Vitamin B12 (cobalamin)

Vitamin B12 has the largest and most complex chemical structure of all the vitamins. It is unique among vitamins in that it contains a metal ion, cobalt. Cyanocobalamin is the form used in most nutritional supplements and fortified foods.

Cobalamin helps keep the body’s nerve and blood cells healthy (it is essential for the preservation of the myelin sheath around neurons and for the synthesis of neurotransmitters) and helps make DNA, the genetic material in all cells.

Vitamin B12 has vital role in Krebs cycle witch happens in cell mitochondria in order to produce energy.

Vitamin B12 and folate are important for homocysteine metabolism. Elevated homocysteine levels in blood are a risk factor for cardiovascular disease (CVD). Although B vitamin supplementation has been proven effective to control homocysteine levels, current data from intervention trials have not shown that lowering homocysteine levels decreases CVD risk.

Vitamin B12 deficiency

The body requires both stomach acid and a protein made in the stomach, called intrinsic factor, to digest vitamin B12. First, stomach acid breaks off a protein that comes attached to vitamin B12. Then, the intrinsic factor is attached to the vitamin B12 molecule. Once in this form, vitamin B12 can be used in cells throughout the body. A problem with either stomach acid or intrinsic factor can lead to a vitamin B12 deficiency.

Who is at risk having B12 deficiency?

  • people over age 50 lose the ability to absorb vitamin B12 from foods
  • people who follow a vegetarian or vegan diet should try to eat vitamin B12-fortified foods or talk to their health care provider about taking B12 supplements
  • people who have had gastrointestinal surgery, such as weight loss surgery, lose the ability to absorb vitamin B12
  • People who have digestive disorders, such as celiac disease, Crohn’s disease or may not absorb enough vitamin B12.

Low levels of B12 can cause:

  • Anemia and pernicious anemia
  • Numbness or tingling in the arms and legs
  • strange sensations, numbness, or tingling in the hands, legs, or feet
  • difficulty walking (staggering, balance problems)
  • a swollen, inflamed tongue
  • yellowed skin (jaundice)
  • difficulty thinking and reasoning (cognitive difficulties), or memory loss
  • paranoia or hallucinations
  • weakness
  • fatigue

In healthy adults, vitamin B12 deficiency is uncommon, mainly because total body stores can exceed 2,500 μg (body can store vitamin B12 for years in the liver), daily turnover is slow, and dietary intake of only 2.4 μg/day is sufficient to maintain adequate vitamin B12 status.

Food sources of vitamin B12

Vitamin B12 is found naturally in a wide variety of animal proteins. Plant foods have no vitamin B12 unless they are fortified.

You can get the recommended amounts of vitamin B12 by eating a variety of the foods including:

  • Organ meats (beef liver)
  • Shellfish (clams)
  • Meat, poultry, eggs, milk and other dairy foods
  • Some breakfast cereals and nutritional yeasts

Some Food Sources of Vitamin B12

Food Serving Vitamin B12 (μg)
Clams (steamed) 3 ounces 84.1
Mussels (steamed) 3 ounces 20.4
Mackerel (Atlantic, cooked, dry-heat) 3 ounces* 16.1
Crab (Alaska king, steamed) 3 ounces 9.8
Beef (lean, plate steak, cooked, grilled) 3 ounces 6.9
Salmon (chinook, cooked, dry-heat) 3 ounces 2.4
Rockfish (cooked, dry-heat) 3 ounces 1.0
Milk (skim) 8 ounces 0.9
Turkey (cooked, roasted) 3 ounces 0.8
Brie (cheese) 1 ounce 0.5
Egg (poached) 1 large 0.4
Chicken (light meat, cooked, roasted) 3 ounces 0.3



The amount of vitamin B12 you need each day depends on your age. Average daily recommended amounts for different ages are listed below in micrograms (mcg):

Life Stage Recommended Amount
Birth to 6 months 0.4 mcg
Infants 7–12 months 0.5 mcg
Children 1–3 years 0.9 mcg
Children 4–8 years 1.2 mcg
Children 9–13 years 1.8 mcg
Teens 14–18 years 2.4 mcg
Adults 2.4 mcg
Pregnant teens and women 2.6 mcg
Breastfeeding teens and women 2.8 mcg


Can vitamin B12 be harmful?

There are no reported side effects after large intakes of vitamin B12 from food or supplements in healthy people.

When orally given high doses of vitamin B12, only a small percentage can be absorbed, which may explain the low toxicity. Because of the low toxicity of vitamin B12, no tolerable upper intake level has been set by the US Food and Nutrition Board.


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