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What Is Vitamin B12 and Why Is It Essential to Your Health?

Written by Dr. Group, DC Founder
An individual running through the field. Your body requires vitamin B12 to function properly.

Vitamin B12 is an essential vitamin, which means the human body cannot make it on its own and must obtain it through food, supplementation, or, in some cases, prescription medication. Known primarily as the “energy vitamin,” this vitally important nutrient plays a role in numerous bodily functions and is a cofactor and catalyst “helper” compound in certain enzymatic processes. Aside from those functions, every cell in your body needs vitamin B12.

What Is Vitamin B12?

Vitamin B12 is one of the eight B vitamins collectively referred to as the B complex. Because B12 contains the element cobalt in its molecular structure, it is also called cobalamin. Most people get this water-soluble vitamin from animal products (i.e., eggs, milk, fish, and shellfish). In contrast to most other nutrients, it is not readily available in many plants or fungi.

Vitamin B12 is actually synthesized in nature by bacteria and single-celled microbes, the only living organisms that possess the enzymes needed for its synthesis. Since the human body cannot manufacture B12 yet it’s an essential nutrient,[1, 2] you must get it from an outside source. This includes through diet, it in supplement form, or, under certain conditions, as a prescription medication. Vitamin B12 is the most complex in chemical structure of all vitamins.[3, 4]

The Four Forms of Vitamin B12

There are four forms of vitamin B12.

1. Hydroxocobalamin

Hydroxocobalamin, also called vitamin B12a, is produced naturally by bacteria and is the form found in most food sources. This type is often used in B12 shots. Hydroxocobalamin easily converts to methylcobalamin in the body.

2. Methylcobalamin

Methylcobalamin is the most bioavailable form in the human body, meaning it is the one that is used readily by the body. Methylcobalamin easily crosses the blood-brain barrier to protect the brain and nerve cells. It also helps convert homocysteine to methionine, which is important because high homocysteine levels are linked to a variety of negative health conditions.

3. Cyanocobalamin

Cyanocobalamin is a form of B12 that’s lab-synthesized via bacterial fermentation, which makes it the cheapest option to use in supplements, but it's not the best. It actually contains a cyanide molecule, which is necessary for its stability.

4. Adenosylcobalamin

Adenosylcobalamin (5-deoxyadenosylcobalamin) is a naturally occurring but unstable form of B12. It is not stable in tablet form, but some premier supplement manufacturers have been able to offer it in a stable, liquid formula.

How Is Vitamin B12 Absorbed?

When you consume B12 with food, it is bound to proteins, but is “freed” in the stomach by specialized cells that release gastric acid (When B12 is obtained through supplementation, this process isn’t necessary because the vitamin is already in free form.) After being freed, B12 binds R protein, a compound which transports B12 from the stomach into the duodenum, the first section of the small intestine. In the duodenum, cobalamin leaves R factor and binds with intrinsic factor, a glycoprotein produced by the stomach and salivary glands.[1, 2]

Once bound to intrinsic factor, B12 is absorbed by the ileum, which is the last part of the small intestine. It then enters blood circulation as a transcobalamin complex – B12 bound to a protein – which delivers B12 to the cells that need it. Because B12 requires intrinsic factor to work, the amount your body can absorb from food or supplements is limited to the amount of intrinsic factor your body can produce. Unused, excess B12 is eliminated in urine or feces.

What Does Vitamin B12 Do for the Body?

Vitamin B12 supports many important functions in the body. The body uses B12 for cell metabolism and cell division. B12 helps cells produce hemoglobin, an iron-containing protein that transports oxygen throughout the body in red blood cells.[4] In the form of methylcobalamin, B12 helps the body synthesize protein, DNA, RNA, lipids, and hormones.[2, 4]

  • Boosts energy and metabolism
  • Promotes heart and cardiovascular health
  • Ensures healthy brain and nervous system function
  • Normalizes mood
  • Promotes normal bone growth and development
  • Assists in DNA production
  • Helps cells divide and grow normally

Vitamin B12, Energy, & Metabolism

People often take B12 to help boost energy because it’s a coenzyme that is used during the Krebs cycle, which is the process by which the body produces energy. Vitamin B12 helps break down carbohydrates, particularly glucose sugars. A lack of B12 can lead to high blood sugar.[5] B12 also helps the body process fat, and a B12 deficiency can lead to an unhealthy lack of fat.[5]

Vitamin B12 & the Brain

Vitamin B12 is important for brain health, including cognitive function and memory. A white, fatty substance called myelin protects brain cells from free radicals and other toxins. A myelin sheath composed of lipids and proteins wraps around a nerve cell much like insulation on a hot water pipe. Insufficient vitamin B12 leads to impaired brain and nerve functioning due to decreased production and maintenance of myelin. In other words, holes in this protective insulation may cause nerves to “leak” electrical current, deteriorating brain function.[4]

Vitamin B12 & Cognitive Health

Research shows that patients with impaired cognitive function and memory loss are often deficient in B12.[2, 6, 7] Scientists think that a B12 deficiency might reduce the production of certain neurotransmitters, which are chemicals that transmit information throughout the nervous system. Although vitamin B12 is crucial to the nervous system, scientists do not have evidence that B12 supplementation is effective after cognitive decline occurs. This point underscores the value of ensuring sufficient intake throughout life.

