Vitamin B12 is synthesized by bacteria. The vitamin is seldom found in foods derived from plants; therefore, those that consume strict vegetarian diets are very likely to have suboptimal intakes of vitamin B12 which, if prolonged and uncorrected, will lead to anaemia and, ultimately, to peripheral neuropathy. Low vitamin B12 status also occurs in individuals with deficiencies in proteins involved in vitamin B12 transport and/or metabolism, and in individuals with compromised gastric parietal cell function. Low vitamin B12 status impairs the metabolic utilization of folate and contributes to high levels of homocysteine-homocysteinemia-a risk factor for vascular disease. Low vitamin B12 status has been estimated to affect a large portion of the general population, including 30–40% of older adults. Deficiency of Vitamin B12 can be produced by several factors
Strict vegetarian diets, containing no meats, fish, animal products (e.g., milk, eggs) contain practically no vitamin B12. Therefore, individuals consuming such diets typically show very low circulating levels of the vitamin; one study found 56% of vegetarian American women to have low serum concentrations of vitamin B12 (1,2). Nevertheless, clinical signs among such individuals appear to be rare and may not manifest for many years, although they are more common among breastfed infants (3). The vitamin B12 content of breast milk has been found to vary inversely with the length of maternal vegetarian practice. Serum vitamin B12 concentrations have also been found to vary inversely with the length of time of vegetarian practice, showing progressive declines through about 7 year (4). This time compares very favourably with the estimated drawdown of hepatic stores of the vitamin.
It should be remembered that not all vegetarians are strict vegans; many consume plant-based diets that also contain servings of dairy products, eggs or fish to varying extents. Studies have shown that the occasional consumption of animal products (e.g., once per month) will support serum vitamin B12 levels comparable to those of people eating traditional mixed diets. In addition, such foods as Nori sp. and Chlorella sp. seaweeds, which may be eaten by vegetarians, appear to contain vitamin B12.
Loss of Gastric Parietal Cell Function -Vitamin B12 malabsorption occurs when Intrinsic Factor (IF) (see below) production by gastric parietal cells is inadequate. Such conditions can have several causes:
Pernicious anaemia. It is a disease of later life, 90% of cases being diagnosed in individuals 40 years of age. The anaemia is the end result of autoimmune gastritis, also called type A chronic atrophic gastritis or gastric atrophy. This causes progressive atrophy of parietal cells leading to hypochlorhydria and loss of IF production, and resulting in severe vitamin B12 malabsorption. The condition presents as megaloblastic anaemia within two to seven years. The disorder is likely to be widely underdiagnosed, as affected subjects may have neurological rather than haematological disease.
Heliobacter pylori infection produces damage mostly to the stomach, referred to as Type B chronic atrophic gastritis. The condition involves hypochlorhydria, which facilitates the proliferation of bacteria in the intestine. Both conditions limit and compete for the enteric absorption of vitamin B12.
Other gastric diseases. Vitamin B12 utilization can be affected by disease involving damage to the gastric parietal cells, and thus reduced production of stomach acid and IF. Such damage can result in megaloblastic anaemia or, frequently, hypochromic anaemia due to impaired iron absorption caused by the resulting hypoacidic condition. Such damage occurs in patients with simple (non-autoimmune) atrophic gastritis as well as those undergoing gastrectomy. After bariatric surgery, 10–15% of patients develop vitamin B12 deficiency within a few years; all patients undergoing complete gastrectomy are placed in need of supplemental vitamin.
Chronic use of proton-pump inhibitors. Chronic inhibition of parietal cell acid production reduces the dissociation of vitamin B12 from the proteins to which it is bound in food matrices.
Pancreatic Insufficiency-The loss of pancreatic function can impair the utilization of vitamin B12. For example, about one-half of all human patients with pancreatic insufficiency show abnormally low enteric absorption of the vitamin. This effect can be corrected by pancreatic enzyme replacement therapy, using oral pancreas powder or pancreatic proteases.
Intestinal Diseases-Tropical sprue and ileitis can cause the loss of ileal IF receptors. Intestinal parasites such as the fish tapeworm can effectively compete with the host for uptake of the vitamin. Explosively growing bacterial floras can do likewise. Protozoal infections such as Giardia lamblia, which cause chronic diarrheoa, appear to cause vitamin B12 malabsorption in malnourished individuals.
