Vitamin B12

Introduction

Vitamin B12 is the largest and most complex of all the vitamins. The name vitamin B12 is generic for a specific group of cobalt-containing corrinoids with biological activity in humans. Interestingly it is the only known metabolite to contain cobalt, which gives this water-soluble vitamin its red colour. This group of corrinoids is also known as cobalamins. The main cobalamins in humans and animals are hydroxocobalamin, adenosylcobalamin and methylcobalamin, the last two being the active coenzyme forms. Cyanocobalamin is a form of vitamin B12 that is widely used clinically due to its availability and stability. It is transformed into active factors in the body.

In 1934, three researchers won the Nobel prize in medicine for discovering the lifesaving properties of vitamin B12. They found that eating large amounts of raw liver, which contains high amounts of vitamin B12, could save the life of previously incurable patients with pernicious anaemia. This finding saves 10,000 lives a year in the US alone. Vitamin B12 was isolated from liver extract in 1948 and its structure was elucidated 7 years later.

Functions

Vitamin B12 is necessary for the formation of blood cells, nerve sheaths and various proteins. It is therefore, essential for the prevention of certain forms of anaemia and neurological disturbances. It is also involved in fat and carbohydrate metabolism and is essential for growth. In humans, vitamin B12 functions primarily as a coenzyme in intermediary metabolism. Two metabolic reactions are dependent on vitamin B12:

1. The methionine synthase reaction with methylcobalamin
2. The methylmalonyl CoA mutase reaction with adenosylcobalamin

In its methylcobalamin form vitamin B12 is the direct cofactor for methionine synthase, the enzyme that recycles homocysteine back to methionine. There is evidence that vitamin B12 is required in the synthesis of folate polyglutamates (active coenzymes required in the formation of nerve tissue) and in the regeneration of folic acid during red blood cell formation.

Methylmalonyl CoA mutase converts 1-methylmalonyl CoA to succinyl CoA (an important reaction in lipid and carbohydrate metabolism). Adenosylcobalamin is also the coenzyme in ribonucleotide reduction (which provides building blocks for DNA synthesis).

Dietary sources

Vitamin B12 is produced exclusively by microbial synthesis in the digestive tract of animals. Therefore, animal protein products are the source of vitamin B12 in the human diet, in particular organ meats (liver, kidney). Other good sources are fish, eggs and dairy products. In foods, hydroxo-, methyl- and 5'-deoxyadenosyl-cobalamins are the main cobalamins present. Foods of plant origin contain no vitamin B12 beyond that derived from microbial contamination. Bacteria in the intestine synthesise vitamin B12, but under normal circumstances not in areas where absorption occurs.

Absorption and body stores

Vitamin B12 from food sources is bound to proteins and is only released by an adequate concentration of hydrochloric acid in the stomach. Free vitamin B12 is then immediately bound to glycoproteins originating from the stomach and salivary glands. This glycoprotein complex protects vitamin B12 from chemical denaturation. Gastrointestinal absorption of vitamin B12 occurs in the small intestine by an active process requiring the presence of intrinsic factor, another glycoprotein, which the gastric parietal cells secrete after being stimulated by food. The absorption of physiological doses of vitamin B12 is limited to approximately 10µg/dose. The vitamin B12 intrinsic factor complex is then absorbed through phagocytosis by specific ileal receptors. Once absorbed, the vitamin is transferred to a plasma-transport protein which delivers the vitamin to target cells. A lack of intrinsic factor results in malabsorption of cobalamin. If this is untreated, potentially irreversible neurological damage and life-threatening anaemia develops (see deficiency).

Regardless of dose, approximately 1% of vitamin B12 is absorbed by passive diffusion, so this process becomes quantitatively important at pharmacological levels of exposure. Once absorbed, vitamin B12 is stored principally (60%) in the liver. The average B12 content is approximately 1.0 mg in healthy adults, with 20-30 µg found in the kidneys, heart, spleen and brain. Estimates of total vitamin B12 body content for adults range from 0.6 to 3.9 mg with mean values of 2-3 mg. The normal range of vitamin B12 plasma concentrations is 150-750 pg/ml, with peak levels achieved 8-12 hours after ingestion.