Vitamin B12 & Mood

Research shows that people who suffer from unhappy moods are sometimes deficient in vitamin B12.[7, 8] This may be due, in part, to a reduction in the myelin sheaths from insufficient B12. Although it’s unclear of the effect vitamin B12 has on major depression, evidence does suggest that B12 supplementation may normalize mood.[7, 8] Vitamin B12 is necessary for the body to produce and release mood-regulating neurotransmitters, namely dopamine, serotonin, and melatonin. Other mood-related disorders are linked to B12 deficiency as well.[5]

Vitamin B12 & Cardiovascular Health

Vitamin B12 is important for heart health; deficiency is linked with heart conditions in some cases.[9] Low B12 is also linked to a condition known as macrocytosis, which is when red blood cells are larger than normal.[9, 10] With macrocytosis, hemoglobin — the molecule that carries oxygen — decreases. With less hemoglobin, less oxygen is delivered to tissues and organs, which creates a form of anemia. B12 deficiency-induced macrocytosis is associated with poor circulation and a higher risk of heart disease and stroke.[9]

Vitamin B12 & Severe Illness

A lack of vitamin B12 can compromise gene expression, and change cellular chromatin, the “recipe” of DNA, RNA (ribonucleic acid), and proteins that make up chromosomes. These changes are commonly observed in unhealthy cells that grow both abnormally and too quickly.[11, 12] Combined with vitamin B6 and B9 (folate), vitamin B12 appears to reduce the risk of uncontrolled unhealthy cell proliferation.[11, 13]

Vitamin B12 & Bone Health

Elevated homocysteine — which occurs when B12 levels are too low — interferes with bone health. Specifically, elevated homocysteine affects bone density and cell formation, increasing the risk of bone fracture in aging individuals. Study results as to whether vitamin B12 supplementation leads to improved bone strength and decreased risk of fracture are mixed. However, since high homocysteine levels indicate vitamin B12 deficiency, medical scientists have recommended that the B12 status in elderly populations should be evaluated periodically as a means to monitor bone health.[12]

How Much Vitamin B12 Do You Need?

Age RDA Pregnant Lactating
1-3 years 0.9 mcg
4-8 years 1.2 mcg
9-13 years 1.8 mcg
14+ years 2.4 mcg 2.6 mcg 2.8 mcg

The Institute of Medicine's Food and Nutrition Board — the scientific health body sanctioned by the U.S. government on matters of nutrient intake — set daily dose recommendations of vitamin B12 for normal adults at 2.4 mcg/day, with 2.6 mcg/day for pregnant and 2.8 mcg/day for lactating women.[2]

Keep in mind these numbers are absolute minimums. Research suggests that the average vitamin B12 intake among the U.S. population is about 3.4 mcg/day, clearly above the adequate recommended daily intake.[3] Regardless, due to digestion and absorption issues, health status, and prescription medication interference, some data suggests a sizable proportion of the population is B12 deficient.

The typical human body can absorb up to 1.5 mcg of vitamin B12 from food, but supplementation has been shown to allow for higher absorption rates by bypassing the digestive process. Although some will tell you that humans store between 2 to 5 mg of vitamin B12 (mostly in the liver), which can last up to five years in the absence of daily intake, I wouldn’t advise taking that chance.[4]

Are There Side Effects From Taking B12?

Vitamin B12 is generally safe and has few, if any, side effects. The biological half-life is estimated at 6 days, meaning it will clear the body after that time. Because it is a water-soluble vitamin, the body eliminates what it can't use or absorb. Some people experience minor side effects from B12 shots, such as pain at the injection site, as well as headaches, nausea, diarrhea, and dizziness.

How Can You Get B12?

You can get B12 from various types of foods, or you can obtain it as a nutritional supplement. It comes in tablet form, sublingual form (a tablet that dissolves under the tongue), liquid, and as injections. Injections of either hydroxocobalamin or cyanocobalamin are most common. Although injections carry more side effects than tablet or liquid supplements, they’re helpful in cases of more serious deficiency and some people rely on them for a periodic energy boost.