Chemical Factors-Several other factors can impair the utilization of vitamin B12: Anti-diabetic medications (Biguanides), alcohol, and smoking can damage the ileal epithelium and cause the loss of ileal IF receptors. The use of contraceptive steroids has been shown to cause a slight drop in plasma vitamin B12 concentration; however, no signs of impaired function have been reported.
Vitamin B12 has no appreciable toxicity. Dietary levels of at least several hundred times the nutritional requirements are safe. High plasma levels of the vitamin are indicative of disease, rather than hypervitaminosis B12.
SOURCES OF VITAMIN B12
Because the synthesis of vitamin B12 is limited almost exclusively to bacteria, the vitamin is found only in foods that have been bacterially fermented and those derived from the tissues of animals that have obtained it from their ruminal or intestinal microflora. Animal tissues that accumulate vitamin B12 (e.g., liver) are therefore excellent food sources of the vitamin. The richest food sources of vitamin B12 are dairy products, meats, eggs, fish, and shellfish. The principle vitamers in foods are methylcobalamin, deoxyadenosylcobalamin, and hydroxycobalamin.
Trace amounts of the vitamin have been found in several vegetables, including broccoli, asparagus, and mung bean sprouts. This appears to reflect the ability of plants to take up vitamin B12 from organic fertilizers (5). While soybeans contain little, if any, vitamin B12, fermented soy products (e.g., tempe, natto) can contain significant amounts, likely due to the fermentation process. Some species of edible green algae and purple laver ( i.e., nori) contain large amounts of vitamin B12; however, studies have indicated that the vitamin is not available from those foods to humans. Edible cyanobacteria (Spirulina) are often cited as containing vitamin B12; however, they often contain large amounts of pseudovitamin B12 (7-adeninyl cyanocobamide), which is biologically inactive (6).
Human milk contains vitamin B12 almost exclusively bound to protein. Initial levels, decline by half after the first 12 weeks of lactation. Breast-milk vitamin B12 levels of women consuming strict vegetarian diets are less than those of women consuming mixed diets, and tend to be inversely correlated with the length of time on the vegetarian diet.
Vitamin B12 is bound to two vitamin B12-dependent enzymes and carrier proteins in foods. Therefore, the utilization of ingested vitamin B12 depends on the nature of the food/meal matrix and the host’s ability to release the vitamin and bind it to proteins that facilitate its absorption. The naturally occurring vitamin B12 in foods is bound in coenzyme form to proteins. The vitamin is released from such complexes on heating, gastric acidification, and/ or by the action of pepsin. Thus, impaired gastric parietal cell function, as in achlorhydria or with chronic use of proton pump inhibitors, impairs vitamin B12 utilization. Free vitamin B12 is bound to proteins secreted by the gastric mucosa. These proteins are found in human saliva, gastric juice, and intestinal contents. The saliva is the first to bind vitamin B12 released from the food matrix. Vitamin B12 binds proteins in the acidic conditions of the stomach, but the salivary form is normally digested in the small intestine to release the vitamin to be bound by intrinsic factor (IF). Patients with pancreatic insufficiency, and consequent deficiencies of enzymatic activities in the intestinal lumen, can achieve high concentrations of proteins that render the vitamin poorly absorbed.
Intrinsic Factor (IF)
IF is a glycoprotein synthesized and secreted in most animals (including humans) by the gastric parietal cells in response to histamine, gastrin, pentagastrin, and the presence of food. Individuals with loss of gastric parietal cell function may be unable to use dietary vitamin B12, as these cells produce both IF and acid, both of which are required for the enteric absorption of the vitamin. For this reason, geriatric patients, many of whom are hypoacidic, may be at risk of low vitamin B12 status. IF binds the four cobalamins (methylcobalamin, adenosylcobalamin, cyanocobalamin, and aquocobalamin) with comparable, high affinities under alkaline conditions. The binding of vitamin B12 by IF produces a complex to protect the vitamin from modification by intestinal bacteria, while also protecting IF from attack by pepsin and chymotrypsin.
Mutations in the IF gene have been identified that result either in failure of its expression or in expression of a defective protein incapable of binding vitamin B12. Affected individuals show normal gastric mucosa and acid production. They show megaloblastic anaemia within the first three years of life, which responds to large doses of vitamin B12 administered orally or by intramuscular injection.
The absorption of vitamin B12 is highly efficient and quantitatively important at low doses (1–2 μg). Such doses appear in the blood within three to four hours of consumption. Absorption of vitamin B12 involves the cellular uptake of the vitamin–IF complex, with IF being degraded and the vitamin being transferred to a specific carrier protein, transcobalamin (TC), for secretion into the portal circulation. Patients lacking IF have very poor abilities to absorb vitamin B12, excreting 80–100% of oral doses in the faeces.
