Vitamin b12 deficiency

[Merna]

Vitamin B12 deficiency can occur in vegans who do not take supplements or as a result of an absorption disorder.

Anemia develops, causing paleness, weakness, fatigue, and, if severe, shortness of breath and dizziness.
A severe vitamin B12 deficiency may damage nerves, causing tingling or loss of sensation in the hands and feet, muscle weakness, loss of reflexes, difficulty walking, confusion, and dementia.
The diagnosis of vitamin B12 deficiency is based on blood tests.
When high doses of vitamin B12 supplements are taken, symptoms due to anemia tend to resolve.
Symptoms due to nerve damage and dementia in older people, may persist.

Vitamin B12 (cobalamins), with folate, is necessary for the formation and maturation of red blood cells and the synthesis of DNA (deoxyribonucleic acid), which is the genetic material of cells. Vitamin B12 is also necessary for normal nerve function. Good sources of vitamin B12 include meats (especially beef, pork, liver, and other organ meats), eggs, fortified cereals, milk, clams, oysters, salmon, and tuna. (See also Overview of Vitamins.)
Unlike most other vitamins, B12 is stored in substantial amounts, mainly in the liver, until it is needed by the body. If a person stops consuming the vitamin, the body’s stores of this vitamin usually take about 3 to 5 years to exhaust.
People should not take high doses of vitamin B12 as a cure-all, but otherwise the vitamin does not appear to be toxic, so consuming excess amounts of B12 is not a concern.
Vitamin B12 occurs in foods that come from animals. Normally, vitamin B12 is readily absorbed in the last part of the small intestine (ileum), which leads to the large intestine. However, to be absorbed, the vitamin must combine with intrinsic factor, a protein produced in the stomach. Without intrinsic factor, vitamin B12 moves through the intestine and is excreted in stool.
Because vitamin B12 is necessary for the formation of mature blood cells, deficiency of this vitamin can result in anemia. The anemia is characterized by abnormally large red blood cells (macrocytes) and abnormal white blood cells. Anemia may not develop until 3 to 5 years after the deficiency begins because a large amount of vitamin B12 is stored in the liver.
Vitamin B12 deficiency can cause nerve damage (neuropathy) even when no anemia develops.

[Julian]

Dietary vitamin B12 deficiency usually results from inadequate absorption, but deficiency can develop in vegans who do not take vitamin supplements. Deficiency causes megaloblastic anemia, damage to the white matter of the spinal cord and brain, and peripheral neuropathy. Diagnosis is usually made by measuring serum vitamin B12 levels. The Schilling test helps determine etiology. Treatment consists of oral or parenteral vitamin B12. Folate (folic acid) should not be used instead of vitamin B12 because folate may alleviate the anemia but allow neurologic deficits to progress.
Cobalamin is a general term for compounds with biologic vitamin B12 activity. These compounds are involved in nucleic acid metabolism, methyl transfer, and myelin synthesis and repair. They are necessary for the formation of normal red blood cells and normal neural function (see table Sources, Functions, and Effects of Vitamins).
Dietary sources of vitamin B12 include meats (especially beef, pork, and organ meats [eg, liver]), poultry, eggs, fortified cereals, milk and milk products, and seafood such as clams, oysters, mackerel, and salmon. Food-bound vitamin B12 is released in the stomach’s acid environment and is bound to R protein (haptocorrin). Pancreatic enzymes cleave this B12 complex (B12-R protein) in the small intestine. After cleavage, intrinsic factor, secreted by parietal cells in the gastric mucosa, binds with vitamin B12. Intrinsic factor is required for absorption of vitamin B12, which takes place in the terminal ileum.
Vitamin B12 in plasma is bound to transcobalamins I and II. Transcobalamin II is responsible for delivering vitamin B12 to tissues. The liver stores large amounts of vitamin B12. Enterohepatic reabsorption helps retain vitamin B12. Liver vitamin B12 stores can normally sustain physiologic needs for 3 to 5 years if B12 intake stops (eg, in people who become vegans) and for months to 1 year if enterohepatic reabsorption capacity is absent.
Large amounts of vitamin B12 seem to be nontoxic but are not recommended for regular use (ie, as a general tonic).
(See also Overview of Vitamins.)