The entire complex of B vitamins is essential for health, and they're known as the B complex for the very good reason that they interact closely together in the performance of all their vital functions. All the vitamins of the complex are usually found together in food, and since all are water soluble, they may also be excreted together from the body. A deficiency in one of the complex is therefore usually accompanied by a corresponding deficiency in each of the others.
That said, there are particular deficiency characteristics which can be identified for each B vitamin.
Vitamin B12 (sometimes known as cobalamin because of the cobalt ion in its composition) is one of the most important of the complex, and also one of those in which deficiencies are most likely. These deficiencies are largely attributable to difficulties in absorption of the vitamin, which are a particular problem for the elderly.
Vitamin B12 is required for the body's manufacture of the essential amino acid, methionine, and the associated enzyme, methionine synthase. An inadequate supply of this enzyme may lead to an excess of homocysteine, a naturally occurring protein within the body, which has been associated with an increased risk of cardiovascular disease. At least 80 separate studies have noted this association and it seems that the increase in risk is not negligible. In fact a large study in Europe in 1997 found that people with blood homocysteine levels in the top 20% of the range faced a more than doubled risk of cardiovascular disease.
Whilst elevated homocysteine levels are strongly associated with a deficiency of vitamin B12, deficiencies of vitamins within the B complex are seldom if ever found in isolation, and are therefore seldom remedied by single vitamin supplementation. It is not just vitamin B12, but vitamins B6 and folic acid, for example, that have been found to be associated with excess homocysteine, and which should therefore always be taken together, preferably as part of a supplement containing the whole B complex.
Given that elevated homocysteine levels are accepted as a risk factor for cardiovascular disease, and that supplementation with the vitamins mentioned above is known to reduce homocysteine levels, you might think it only commonsense to suppose that an adequate intake of these vitamins, including B12, would be a protector against cardiovascular disease. And there are many nutritionists, of course, who strongly agree with this apparently logical position. Perhaps surprisingly, then, conventional medicine remains reluctant to accept it pending the outcome of further large scale placebo trials.
Science must have its definitive answer no doubt, and it is a quest which must be respected, but in this case, given the relatively small amounts of these vitamins required by the body, and the absence of any reported toxicity or adverse side effects, it might be wondered whether from the point of view of the seeker after health, particularly those no longer in the first flush of youth, it is really worth running the risk of a deficiency.
And even orthodox opinion recognises that the risk is real, even though it is unlikely to be caused by an inadequate dietary supply. The Recommended Dietary Allowance (RDA) for vitamin B12 is only 2.4 mcg a day, an amount which should be readily obtainable, except perhaps for those following a strict vegetarian regime, given that a single 3 oz serving of fish or red meat may provide this quantity. For those with a taste for it, sea food may provide a great deal more, and chicken, turkey, eggs, milk and cheese are also useful, though less lavish, sources.
Deficiencies may well result, however, from problems with the absorption of the vitamin, particularly in older people, and it has been estimated that deficiency may affect 10% -15% of individuals over the age of 60.
This is because good absorption of vitamin B12 from food is heavily dependent on the normal stomach acid and digestive enzymes, the quantity and effectiveness of which decreases substantially as the body ages, and on the presence in the stomach of a specialised protein known as Intrinsic Factor . The correct action of Intrinsic Factor requires the presence of adequate calcium in the body, another nutrient in which the elderly, of course, are notoriously likely to be deficient.
Absorption of B12 from supplements is much less problematic, however, because stomach acid and digestive enzymes are not required to release the vitamin from its protein bindings. So this is a rare case in which even conventional medical "wisdom" recognises the value of supplementation, at any rate for the over 50s.
But given that sub-optimal levels of vitamin B12, if not outright deficiencies, are commonly found even in younger individuals, and that cardiovascular disease is a problem which may develop insidiously and without symptoms over many years, there would seem to be strong arguments for supplementation even amongst the wider population.