What is vitamin B12?

Vitamin B12 or cobalamin is easily the most well-known of all the B vitamins. It is vital for nervous system function, DNA production, and red blood cell development. It is not produced in the body and therefore it has to be obtained from our foods. It is naturally found in foods of animal origin like meats, eggs, seafood, and dairy.

It belongs to the vitamin B family, a water-soluble vitamin, and is an essential micronutrient. It functions as a coenzyme in cellular activities, being particularly important in the gastrointestinal tract, nervous system, and bone marrow.

It is an important cofactor in DNA synthesis, regeneration of methionine for protein synthesis and methylation, and preventing the accumulation of homocysteine, which helps to reduce the risk of dementia, heart disease and stroke.

Since vitamin B12 contains the mineral cobalt, compounds with vitamin B12 activity are collectively called “cobalamins”

How is B12 produced and absorbed in humans?

Vitamin B12 or cobalamin is synthesized by two groups of organisms, Pseudomonas and Klebsiella sp, and archaeon (a type of a microorganism)  in the colon where B12 cannot be taken up by the organisms. It is a result of bacterial fermentation in the gut.

However, since B12 is absorbed in the ileum, the last part of the small intestine, it does not get absorbed in the colon and is therefore excreted through the feces.

Thus, cobalamin has to be taken up from the food and only animal foods gives you sufficient amounts.

The body absorbs vitamin B12 from food in two steps:

  • First, hydrochloric acid in the stomach removes vitamin B12 from the protein that it’s combined with.
  • Second, the isolated vitamin B12 then combines with the intrinsic factor, a protein made by the stomach, so that the body can absorb them together.

Types of vitamin B12

Vitamin B12 exists in four chemically similar forms:

  • Methylcobalamin
  • Adenosylcobalamin
  • Cyanocobalamin
  • Hydroxocobalamin

Methylcobalamin and adenosylcobalamin are the metabolically active forms of vitamin B12 in the body. Hydroxycobalamin and cyanocobalamin, become active after they are converted to methylcobalamin or adenosylcobalamin.


Methylcobalamin is the most readily absorbed type of Vitamin B12 by the body. It is the most active form of Vitamin B12 and occurs naturally in animal-based foods such as meat, fish, milk, and eggs.


Adenosylcobalamin is also a naturally occurring, co-enzyme form of this vitamin. It along with Methylcobalamin, work together to fulfill the body’s need of B12.

Adenosylcobalamin promotes the functioning of the mitochondria in body cells, which Methylcobalamin cannot do. The mitochondria are like ‘powerhouses’ in each cell and produce energy for cellular activities. It helps the mitochondria metabolize food efficiently to produce energy and boost metabolism.

Additionally, Adenosylcobalamin is an important component of the myelin sheath, an insulating layer that forms around nerves, including those in the brain and spinal cord. It protects the nerve cells and enables them to respond very quickly to stimuli.


Hydroxocobalamin is naturally produced by bacteria in the digestive tract and it is also manufactured synthetically. Its supplement is available under prescription and is usually given parenterally (by injection).

Hydroxocobalamin is readily converted by the body into Adenosylcobalamin and Methylcobalamin so that it can be readily absorbed and used by the cells.


Cyanocobalamin is a synthetic form of Vitamin B12, which the body converts into the active forms of Methylcobalamin and Adenosylcobalamin for ready use.

Cyanocobalamin B12 contains a cyanide molecule that gives it stability during shelf life and in the body.

Cyanocobalamin is artificially made and does not exist naturally in foods. You will only find this form in supplements. It is comparatively cheap and becomes highly active once it is broken down in the body.

How much vitamin B12 do I need?

The amount of vitamin B12 you need each day depends on your age. The  following table gives the daily recommended amounts for different ages in micrograms (mcg): 

Age Recommended Amount
Up to 6 months 0.4 mcg
Infants 7 to 12 months 0.5 mcg
Children 1 to 3 years 0.9 mcg
Children 4 to 8 years 1.2 mcg
Children 9 to 13 years 1.8 mcg
Teens 14 to 18 years 2.4 mcg
Adults 2.4 mcg
During pregnancy 2.6 mcg
During lactation 2.8 mcg

Those older than 50 years should consume vitamin B12-fortified foods vitamin and B12 supplements, because some older people may not be able to absorb naturally occurring B12. This will enable them to meet their daily recommended amounts and not become vitamin B12 deficient.

Food sources of vitamin B12

Fish and Shellfish: B12 from 3-ounce servings:

  • Cooked clams: 84.1 micrograms
  • Steamed mussels: 20.4 micrograms
  • Cooked Atlantic mackerel: 16.1 micrograms
  • Cooked wild rainbow trout 5.4 micrograms
  • Cooked salmon: 2.4 micrograms

Red Meat: B12 from 3-ounce servings

  • Cooked beef liver: 70.7 micrograms
  • Grilled lean beef, steak: 6.9 micrograms


The proportion of B12 that enters the circulation after consumption (bioavailability) appears to be about three times higher in dairy products than in meat, fish, and poultry,

  • One cup of low-fat milk: 1.2 micrograms
  • Eight ounces of low-fat yogurt: 1.2 micrograms
  • One ounce of Swiss cheese: 0.9 micrograms


  • Three ounces of cooked turkey liver: 23.9 micrograms
  • One ounce cooked chicken liver: 4.7 micrograms
  • Three ounces of cooked ground lean turkey: 1.6 micrograms
  • Three ounces of roasted turkey: 0.8 micrograms
  • Three ounces of roasted chicken breast: 0.3 micrograms


  • One hard-boiled egg: 0.6 micrograms. Most of the B12 is present in the yolk.

Vegan or Vegetarian Sources

If you’re on a strict plant-based diet, it may be difficult to get enough of vitamin B12 from your diet. However, you can get B12 from some fortified plant-based foods. Some examples include:

  • One cup of fortified non-dairy milks, such as soy or oat, contains 0.6-2.07 micrograms
  • One serving of fortified cereals contains 0.6-2.1 micrograms
  • One tablespoon of nutritional yeast contains 4.8 micrograms

Benefits of vitamin B12

Vitamin B12 is a vital nutrient for various bodily functions to take place. They include:

  • Normal functioning of the brain and nervous system
  • Normal cognitive functioning
  • Production of healthy red blood cells in the bone marrow
  • Helping in the creation and normalizing of DNA
  • Preventing congenital abnormalities
  • Helping protect the eyes from macular degeneration promoting healthy eyesight and preventing
  • Acular degeneration (loss of eyesight) especially in older people
  • Helps in energy production
  • Improves your sleep and wake cycle