Source: What Is Vitamin B-12?
by Dr. Edward Group

Vitamin B12 is a vital nutrient that supports normal energy levels, cardiovascular health, and the nervous system.Vitamin B-12 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, virtually every cell in your body needs vitamin B-12.

What Is Vitamin B-12?

Vitamin B-12 is one of the eight B vitamins collectively referred to as B complex. Because B-12 contains the element cobalt in its molecular structure, it is also called cobalamin. This water-soluble vitamin occurs naturally in certain animal products (i.e., eggs, milk, fish, and shellfish) but, in contrast to most other nutrients, it is not readily available in many plants or fungi.

Vitamin B-12 is exclusively manufactured in nature by bacteria and single-celled microbes, the only living organisms that possess the enzymes needed for its synthesis. Since your body cannot manufacture vitamin B-12 and it’s an essential nutrient,[1,2] you must get it through a balanced diet, take it in supplement form, or, under certain conditions, as a prescription medication. In fact, vitamin B-12 is the most complex in chemical structure of all vitamins.[3,4]

The Four Forms of Vitamin B-12

There are four forms of vitamin B-12.

1. Hydroxycobalamin

Hydroxycobalamin (hydroxocobalamin), also called vitamin B-12a, is produced naturally by bacteria, and is the form found in most food sources since they contain the bacterial byproduct. This type is often used in B-12 shots. Hydroxycobalamin 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 B-12 that’s lab-synthesized via bacterial fermentation, which makes it the cheapest option to use in supplements, but it’s far from the best. It actually contains a cyanide molecule, which is necessary for stability.

4. Adenosylcobalamin

Adenosylcobalamin (5-deoxyadenosylcobalamin) is a naturally occurring but unstable form of B-12. It is not stable in tablet form, but some premier supplement manufacturers have been able to offer it in a stable, liquid formula B-12.

How Is Vitamin B-12 Absorbed?

When you consume B-12 with food, it is bound to proteins, but is “freed” in the stomach by specialized cells that release gastric acid. (When B-12 is obtained through supplementation, this process isn’t necessary because the vitamin is already in free form.) After being freed, B-12 binds with a compound called R protein which transports B-12 from the stomach into the duodenum, the first section of the small intestine. In the duodenum, cobalamin leaves R factor and binds with a glycoprotein known as intrinsic factor which is produced by the stomach and salivary glands.[1],[2]

Once bound to intrinsic factor, B-12 is absorbed by the ileum, which is the last part of the small intestine. It then enters blood circulation as a transcobalamin complex – B-12 bound to a protein – which delivers B-12 to the cells that need it. Because B-12 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 B-12 is eliminated in urine or feces.

What Does Vitamin B-12 Do for the Body?

Vitamin B-12 supports many important functions in the body. The body uses B-12 for cell metabolism and cell division. B-12 helps cells produce hemoglobin, an iron-containing protein that transports oxygen throughout the body in red blood cells.[4] In the form of methylcobalamin, B-12 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 B-12, Energy, & Metabolism

People often take B-12 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 B-12 helps break down carbohydrates, particularly glucose sugars. A lack of B-12 can lead to high blood sugar.[5] B-12 also helps the body process fat, and a B-12 deficiency can lead to an unhealthy lack of fat.[5]

Vitamin B-12 and the Brain

Vitamin B-12 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 B-12 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 B-12 and Cognitive Health

Research shows that patients with impaired cognitive function and memory loss are often deficient in B-12.[2],6],[7] Scientists think that a B-12 deficiency might reduce the production of certain neurotransmitters, which are chemicals that transmit information throughout the nervous system. Although vitamin B-12 is crucial to the nervous system, scientists do not have evidence that B-12 supplementation is effective after cognitive decline occurs. This point underscores the value of ensuring sufficient intake throughout life.

Vitamin B-12 and Mood

Research shows that people who suffer from unhappy moods are sometimes deficient in vitamin B-12.[7],[8] This may be due, in part, to a reduction in the myelin sheaths from insufficient B-12. Although it’s unclear of the effect vitamin B-12 has on major depression, evidence does suggest that B-12 supplementation may normalize mood.[7],[8] Vitamin B-12 is necessary for the body to produce and release mood-regulating neurotransmitters, namely dopamine, serotonin, and melatonin. Other mood-related disorders are linked to B-12 deficiency as well.[5]

Vitamin B-12 and Cardiovascular Health

Vitamin B-12 is important for heart health; deficiency is linked with heart conditions in some cases.[9] Low B-12 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. B-12 deficiency-induced macrocytosis is associated with poor circulation and a higher risk of heart disease and stroke.[9]

Vitamin B-12 and Severe Illness

A lack of vitamin B-12 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 B-12 appears to reduce the risk of uncontrolled unhealthy cell proliferation.[13],[2]

Vitamin B-12 and Bone Health

Elevated homocysteine – which occurs when B-12 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 B-12 supplementation leads to improved bone strength and decreased risk of fracture are mixed. However, since high homocysteine levels indicate vitamin B-12 deficiency, medical scientists have recommended that the B-12 status in elderly populations should be evaluated periodically as a means to monitor bone health.[12]

How Much Vitamin B-12 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 and the Food and Nutrition Board – the premiere scientific health bodies officially sanctioned by the U.S. government on matters of nutrient intake – set daily dose recommendations of vitamin B-12 for normal adults at 2.4 mcg/day, with up to 3 mcg/day for pregnant and lactating women.[2]

Keep in mind these numbers are absolute minimums. Research suggests that the average vitamin B-12 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 number of the population is B-12 deficient.

