Vitamin B₁₂also called cobalamin is a water-soluble vitamin that has a key role in the normal functioning of the brain and nervous system, and the formation of red blood cells. It is one of eight B vitamins. It is involved in the metabolism of every cell of the human body, especially affecting DNA synthesis, fatty acid and amino acid metabolism. No fungi, plants, or animals (including humans) are capable of producing vitamin B₁₂. Only bacteria and archaea have the enzymes needed for its synthesis. Some substantial sources of B₁₂ include animal products (shellfish, meat), fortified food products, and dietary supplements.B₁₂ is the largest and most structurally complicated vitamin and can be produced industrially only through bacterial fermentation synthesis, typically used to manufacture B₁₂ for fortified foods and supplements.

                            Vitamin B12

 

Structurally vitamin B12  are a large mollecules & in this reason it inter in the body  by converting into cyanocobalamone.Booster dose such as injection form are the best for patient benefit.

Causes

• Inadequate dietary intake of vitamin B₁₂. Vitamin B₁₂ occurs in animal products (eggs, meat, milk) and in some edible alge. B₁₂ isolated from bacterial cultures is also added to many fortified foods, and available as a dietary supplement. Vegans, and to a lesser degree vegetarians, may also be at risk for B₁₂ deficiency due to inadequate dietary intake of B₁₂, if they do not supplement. Children are at a higher risk for B₁₂ deficiency due to inadequate dietary intake, as they have fewer vitamin stores and a relatively larger vitamin need per calorie of food intake.
• Selective impaired absorption of vitamin B₁₂ due to intrinsic factor deficiency. This may be caused by the loss of gastric parietal cells in chronic atrophic gastritis (in which case, the resulting megaloblastic anemia takes the name of “pernicious anemia”), or may result from wide surgical resection of stomach (for any reason), or from rare hereditary causes of impaired synthesis of intrinsic factor. B₁₂ deficiency is more common in the elderly because gastric intrinsic factor, necessary for absorption of the vitamin, is deficient, due to atrophic gastritis.
• Impaired absorption of vitamin B₁₂ in the setting of a more generalized malabsorption or maldigestion syndrome. This includes any form due to structural damage or wide surgical resection of the terminal ileum (the principal site of vitamin B₁₂ absorption).
• Forms of achlorhydria (including that artificially induced by drugs such as proton pump inhibitors and histamine 2 receptor antagonists) can cause B₁₂ malabsorption from foods, since acid is needed to split B₁₂ from food proteins and salivary binding proteins. This process is thought to be the most common cause of low B₁₂ in the elderly, who often have some degree of achlorhydria without being formally low in intrinsic factor. This process does not affect absorption of small amounts of B₁₂ in supplements such as multivitamins, since it is not bound to proteins, as is the B₁₂ in foods.
• Surgical removal of the small bowel (for example in Crohn’s disease) such that the patient presents with short bowel syndrome and is unable to absorb vitamin B₁₂. This can be treated with regular injections of vitamin B₁₂.
• Long-term use of ranitidine hydrochloride may contribute to deficiency of vitamin B₁₂.
• Untreated celiac disease may also cause impaired absorption of this vitamin, probably due to damage to the small bowel mucosa. In some people, vitamin B₁₂ deficiency may persist despite treatment with a gluten-free diet and require supplementation.
• Some bariatric surgical procedures, especially those that involve removal of part of the stomach, such as Roux-en-Y gastric bypass surgery. (Procedures such as the adjustable gastric band type do not appear to affect B₁₂ metabolism significantly).
• Bacterial overgrowth within portions of the small intestine, such as may occur in blind loop syndrome, (a condition due to a loop forming in the intestine) may result in increased consumption of intestinal vitamin B₁₂ by these bacteria.
• The diabetes medication metformin may interfere with B₁₂ dietary absorption.
• A genetic disorder, transcobalamin II deficiency can be a cause.
• Alcoholism – if a “diet” of excessive alcohol intake is substituted for a diet adequate in sources of B₁₂.
• Nitrous oxide exposure, and recreational use.
• Infection with the Diphyllobothrium latum tapeworm

Symptoms of vitamin B12 deficiency

Anaemia and anaemia caused by a lack of vitamin B12 can result in symptoms which include:

• Extreme tiredness or fatigue
• A lack of energy or lethargy
• Being out of breath
• Feeling faint
• Headache
• Ringing in the ears (tinnitus)
• Lack of appetite

More specific symptoms linked to a lack of vitamin B12 include:

• Yellowing of the skin
• Sore, red tongue
• Mouth ulcers
• Changes or loss of some sense of touch
• Feeling less pain
• Walking problems
• Vision problems
• Mood changes, irritability, depression or psychosis
• Symptoms of dementia

Recommended Intakes

Intake recommendations for vitamin B12 and other nutrients are provided in the Dietary Reference Intakes (DRIs) developed by the Food and Nutrition Board (FNB) at the Institute of Medicine (IOM) of the National Academies (formerly National Academy of Sciences) . DRI is the general term for a set of reference values used for planning and assessing nutrient intakes of healthy people. These values, which vary by age and gender , include:

• Recommended Dietary Allowance (RDA): average daily level of intake sufficient to meet the nutrient requirements of nearly all (97%–98%) healthy individuals.
• Adequate Intake (AI): established when evidence is insufficient to develop an RDA and is set at a level assumed to ensure nutritional adequacy.
• Tolerable Upper Intake Level (UL): maximum daily intake unlikely to cause adverse health effects .

Table 1 lists the current RDAs for vitamin B12 in micrograms (mcg) . For infants aged 0 to 12 months, the FNB established an AI for vitamin B12 that is equivalent to the mean intake of vitamin B12 in healthy, breastfed infants.

Table 1: Recommended Dietary Allowances (RDAs) for Vitamin B12 

Age	Male	Female	Pregnancy	Lactation
0–6 months*	0.4 mcg	0.4 mcg
7–12 months*	0.5 mcg	0.5 mcg
1–3 years	0.9 mcg	0.9 mcg
4–8 years	1.2 mcg	1.2 mcg
9–13 years	1.8 mcg	1.8 mcg
14+ years	2.4 mcg	2.4 mcg	2.6 mcg	2.8 mcg

* Adequate Intake

References