Foods With Vitamin B12

The following represents the foods with the highest vitamin B12 content per serving.[2, 12]

Foods Serving
Clams, steamed, 3 oz. 84.1 mcg
Liver (beef), cooked, 3 oz. 70.7 mcg
Mussels, steamed, 3 oz. 20.4 mcg
Mackerel, cooked, 3 oz. 16.1 mcg
Crab (Alaska king), cooked, 3 oz. 9.8 mcg
Beef (plate steak), cooked, 3 oz. 6.9 mcg
Trout (wild rainbow), cooked, 3 oz. 5.4 mcg
Salmon, (sockeye), cooked, 3 oz. 4.8 mcg
Tuna (light, packed in water), 3 oz. 2.5 mcg

Vitamin B12 Supplements

Many people have difficulty getting vitamin B12 in their diet. Older populations and those with certain health conditions may not get enough B12 due to malabsorption issues. If you fall into one of these groups, you may want to consider a daily supplement.

If you need a B12 supplement, I recommend a blend of methylcobalamin, adenosylcobalamin, and hydroxocobalamin, like our B12. Our Vitamin B12 also contains Energized Trace Minerals™, which gives it amplified bioavailability. It's great for athletes and people who need daily or advanced nutritional support.

What Is Vitamin B12 Deficiency?

A B12 deficiency can arise for reasons as simple as eating a vegetarian diet — since plant foods do not contain B12 — or as complex as an autoimmune disease that prevents B12 absorption. Long-term use of acid-inhibiting medications can reduce levels of intrinsic factor, which results in inhibited B12 absorption.

When the body doesn’t have enough vitamin B12, two chemical processes occur in the body. The first is that homocysteine levels increase, and the second is that methylmalonic acid (MMA) increases. High levels of these substances indicate B12 deficiency and are often measured in lab tests.

What Are the Symptoms of Vitamin B12 Deficiency?

Symptoms of vitamin B12 deficiency vary from person to person and depend on co-existing conditions. They include:

  • Confusion, disorientation, brain fog
  • Memory loss, dementia, or cognitive decline
  • Paresthesia (tingling in the limbs)
  • Peripheral neuropathy (loss of feeling in limbs)
  • Loss of balance
  • Excessive fatigue
  • Bowel/urinary tract incontinence
  • Tongue soreness
  • Appetite loss
  • Constipation
  • Diarrhea
  • Learning or developmental disorders in kids
  • Weak immune system
  • Brittle, flaky nails
  • Dry skin
  • Low red blood cell count (anemia)

Anemia is directly linked to B12 deficiency in certain individuals. There are two types associated with insufficient vitamin B12 — pernicious anemia and megaloblastic anemia.

What Is Megaloblastic Anemia?

Normally, bone marrow produces healthy red blood cells that circulate throughout the body and carry oxygen to organs and tissues. With megaloblastic anemia, the marrow makes megaloblasts, which are oversized, immature red blood cells that lack enough hemoglobin, the molecule that carries oxygen in red blood cells. This happens because there isn’t enough vitamin B12 to convert homocysteine to tetrahydrofolate (THF), or folate, which is crucial for DNA synthesis and cell division. Even with folate supplementation, megaloblastic anemia will persist until the vitamin B12 deficiency is solved.[12, 14]

What Is Pernicious Anemia?

Pernicious anemia is a form of megaloblastic anemia in which the body does not produce intrinsic factor. Pernicious anemia is the end-stage of an autoimmune disorder called atrophic gastritis, wherein the body unleashes antibodies that destroy stomach cells that produce intrinsic factor.[12] Remember, intrinsic factor helps carry the B12 you consume in food or supplements past the acidic environment of the stomach and delivers it to the intestines. Without intrinsic factor, B12 does not get delivered throughout the body, which means the body cannot manufacture enough healthy red blood cells.

Pernicious anemia can take years to develop, and people often mistakenly attribute its symptoms to aging. It’s a condition that often co-occurs with other autoimmune disorders, such as diabetes type I, psoriasis, multiple sclerosis, and thyroid issues.[12, 15] In addition to fatigue, early symptoms of pernicious anemia include brain fog, flaky skin, weakness, and unexplained weight loss, and symptoms may graduate to impaired coordination, neuropathy, depression and behavioral changes as time goes on.

Risk Factors for Vitamin B12 Deficiency

There are several risk factors that may put a person at risk for vitamin B12 deficiency.