Excretion of vitamin B12 is proportional to stores and occurs mainly by urinary and faecal routes. Vitamin B12 is very efficiently conserved by the body, with 65-75% re-absorption in the ileum of the 0.5-5 µg excreted into the alimentary tract per day (mainly into the bile). This helps to explain the slow development (over several years) of deficiency states in subjects with negligible vitamin B12 intake, such as vegans. Subjects with a reduced ability to absorb cobalamin via the intestine (lack of intrinsic factor) develop a deficiency state more rapidly.

Measurement

Measurement of vitamin B12 in plasma is routinely used to determine deficiency, but may not be a reliable indication in all cases. In pregnancy, for example, tissue levels are normal but serum levels are low. Vitamin B12 can be measured by chemical, microbiological or immunoassay isotope dilution methods. Microbiological assays, which are widely used for blood and tissue samples, are sensitive but non-specific. Serum cobalamin concentration is often determined by automated immunoassays using intrinsic factor as a binding agent. These assays have mainly replaced microbiological methods. Data in the literature about vitamin B12 concentration in serum varies. However, values < 110 – 150 pmol/L are considered to reflect deficiency, whereas values > 150 – 200 pmol/L represent an adequate status.

Major vitamin B12-dependent metabolic processes include the formation of methionine from homocysteine, and the formation of succinyl coenzyme A from methylmalonyl coenzyme A. Thus, apart from directly determining vitamin B12 concentration in the blood, elevated levels of both methylmalonic acid (MMA) and homocysteine may indicate a vitamin B12 deficiency.

The Schilling test (which quantifies ileal absorption by measuring radioactivity in the urine after oral administration of isotopically labelled vitamin) enables detection of impaired vitamin B12 absorption. The measurement of urinary methylmalonate (0-3.5 mg/day is normal; a vitamin B12-deficient patient will excrete up to 300 mg/day) can be used as a diagnostic means to assess vitamin B12 status. (Other tests include the cobalamin absorbance test and the serum gastrin deoxyuridine suppression test).

Stability

Vitamin B12 is stable to heat, but slowly loses its activity when exposed to light, oxygen and acid or alkali-containing environments. Loss of activity during cooking is due to the water solubility of vitamin B12 (loss through meat juices or leaching into water) rather than to its destruction.

Interactions

Negative interactions

Absorption of cobalamins is impaired by alcohol and vitamin B6 (pyridoxine) deficiency. Furthermore, a number of drugs reduce the absorption of vitamin B12, and supplementation with the affected nutrient may be necessary:

• Stomach medication: proton pump inhibitors, H2 receptor antagonists
• Liver medication: cholestyramine
• Tuberculostatics: para-aminosalicylic acid
• Anti-gout medication: colchicine
• Antibiotics: neomycin, chloramphenicol
• Anti-diabetics: oral biguanides metformin and phenformin
• Potassium chloride medications
• Oral contraceptives

A number of anticonvulsants – phenobarbitone, primidone, phenytoin and ethylphenacemide – can alter the metabolism of cobalamins in the cerebrospinal fluid and lead to neuropsychic disturbances. Several substituted amide, lactone and lactam analogues of cyanocobalamin compete with binding sites on intrinsic factor and lead to depressed absorption of the vitamin. Nitrous oxide (anaesthetic) also interferes with cobalamin metabolism.

Deficiency

Clinical cobalamin deficiency due to dietary insufficiency is rare in younger people, but occurs more frequently in older people. Vitamin B12 deficiency affects 10-15% of individuals over the age of 60.

Deficiency of vitamin B12 leads to defective DNA synthesis in cells, which affects the growth and repair of all cells. Tissues most affected are those with the greatest rate of cell turnover, e.g. those of the haematopoietic system. This can lead to megaloblastic anaemia (characterised by large and immature red blood cells) and neuropathy, with numerous symptoms including: glossitis, weakness, loss of appetite, loss of taste and smell, impotence, irritability, memory impairment, mild depression, hallucination, breathlessness (dyspnea) on exertion, tingling and numbness (paraesthesia).

Vitamin B12 deficiency can also lead to hyperhomocysteinaemia, a possible risk factor for occlusive vascular disease.

The symptoms of vitamin B12 deficiency are similar to those of folic acid deficiency, the major difference being only that vitamin B12 deficiency is associated with spinal cord degeneration. If folic acid is used to treat vitamin B12 deficiency, anaemia may be alleviated but the risk of damage to the nervous system remains. It is therefore essential to diagnose the deficiency accurately before starting therapy. The Food and Nutrition Board advises adults to limit their folic acid intake (through supplements and fortification) to 1 mg per day.

