-
Vitamin B12 (cobalamin, Cbl) is a water-soluble vitamin.
Vitamin B12 is the generic name for a specific group of cobalt-containing corrinoids with biological activity in humans. - In foods, hydroxo-, methyl- and 5'-deoxyadenosyl-cobalamins are the main cobalamins present.
- Plant foods does not contain vitamin B12.
- Vitamin B12 only occurs in foods of animal origin, likes meat, liver, kidney, yogurt, dairy products, fish, clams, oysters, nonfat dry milk, salmon and sardines.
- The two forms of vitamin B12 having activity in B12-dependent enzymes within the human body are adenosylcobalamin (AdeCbl) and methylcobalamin (MetCbl).
- Vitamin B12 is needed for building proteins in the body, red blood cells and normal function of nervous tissue.
- B12-dependent enzymes play an important role in the folate-dependent methylation of homocysteine to methionine and in the conversion of methylmalonyl-coenzyme A to succinyl-coenzyme A.
- The estimated daily requirements of vitamin B12 are 2-5 mcg (μg).
- A dietary vitamin B12 deficiency can occur in strict vegetarians or after gastrectomy, and other diseases affecting cobalamin absorption.
- Dietary deficiency is rare in younger people living in the community but occurs more frequently in older people, particularly those living in institutional environments.
- Vitamin B12 deficiency can lead to moderate hyperhomocysteinaemia (plasma homocysteine concentrations > 12 μmol/L), a possible risk factor for occlusive vascular disease.
- Pernicious anemia (PA) is a macrocytic anemia that is caused by vitamin B12 deficiency, as a result of intrinsic factor deficiency.
- Causes of the deficiency include, most frequently, foodcobalamin malabsorption syndrome (> 60% of all cases), pernicious anemia (15%–20% of all cases), insufficent dietary intake and malabsorption.
-
Dietary vitamin B12 is absorbed through the Intrinsic Factor (IF) – a glycoprotein secreted by the parietal cells of the stomach – in the ileum of the small intestine.
The IF-cobalamin complex is saturated with dosages between approximately 1.5 and 4 μg of vitamin B12 per meal.
At intakes around 1 μg about 50% is absorbed, at dosages around 25 μg only 5% is absorbed.
Very small amounts (ca 1-1.2%) can be absorbed by passive diffusion, in the absence of intrinsic factor. - In the absence of the intrinsic factor vitamin B12 injections should be given to treat a deficiency.
- The total body pool size is estimated between 1-10 mg or sufficient for 3-5 years. The liver is the predominant storage site for vitamin B12.
- Cyanocobalamin and hydroxycobalamin are the most commonly encountered supplemental forms of vitamin B12.
- The upper tolerable level (UL) for vitamin B12 has not been set yet.
Scientific studies on the relationship between vitamin B12 and disease prevention:
Review articles of randomized, placebo-controlled double-blind clinical trials (RCTs) will answer the following question:
"Is taking dietary supplements make sense?". Yes at a positive conclusion and no at a negative conclusion.
Review articles of cohort studies or case-control studies will answer the following question:
"Should I change my diet?".