The typical human body can absorb up to 1.5 mcg of vitamin B-12 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 B-12 (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 B-12?

Vitamin B-12 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 the B-12 shot, such as pain at the injection site, as well as headaches, nausea, diarrhea, and dizziness.

How Can You Get B-12?

You can get B-12 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 B-12

The minimum recommended daily allowance (RDA) of vitamin B-12 for most adults is 2.4 micrograms (mcg) per day from foods and fortified foods, with a slight increase of 2.6 mcg and 2.8 mcg per day for pregnant and lactating women, respectively. The following represents the foods with the highest vitamin B-12 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 B-12 Supplements

Many people have difficulty getting vitamin B-12 in their diet. Older populations and those with certain health conditions may not get enough B-12 due to malabsorption issues. If you fall into one of these groups, you may want to consider a daily supplement.

Methylcobalamin and adenosylcobalamin are the best absorbed forms of B-12. If you need a B-12 supplement, I recommend a blend of methylcobalamin and adenosylcobalamin. Together, these two naturally occurring forms of B-12 provide the best option. The liquid formula tastes great and is more stable and better absorbed than a pill. It’s a gluten-free, non-GMO, certified organic, vegan formula that’s perfect for anyone.

What Is Vitamin B-12 Deficiency?

A B-12 deficiency can arise for reasons as simple as eating a vegetarian diet – since plant foods do not contain B-12 – or as complex as an autoimmune disease that prevents B-12 absorption. Long-term use of acid-inhibiting medications can reduce levels of intrinsic factor, which negatively affects B-12 absorption.

When the body doesn’t have enough vitamin B-12, 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 B-12 deficiency and are often measured in lab tests.

What Are the Symptoms of Vitamin B-12 Deficiency?

Symptoms of vitamin B-12 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 levels of red blood cells (anemia)

Anemia is directly linked to B-12 deficiency in certain individuals. There are two types associated with insufficient vitamin B-12 – 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 B-12 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 B-12 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 B-12 you consume in food or supplements past the acidic environment of the stomach and delivers it to the intestines. Without intrinsic factor, B-12 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 B-12 Deficiency

There are several risk factors that may put a person at risk for vitamin B-12 deficiency.

Vegetarian Diet

Because animal products provide the only dietary source of vitamin B-12, people who adhere to a plant-based or total vegan diet have a higher risk for vitamin B-12 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 B-12 supplement.[2] Recent research shows that certain plant-based foods, such as fermented beans, and edible mushrooms and algae contain significant amounts of vitamin B-12.[15]

Genetic Predisposition

Several inherited disorders result in vitamin B-12 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 B-12 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 B-12 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 B-12. The surgical removal of any part of the stomach or small intestine, including the distal ileum, also interferes with vitamin B-12 absorption.[1]


Several medications inhibit or reduce B-12 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 B-12 Sufficient or Deficient?

Despite a healthy and balanced diet, physiological (genetics, disease) and environmental (medications for diabetes, etc.) factors can interfere with normal B-12 absorption for many people, leading to depletion or outright deficiency.

Blood level testing is the surest diagnostic method to assess if you are B-12 deficient. Here is a general guideline to follow to determine whether or not you are vitamin B-12 deficient:

Level of vitamin B-12 in Blood Severity
180-914 ng/L Low Range/Normal High
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 B-12 status may be suggested, including homocysteine and methylmalonic acid/MMA (levels of these molecules rise when vitamin B-12 deficiency-symptoms unfold, testing of which often allows for a more thorough assessment, in conjunction with B-12 testing).

Your Story

Where is B-12 on your radar? What’s happened in your life for B-12 to have gotten your attention? How do you ensure that you get enough? Leave a comment below and share your story with us.

Bottom Line

Considering what we know about vitamin B-12 and its importance for our health, the simplest and least expensive insurance against vitamin B-12 deficiency is supplementation. It’s possible to get enough via diet, but it’s difficult and requires a healthy amount of consistency. A B-12 shot is another solution preferred by many and I don’t have much problem with it except that it can get expensive, be difficult to access regularly, and heaven help you if you’re not a fan of needles. Simply supplementing with an absorbable form of B-12 like methylcobalamin or adenosylcobalamin can really help fill the gaps in easy fashion.

Is B-12 a big part of your life? Leave a comment below and share your experience!


Watch an In-Depth Video on
Everything You Need to Know About Vitamin B-12

Video Length: 60 minutes

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.

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