Cause of deficiency is not usually insufficient dietary intake but lack of intrinsic factor secretion. Without intrinsic factor, absorption is not possible and a severe and persistent deficiency develops that cannot be prevented by the usual dietary intakes of vitamin B12. This occurs in people with:

• pernicious anaemia (a hereditary autoimmune disease that chiefly affects persons post middle age),
• food-bound vitamin B12 malabsorption, reported in patients on long-term treatment with certain drugs, and in elderly patients with gastric atrophy.
• after gastrectomy
• after ingestion of corrosive agents with destruction of gastric mucosa.
• lesions of the small bowel (blind loops, stenoses, strictures, diverticula). Bacterial overgrowth may lead to competitive utilisation of available vitamin. Impaired absorption also occurs in patients with small intestinal defects (e.g. sprue, celiac disease, ileitis, ileal resection) and those with inborn errors of cobalamin metabolism, secretion of biologically abnormal intrinsic factor, or Zollinger-Ellison syndrome.
• pancreatic insufficiency
• in alcoholics vitamin B12 intake and absorption are reduced, while elimination is increased
• AIDS also brings an increased risk of deficiency

The risk of nutritional deficiency is increased in vegans; a high intake of fibre has been shown to aggravate a precarious vitamin balance. There have also been reports of vitamin B12 deficiency in infants breast-fed by vegetarian mothers. Strict vegetarians are urged to use a vitamin B12 supplement.

Disease prevention and therapeutic use

Pernicious anaemia

Patients with lack of intrinsic factor secretion can be effectively treated using oral vitamin B12 but require lifetime vitamin B12 therapy. When used alone, oral doses of at least 150 µg/day are necessary, although single weekly oral doses of 1000 µg have proved satisfactory in some cases. Combinations of vitamin B12 and intrinsic factor may be given, but as a variable number of patients become refractory to intrinsic factor after prolonged treatment, parenteral therapy with cyanocobalamin or hydroxocobalamin is preferred.

Hyperhomocysteinaemia

Homocysteine appears to be a nerve and vessel toxin, promoting mortality and cardiovascular disease (CVD) as well as stroke, Alzheimer's disease, birth defects, recurrent pregnancy loss, and eye disorders. Keeping homocysteine at levels associated with lower rates of disease requires adequate B12, folic acid and B6 intake.

Cancer

Vitamin B12 deficiency may lead to an elevated rate of DNA damage and altered methylation of DNA. These are obvious risk factors for cancer. In a recent study, chromosome breakage was minimised in young adults by supplementation with 700µg of folic acid and 7 µg of vitamin B12 daily in cereal for two months.

Recommended Dietary Allowance (RDA)

RDA intakes for vitamin B12 range from 0.3 to 5.0 µg/day in 25 countries. An increase to 2.2 µg/day is recommended during pregnancy and to 2.6 µg/day for lactation to cover the additional requirements of the foetus/infant. The Committee on Nutrition of the American Academy of Paediatrics recommends a daily vitamin B12 intake of 0.15 µg/100 kcal energy intake for infants and preadolescent children. Other authorities have suggested intakes of 0.3-0.5 µg (0-1 year of age), 0.7-1.5 µg (1-10 years of age) and 2 µg (> 10 years). The “average” western diet probably supplies 3-15 µg/day, but can range from 1-100 µg/day.

Safety

Large intakes of vitamin B12 from food or supplements have caused no toxicity in healthy people. No adverse effects have been reported from single oral doses as high as 100 mg and chronic administration of 1 mg (500 times the RDA) weekly for up to 5 years. Moreover, there have been no reports of carcinogenic or mutagenic properties, and studies to date indicate no teratogenic potential. The main food safety authorities have not set a tolerable upper intake level (UL) for vitamin B12 because of its low toxicity.

Supplements and food fortification

The principal form of vitamin B12 used in supplements is cyanocobalamin. It is available in the form of injections and as a nasal gel for the treatment of pernicious anaemia. Cyanocobalamin is also available in tablet and oral liquid form for vitamin B-complex, multivitamin and vitamin B12 supplements.

Vitamin B12 is widely used to enrich cereal products and certain beverages. Dietetic foods such as slimming foods and infant formulas are often fortified with vitamins, including vitamin B12. Fortification with vitamin B12 is especially important for products aimed at people with a low dietary intake, such as vegans.

Industrial production

Vitamin B12 is produced commercially from bacterial fermentation, usually as cyanocobalamin.

History

» History of Vitamin B12