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Vitamin B12 deficiency
Last reviewed: 05.07.2025
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Vitamin B 12 (cobalamin - Cbl) enters the body mainly with animal products (such as meat, milk) and is absorbed. Absorption of vitamin B 12 is a multi-stage process, including:
- proteolytic release of cobalamin from proteins;
- attachment of cobalamin to the protein of gastric secretion (intrinsic factor - IF, Castle factor);
- recognition of the IF-cobalamin complex by receptors of the ileal mucosa;
- transport through iliac enterocytes in the presence of calcium ions;
- release into the portal circulation in combination with trancobalamin II (TC II), a serum protein.
Usually, vitamin B12 deficiency (insufficiency) in young children is caused by insufficient intake of vitamin B12 from food in the mother’s body.
The most common disorder of vitamin B12 absorption is pernicious anemia. This is a chronic disease that develops as a result of impaired cobalamin intake due to IF deficiency in gastric secretion. Insufficient IF content in gastric secretion can be caused by a congenital deficiency of this factor or acquired causes, including immune causes (production of autoantibodies against IF and parietal cells of the gastric mucosa).
To release cobalamin from the protein complex in which the compound enters with food, an acidic reaction of the environment and pepsin activity of the gastric juice are necessary. This is why pernicious anemia develops in some diseases of the stomach (atrophic gastritis, partial gastrectomy).
In the absence or damage of IF, the entry of cobalamin into enterocytes becomes impossible, which leads to the development of pernicious anemia. IF deficiency can be both congenital and acquired.
In many cases, vitamin B12 metabolism disorders develop due to insufficient protein nutrition (kwashiorkor) and liver diseases. Some medications affect the absorption and metabolism of vitamin B12.
[ Vitamin B 12 metabolism
Vitamin B 12 (cobalamin) is found in animal products only: meat, liver, milk, eggs, cheese and others (the vitamin in animal tissues is a derivative of bacteria). Under the influence of cooking and proteolytic enzymes of the stomach, it is released and quickly binds to "R-binders" (trans-cobalamins I and III) - proteins with rapid electrophoretic mobility, compared to the intrinsic factor; to a lesser extent, vitamin B 12 binds to the intrinsic factor (IF, Castle factor) - a glycoprotein produced by the parietal cells of the fundus and body of the stomach.
Pathogenesis
In plasma, vitamin B 12 is present in the form of coenzymes - methylcobalamin and 5'-deoxyadenosylcobalamin. Methylcobalamin is necessary to ensure normal hematopoiesis, namely for the synthesis of thymidine monophosphate, which is part of DNA, and the formation of tetrahydrofolic acid. Disruption of thymidine formation with vitamin B 12 deficiency leads to disruption of DNA synthesis, slowing down of normal processes of maturation of hematopoietic cells (lengthening of phase S), which is expressed in megaloblastic hematopoiesis.
Symptoms of Vitamin B 12 Deficiency
There are hereditary and acquired forms of vitamin B12 deficiency anemia.
Hereditary forms of vitamin B12 deficiency anemia are rare. Clinically, they are characterized by a typical picture of megaloblastic anemia with the presence of symptoms of damage to the gastrointestinal tract and nervous system.
Symptoms of the disease appear gradually. Initially, loss of appetite, aversion to meat, and possible dyspeptic symptoms are noted. The most pronounced anemic syndrome is pallor, slight icterus of the skin with a lemon-yellow tint, subicterus of the sclera, weakness, malaise, rapid fatigue, dizziness, tachycardia, shortness of breath even with little physical exertion.
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Treatment of vitamin B 12 deficiency
Prevention is carried out in case of gastrocnemius and ileal resection.
The initial daily dose of vitamin B 12 is 0.25-1.0 mg (250-1000 mcg) for 7-14 days. As an alternative regimen (if the body is able to store the vitamin for a long time), intramuscular administration of the drug at a dose of 2-10 mg (2000-10,000 mcg) monthly is used. In most cases, therapy is lifelong.
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