Diffusion of the vitamin occurs with low efficiency (~1%) throughout the small intestine, and becomes significant only at higher doses. Such doses appear in the blood within minutes of comsumption. This passive mechanism is utilized in therapy for pernicious anaemia, in which patients are given high doses (500 μg/day) of vitamin B12. For such therapy, the vitamin must be given an hour before or after a meal to avoid competitive binding of the vitamin food.
Vitamin B12 is the best stored of the vitamins. Under conditions of non-limiting intake, the vitamin accumulates to very appreciable amounts in the body, mainly in the liver and muscles. Body stores vary with the intake of the vitamin, but tend to be greater in older subjects. The greatest concentrations of vitamin B12 occur in the pituitary gland; the kidneys, heart, spleen, and brain also contain substantial amounts – in humans. The great storage and long biological half-life (350–400 days in humans) of the vitamin provide substantial protection against periods of deprivation. The low reserve of the human infant (~25 μg) is sufficient to meet physiological needs for about a year.
Secretion of vitamin B12 in the bile is a major means by which it is recycled by reabsorption. Significant amounts (0.5–5 μg) of the vitamin enter the bile each day, and are thus made available for reabsorption from the gut (7).
Vitamin B12-deficient subjects show reduced availability of methionine. Methionine is essential for the synthesis of proteins and polyamines, and is the precursor of S-adenosylmethionine (SAM), which serves more than 100 enzymatic reactions that have critical roles in metabolism. SAM also serves as a key regulator of the folate-dependent enzyme methylenetetrahydrofolate reductase (MFTHR). Losses of SAM lead to impairments in the synthesis of creatine, phospholipids, and the neurotransmitter acetylcholine, all of which have broad impacts on physiological function.
Interrelationship with Folate
Methyl-B12 serves as the immediate methyl donor for converting homocysteine (Hcy) to methionine. Without adequate vitamin B12 to accept methyl groups from 5-methyl-FH4, that metabolite accumulates at the expense of the other metabolically active folate pools. This is known as the “methyl-folate trap.” This blockage results in the accumulation of the intermediate formiminoglutamic acid (FIGLU). Thus, an elevated urinary FIGLU level after an oral histidine load is diagnostic of vitamin B12 deficiency.
VITAMIN B12 IN HEALTH AND DISEASE
Neural Tube Defects (NTDs)
It is possible that vitamin B12 may be involved in the residual incidence of NTDs not prevented by folate supplementation. Studies have shown lower vitamin B12 levels in amniotic fluid from NTD pregnancies compared to healthy ones, while mothers’ serum vitamin B12 levels are comparable. This suggests a limitation in the maternal capacity to provide the fetus with an adequate supply of the vitamin (8).
Subclinical vitamin B12 deficiency can result in a moderate to intermediate elevation of plasma Hcy concentrations. This condition, called homocysteinemia, can also be produced by folate deficiency. Vitamin B12 deficiency may be the primary cause of homocysteinemia in many people; almost two-thirds of elderly subjects with homocysteinemia also show vitamin B12 deficiency.
Epidemiologic studies show associations of moderately elevated plasma Hcy and risks of coronary, peripheral, and carotid arterial thrombosis and atherosclerosis; venous thrombosis; retinal vascular occlusion; carotid thickening; and hypertension. A meta-analysis of the results of 27 cross-sectional and case–control studies attributed 10% of total coronary artery disease to homocysteinemia (9). However, the results of clinical trials have shown that Hcy-lowering by folate treatment does not reduce cardiovascular disease risk, casting doubt on the role of Hcy in the etiology of cardiovascular disease. It has been suggested that cardiovascular pathogenesis may actually involve the metabolic precursor of Hcy, S-adenosylhomocysteine (SAH). SAH is reversibly converted with Hcy, the equilibrium favoring SAH.
Neuropathies and neurological abnormalities develop in most species as a result of vitamin B12 deficiency. . They typically manifest with relatively late onset, due to the effective storage and conservation of the vitamin. Neurological lesions of vitamin B12 deficiency involve diffuse and progressive nerve demyelination, manifested as progressive neuropathy (often beginning in the peripheral nerves) and progressing eventually to the spinal cord. The neurological signs of vitamin B12 deficiency in humans include peripheral neuropathy, characterized by numbness of the hands and feet, and losses of proprioreception and vibration sense of the ankles and toes. Associated psychiatric signs can also be seen: memory loss, depression, irritability, psychosis, and dementia. Marginal deficiencies of vitamin B12 are estimated to be at least 10-fold more prevalent than frank deficiencies characterized by classical signs.
