Vitamin B12

Vitamin B12, also referred to as cobalamin, is primarily produced by bacteria in the large intestine below the ileum. This makes animals the primary dietary sources of this essential nutrient. Because the vitamin is produced below the ileum it can not be absorbed by the body and it is therefore often the case that Vegans may undergo Vitamin B12 deficiency unless they take supplements or eat fortified foods.

The water-soluble vitamin B12 has the largest and the most complex chemical structure among all the other vitamins. The forms of this nutrient that are used in the human body are 5-deoxyadenosyl cobalamin and methylcobalamin.

In most supplements, the cobalamin used is under the form of cyanocobalamin; these are later converted readily into methylcobalamin and 5-deoxyadenosyl in the body. Cobalamin in mammals is a cofactor for only two enzymes, namely: L-methylmalonyl-CoA mutase and methionine synthase.

The role of Vitamin B12 in the body

Vitamin B12 is vital for the absorption of folate, activation and storage to its coenzyme forms. As the vitamin works in conjunction with folate in order to support cellular replication, it follows that a deficiency in either vitamin can greatly affect this function.

Furthermore a unique role for this vitamin is the ability to synthesize myelin, which is a constituent of the sheaths than is protecting the nerve fibers. In general, some of the identified functions of vitamin B12 include:

  1. Plays a vital role in fat, protein, and carbohydrate metabolism.
  2. Assists in the neurotransmitter production; these chemicals are responsible in regulating pain, mood, and sleep.
  3. Enables proper DNA replication.
  4. Prevents anemia by aiding in the production of red blood cells.
  5. Helps in catalyzing the reaction produced by a chemical needed for the synthesis of hemoglobin, called succinyl-CoA.
  6. Aids in the homocysteine conversion into methionine.

Vitamin B12 Stability

Cobalamin is generally stable and is only destroyed minimally when undergoing normal methods of processing and cooking. However, studies have shown that prolonged cooking time can destroy up to third of this vitamin. Although this vitamin is absorbed by both facilitated and passive diffusion only trace amounts of vitamin B12 are passively absorbed, thus in order to have optimal absorption, facilitated diffusion that involves binding to intrinsic factor is required. It therefore follows that decreased gastric acidity as a consequence of age or certain medication can significantly compromise the status of vitamin B12.

Vitamin B12 Deficiency

Deficiency of vitamin B12 is estimated to affect 10% to 15% of individuals who are 60 years and older. Proper absorption of this nutrient from food requires the full functioning of the small intestine, pancreas, and the stomach.

In the body enzymes and stomach acid enable vitamin B12 to be freed from food, allowing the vitamin to bind to proteins known as R proteins. The R proteins are degraded by pancreatic enzymes in the alkaline environment of the small intestine further freeing vitamin B12 to become bound to the protein secreted by the specialized cells found in the stomach known as intrinsic factor (IF).

Food-bound vitamin B12 malabsorption and pernicious anemia (an autoimmune condition) are the most common causes of vitamin B12 deficiency. Both are separated conditions although both causes often occur with age. Not having sufficient amounts of cobalamin in the body can give rise to:

  1. Nerve damage with symptoms of tingling and numbness in the hands and feet.
  2. Pernicious Anemia, which is a type of anemia that is characterized by having very large red blood cells. Poor resistance to infections, listlessness, fatigue, and breathlessness are some of the common symptoms.
  3. Delayed growth and development, anemia, and movement disorders in infants.
  4. Other problems: poor memory, dementia, excessive weight loss, difficulty keeping balance, constipation, confusion, and loss of appetite.

Most of these deficit symptoms occur due to the difficulty of absorbing vitamin B12 in the intestine. Strict vegetarians, who don’t consume any form of animal products, are mostly susceptible to this deficiency.

Vitamin B12 toxicity is very rare. Large doses of this nutrient from supplements and food sources are not associated with any forms of toxicity or adverse effect in healthy individuals.

Recommended Dietary Allowance (RDA)

The Food and Nutrition Board (FNB) of the Institute of Medicine revised the current RDA in 1998 due to an increased risk of food-bound malabsorption of vitamin B12 in older adults.

Recommended Dietary Allowance (RDA) for Vitamin B12

AGE (µg/day)
0 to 6 months 0.4 (AI)
7 to 12 months 0.5 (AI)
1 to 3 years 0.9
4 to 8 years 1.2
9 to 13 years 1.8
14 to 18 years 2.4
19 years and older 2.4
51 years and older 2.4
Pregnant 2.6
Lactating 2.8

Food Sources of Vitamin B12

Vitamin B12 is only present in animal products as it can only be synthesized by bacteria. Poultry, meat, fish (including shellfish) are good sources of cobalamin, but this nutrient is not generally found in yeasts or plant products. Mussels, clams, salmon, crab, beef, turkey, and chicken contain significant amounts of vitamin B12. A good source of this vitamin for Vegans is B12 fortified soymilk and cereals, or the nutritional yeast known as Red Star T-6635+.


LPI; UMM; Harvard; Illinois; Northwestern; University of North Dakota.