mcg.

Sources of B12 include food, supplements, injections, and nasal sprays.

B12 Supplements

B12 is available in oral supplement form, through intramuscular injection (IM), and via nasal spray. In supplements, B12 can be found singularly, in multivitamins, or as a part of B complex vitamins. B12 levels in multivitamins usually range from 5-25 mcg. B12 levels are often higher in stand-alone supplements and B complex vitamins. The forms of B12 found in supplements are cyanocobalamin, hydroxocobalamin, adenosylcobalamin, and methylcobalamin. As discussed previously, methylcobalamin and adenosylcobalamin are the most active forms of B12. Supplement administration may be in the form of capsules, lozenges, or tablets. For injections, B12 is usually in the form of cyanocobalamin and hydroxocobalamin. Prescriptive B12 nasal sprays also exist.



Vitamin B12 is an essential B vitamin required for the central nervous system, red blood cells, and DNA formation. B12 is primarily found in animal sources, and thus people with low animal product intake should be aware of B12 deficiency symptoms such as fatigue, dizziness, muscle weakness, and more. Testing B12 levels along with MMA and homocysteine can help providers understand B12 status in the body. Once the cause and level of B12 are discovered, a treatment plan that may include food sources, supplements, injections, or nasal sprays can be composed.

The information provided is not intended to be a substitute for professional medical advice. Always consult with your doctor or other qualified healthcare provider before taking any dietary supplement or making any changes to your diet or exercise routine.
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