Insufficient vitamin B12 status is thought to lead to neurodegeneration as a result of abnormal neuronal lipids including those in myelin sheaths, and reduced synthesis of choline, the precursor of the neurotransmitter acetylcholine.
Serum Hcy levels have been found to be negatively correlated with neuropsychological test scores. Low serum vitamin B12 levels have been associated with poor or declining cognition in older subjects, and have been observed more frequently in patients with senile dementia of the Alzheimer’s type than in the general population. Nevertheless, there is little evidence that vitamin B12 treatment can improve cognitive function in most impaired patients, although a review of clinical experience in India suggested value of the vitamin in improving frontal-lobe and language function in patients (10).
Low plasma levels of vitamin B12 (and folate) have been reported in nearly a third of patients with depression, who also tend to show homocysteinemia. A recent study suggested that patients with high vitamin B12 status may have better treatment outcomes, but randomized clinical trials of vitamin B12 treatment have not been reported (11).
It has been suggested that low vitamin B12 status may exacerbate multiple sclerosis by enhancing the processes of inflammation and demyelination, and by impairing those of myelin repair. Pertinent to this hypothesis are the results of a study that found combination therapy with interferon-β and vitamin B12 to produce dramatic improvements in an experimental model of the disease.
That vitamin B12 may affect bone health was suggested by the finding that osteoporosis was more prevalent among elderly Dutch women of marginal or deficient status with respect to the vitamin (12). Subsequent studies have revealed positive associations of serum vitamin B12 level and bone mineral density, markers of bone turnover, and risks of osteoporosis and hip fracture. It has been suggested that homocysteinemia may underlie these relationships. Intervention with both vitamin B12 and folate, which reduced plasma Hcy, reduced hip fracture risk (13).
Serum vitamin B12 levels have been reported to be lower in subjects with hearing loss compared to normal-hearing controls, and vitamin B12 supplementation has been reported to lessen tinnitus in chronically affected subjects (14).
Cobalamins can bind cyanide, to produce the non-toxic cyanocobalamin. For that reason hydroxocobalamin is a well recognized cyanide antidote. Thus, it has been proposed that vitamin B12 may have a role in the inactivation of the low levels of cyanide consumed in many fruits, beans, and nuts.
Vitamin B12 deficiency causes delay or failure of normal cell division, particularly in the bone marrow-which manifests itself as a characteristic type of anaemia in which such enlarged cells are found (megaloblastic anaemia)-and also in the intestinal mucosa (15).
Some clinical signs (e.g., macrocytic anaemia) can result from deficiencies of either vitamin B12 or folate. The only metabolic process that is common to the two vitamins is the methyl group transfer from 5-methyl-FH4 to methylcobalamin for the subsequent methylation of Hcy to yield methionine and the return of folate to its most important central metabolite, FH4. Thus, deficiencies of either vitamin will reduce the FH4 pool either directly by deprivation of folate or indirectly via the methyl-folate trap, resulting from deprivation of vitamin B12. This limits the production of thymidylate and thus of DNA, resulting in impaired mitosis, which manifests itself as anaemia.
While supplemental folate can mask the anaemia associated with vitamin B12 deficiency by maintaining FH4 in spite of the methyl-folate trap, supplemental vitamin B12 does not affect the anaemia (or other signs) of folate deficiency. Although such signs as macrocytic anaemia, and subnormal circulating folate concentrations are therefore not diagnostic for either vitamin B12 or folate deficiency (these deficiencies cannot be distinguished on the basis of these signs), the urinary excretion of methylmalonic acid can be used for that purpose.
Pernicious anaemia is a disease of later life, 90% of cases being diagnosed in individuals 40 years of age. The anaemia is the end result of autoimmune gastritis, also called type A chronic atrophic gastritis or gastric atrophy. This causes progressive atrophy of parietal cells leading to hypochlorhydria and loss of IF production, and resulting in severe vitamin B12 malabsorption. The condition presents as megaloblastic anaemia within two to seven years. The disorder is likely to be widely underdiagnosed, as affected subjects may have neurological rather than haematological disease.
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Keywords-vitamin B12, neural tube defects, cardiovascular disease, cognitive function, depression, multiple sclerosis, osteoporosis, tinnitus, nervous system (neuropathy), anaemia