Vegetarian Diet

Because animal products provide the only dietary source of vitamin B12, people who adhere to a plant-based or total vegan diet have a higher risk for vitamin B12 deficiency. Experts recommend vegetarians consume vitamin-fortified foods like breakfast cereal, plant-based milk, and certain soy and nutritional yeast products, as well as a vitamin B12 supplement.[2] Recent research shows that certain plant-based foods, such as fermented beans, and edible mushrooms and algae contain significant amounts of vitamin B12.[16]

Genetic Predisposition

Several inherited disorders result in vitamin B12 malabsorption, such as being born with a mutation of the MTHFR gene, which would otherwise allow the body to use vitamin B9 (folic acid). Other congenital disorders known to cause B12 deficiency include Imerslund-Gräsbeck syndrome and congenital pernicious anemia, also known as hereditary IF (intrinsic factor) deficiency.[12]


People over the age of 50 produce less hydrochloric acid in the stomach, which can lead to a B12 deficiency. Sometimes this is due to the use of proton pump inhibitors prescribed to treat peptic ulcers and gastroesophageal reflux disease (GERD).

Gastrointestinal Disease

People who have Crohn’s disease or celiac disease may also be unable to properly absorb vitamin B12. The surgical removal of any part of the stomach or small intestine, including the distal ileum, also interferes with vitamin B12 absorption.[1]


Several medications inhibit or reduce B12 absorption, most notably the diabetes medication metformin, proton pump inhibitors (i.e., omeprazole and lansoprazole) used to treat GERD and Zollinger-Ellison syndrome, and histamine H2 receptor antagonists (cimetidine, famotidine and ranitidine) used to treat peptic ulcers.[2, 12]

Are You Vitamin B12 Sufficient or Deficient?

Physiological (genetics, disease) and environmental (medications for diabetes, etc.) factors can interfere with normal B12 absorption for many people, leading to depletion or outright deficiency.

Blood level testing is the surest diagnostic method to assess if you are B12 deficient. Here is a general guideline to follow to determine whether or not you are vitamin B12 deficient:

Level of vitamin B12 in Blood Severity
450+ ng/L Healthy/optimal
180-400 ng/L Conditionally low levels
150-180 ng/L Low levels where disease symptoms start

Depending on test results and potential concurrent symptoms, other simple follow-up blood tests which indirectly test for B12 status may be suggested, including homocysteine and methylmalonic acid/MMA (levels of these molecules rise when vitamin B12 deficiency-symptoms unfold, testing of which often allows for a more thorough assessment, in conjunction with B12 testing).

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References (16)
  1. O’Leary F, Samman S. "Vitamin B12 in Health and Disease." Nutrients. 2010;2(3),299-316.
  2. "Vitamin B-12: Fact Sheet for Professionals." National Institutes of Health Office of Dietary Supplements. 2 Mar. 2018.
  3. LibraTexts: Vitamin B-12: Cobalamin
  4. Mahmood L. The metabolic processes of folic acid and Vitamin B-12 deficiency." J Health Res Rev 2014;1:5-9
  5. Maryam Valizadeh, Nasim Valizadeh. "Obsessive Compulsive Disorder as Early Manifestation of B-12 Deficiency." ndian J Psychol Med. 2011 Jul-Dec; 33(2): 203–204.
  6. " Mayo Clinic: Vitamin B-12.
  7. Ehsan Ullah Syed, Mohammad Wasay, Safia Awan. "Vitamin B-12 Supplementation in Treating Major Depressive Disorder: A Randomized Controlled Trial." Open Neurol J. 2013; 7: 44–48.
  8. Galizia I, Oldani L, Macritchie K. "S-adenosyl methionine (SAMe) for depression in adults." Cochrane Database Syst Rev. 2016 Oct 10;10:CD011286.
  9. Pawlak R. "Is vitamin B-12 deficiency a risk factor for cardiovascular disease in vegetarians?" Am J Prev Med. 2015 Jun;48(6)
  10. Joyce Kaferle, Cheryl Strozda. "Evaluation of Macrocytosis." Am Fam Physician. 2009 Feb 1;79(3):203-208.
  11. Memorial Sloan Kettering Cancer Center: Vitamin B-12."
  12. Oregon State University:"Vitamin B-12."
  13. Piyathilake CJ, Macaluso M, Chambers MM, et al. "Folate and vitamin B-12 may play a critical role in lowering the HPV 16 methylation-associated risk of developing higher grades of CIN." Cancer Prev Res (Phila). 2014 Nov;7(11):1128-37
  14. National Organization for Rare Disorders:" Anemia, Megaloblastic."
  15. Pernicious Anemia Society: Symptoms of Pernicious Anemia."
  16. Watanabe F, Yabuta Y, Tanioka Y, Bito T. "Biologically active vitamin B-12 compounds in foods for preventing deficiency among vegetarians and elderly subjects." J Agric Food Chem. 2013 Jul 17;61(28):6769-75.

†Results may vary. Information and statements made are for education purposes and are not intended to replace the advice of your doctor. If you have a severe medical condition or health concern, see your physician.


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