Vitamin B12 (a.k.a. cobalamin, cyanocobalamin) is an essential, water-soluble group of biochemically related compounds required for normal blood cell formation and various neurological functions.
      More specifically, the cobalamin related compounds are involved in two major biochemical pathways that involve the conversion of coenzyme A (CoA) into its active component, succinyl-CoA, and the conversion of homocysteine to methionine, a process in which folate is also involved.

      The recent National Academy of Sciences Dietary Reference Intakes study has reported a Recommended Dietary Allowance (RDA) for Vitamin B12 of 2.4 mcg/day for adults, an amount somewhat less than the current U.S. Food and Drug Administration’s RDI of 6 mcg. Although a thorough and statistically accurate report, this new, lower recommended daily intake appears to be somewhat unsupportive of other, concurrent findings for this vitamin (see Supplements below).

      Vitamin B12 is derived almost entirely from animal products and foods fortified with B12, with very little available from vegetable sources. The amount of this vitamin that is actually absorbed in the human gastrointestinal tract varies greatly depending upon its food source, so establishing dietary intake from reported amounts of B12 in one food to another can be somewhat misleading.
      Among the meat, poultry and fish sources in which B12 is found in the highest concentrations, the most efficiently absorbed are lamb, chicken, trout, eggs, liver, and shellfish. Other dairy products, especially fresh milk, and fortified foods such as breakfast cereals, are other good sources of this vitamin.

      The primary result of vitamin B12 deficiency is the body’s inability to produce adequately formed and functional red blood cells. The early symptoms of this disorder can be similar to other, unrelated anemias: general fatigue, lack of energy, shortness of breath, etc. In some cases, observable symptoms include only subtle neurological abnormalities, such as tingling or numbness in the extremities, motor problems, such as abnormal gait, and in some cases, lack of concentration, memory loss or even dementia. The condition ultimately leads to a disease known as pernicious anemia. (See folate deficiency for a similar red blood cell disorder.)
      This disorder can result from either prolonged, inadequate dietary intake of vitamin B12, common in strict vegetarians who have not supplemented their diet with this vitamin or, more commonly, from malabsorption of B12 due to the absence of a compound known as intrinsic factor, as well as other forms of malabsorption found in adult populations. Left untreated, the disease can lead to irreversible neurological damage, and ultimately life-threatening anemia.
      Another common cause of deficiency is food-bound malabsorption, a condition which tends to occur in some 10% to 30% of adults over the age of 50 years who are unable to properly absorb the naturally occurring vitamin B12 present in their diets.
      Individuals with an otherwise normal diet and, in particular, a normal dietary consumption of folate, should be particularly cautious as vitamin B12 deficiency is much more difficult to assess and diagnose in the presence of normal folate levels.

      In North America, vitamin B12 is generally available as cyanocobalamin, either as a standalone supplement, a component of a B complex or, most commonly, as part of a multivitamin.
      Supplemental vitamin B12 is probably a wise choice for many individuals. Diets that are either strict or primarily vegetarian, low in meat and meat based products for reasons of fat and cholesterol content, or those that are simply inadequately absorbed, all have something to gain by dietary supplements of this vitamin.
      Additionally, there is mounting scientific evidence that supplemental vitamin B12, particularly in combination with other vitamins of the B complex (folate and vitamin B6, in particular), may have a significant effect on reducing the risk of heart disease (see Current Issues).

      There appears to be little risk associated with excess vitamin B12 intake from dietary and supplemental sources. This is due, in part, to the gastrointestinal tract’s inability to effectively absorb large amounts of this vitamin ingested at one time.
      Although there are some conflicting reports about the effects of excessively high intakes of this vitamin, extremely large amounts of vitamin B12 administered in clinical situations as treatment for pernicious anemia support the judgement that moderate to large amounts of B12 in supplemental form are relatively safe for normal, healthy adults.
      The most recent National Academy of Sciences Dietary Reference Intakes study has not set a Tolerable Upper Intake Level (UL) for vitamin B12 because of the lack of consistency across the range of work that has been done on this vitamin. They have, however, established an “exposure assessment level” (an amount that appears safe based on reports of supplemental consumption in mixed gender and age groups) of 17 mcg/day. At the extreme, approximately double that amount was noted to be absent of ill effects in pregnant females of mixed age.

      Cardiovascular disease. The relationship between some forms of cardiovascular disease and high levels of homocysteine has been demonstrated in numerous studies dating back to the 1970s. The risk of coronary heart disease, for example, has repeatedly been shown to be greater in the presence of high homocysteine levels, even in the absence of tobacco smoking.
      Although there are congenital causes of homocystinuria (high levels of homocysteine) that are not related to diet, it has been well documented that vitamin B12, as well as vitamin B6 or folate deficiency, can, and does lead to markedly elevated levels of homocysteine which, in turn, increases the risk potential of coronary heart disease.

      Vegetarian diets. It is a well established fact that strict vegetarians are at risk for vitamin B12 deficiency, due to the relative absence of the vitamin in this type of diet. Left unchecked, such a diet can lead to serious medical problems that may not be reversible. Even Alzheimer's disease-like dementia has been diagnosed as the result of prolonged vitamin B12 deficiency (see Deficiencies). The best remedy for the avoidance of diet-induced deficiency for those who are at risk is to supplement the diet with vitamin B12 fortified foods and dietary supplements containing this vitamin.

      Elderly individuals. Elderly individuals are typically subject to any number of gastrointestinal changes that are, by themselves benign, but result in the inability to properly absorb food-bound vitamin B12. Modification of the diet to include more vitamin B12 in situations of this kind is not the remedy. Since most individuals so affected may not be aware of such changes, it is advised that older people consider a supplemental source of this vitamin as part of their normal diet. It is estimated that upwards to 30% of the population over 50 years of age in North America are so affected, and some studies suggest that the observation is more prevalent in men than women.

      Smoking. There have been a number of reports suggesting that the by products of tobacco smoke may have a negative effect on vitamin B12 metabolism. While some smokers were seen to have lower vitamin B12 levels than some non-smokers, the evidence from the research in this area has not been able to establish a causative effect between smoking and vitamin B12 deficiency.