First isolated in the 1930s, vitamin B6 is a highly water soluble vitamin of vital importance to human health. An adequate dietary supply is essential because the vitamin cannot be manufactured by the body, and is required for the proper action of more than 100 enzymes involved in vital biochemical reactions.

Amongst its many functions, in its coenzyme form of pridoxal 5'-phosphate (PLP), vitamin B6 is critical for the release of energy within the body, for the proper formation and function of oxygen carrying red blood cells, and for the synthesis of neuro-transmitting hormones within the brain.

There is also some evidence that PLP may play a role in mitigating the potentially harmful effects of the steroid (sex) hormones, such as oestrogen and testosterone, within the body, so that levels of vitamin B6 may even be related to the incidence of the characteristic cancers eg of the breast and prostate, which are most strongly associated with the hormones.

Perhaps most important, however, is vitamin B6's role in the regulation of the protein, homocysteine, even sightly raised blood levels of which have been repeatedly identified as a risk factor for cardiovascular disease and stroke. Homocysteine is a perfectly normal by-product of the body's digestion of protein foods and should normally be removed harmlessly from the body. But the risk of excess build up is not negligible.

A large 1997 European study of young and middle aged adults showed a more than doubled risk of cardiovascular disease and stroke for individuals whose blood homocysteine levels were in the top fifth of the normal range. And some sources attribute as many as 10% of heart attack fatalities and an even higher proportion of stroke deaths directly to high homocysteine levels. Since these are still two of the biggest causes of premature death in the affluent Western world, such figures are particularly alarming.

The B vitamin complex is known to be vital for the prevention of excess homocysteine. Vitamins B12 and folic acid operate to convert homocysteine into the vital amino acid, methionine; whilst B6 in the form of PLP converts it into another amino acid, cysteine.

Not surprisingly then, research studies have demonstrated a clear link between low intakes of vitamin B6, higher levels of blood homocysteine and enhanced risk of cardiovascular disease. One such study, in particular, showed that the daily consumption of 4.6 mg of vitamin B6 reduced the incidence of heart disease by more than a third as compared with the consumption of 1.1 mg (slightly below the 1.3 mg Recommended Dietary Allowance (RDA) for adults under 50).

In addiition to its important role in regulating blood homocysteine levels, there is also good evidence to associate vitamin B6 with the proper functioning of the immune system and indivduals deficient in B6 have been shown also to have reduced numbers of vital immune system cells. It is notable, too, that the amounts of vitamin B6 required to restore optimum immune function, particularly in the elderly, have been shown to be well in excess of the RDA. Other benefits of vitamin B6 for which there is some research evidence include improved memory, reduced incidence of kidney stones, the alleviation of pre-menstrual syndrome, the side effects of the contraceptive pill and morning sickness in pregnancy.

Severe deficiencies of vitamin B6 are thought by most conventional medical sources to be highly unlikely in Western societies and indeed the following food values would suggest that the RDAs of 1.3 mg for adults and 1.7 mg for the over 50s should be easily achievable. A cup of fortified cereal will provide between 0.5 and 2.5 mg and even a single banana perhaps as much as 0.4 mg. Meat, particularly poultry such as chicken and turkey, will provide up to 0.5 mg per 3 oz serving, as will fish. Vegetables, especially potatoes, and nuts are also useful sources.

But despite this wide range of possible sources, surveys have shown that the typical adult's daily intake of B6 is only around 2 mg per day, and for older individuals this reduces to 1- 1.2 mg. Both figures are worrying, because they suggest not only that many older people are falling below the RDA, but that many younger adults are perilously close to doing so. It has to be remembered, too, that the RDAs are in any case at levels only sufficient to avoid outright deficiency, which is not at all the same thing as optimum health.

Being water soluble, vitamin B6 is easily excreted from the body and the consumption of alcohol and other diuretic drinks such as tea and coffee may dramatically increase the body's need for the vitamin. The requirement also increases in line with the amount of protein consumed in the diet.

For these reasons, even conventional medicine is inclined to recognise the value of supplementation with B6 up to a level of 2 mg per day. Some nutritional therapists, however, recommend doses of as much as 100 mg a day in view of B6's significant role in combatting the increased risk of heart disease and stroke attributable to elevated blood homocysteine levels.

And indeed there are no known toxic effects of vitamin B6 at this level of intake, which is in fact the upper safe limit recommended by the US Food and Nutrition Board. As always with the B complex vitamins, however, their close interdependence means that such doses should only be taken as part of a supplement containing the whole complex. And to enable the vitamins to do their work to maximum effect, a comprehensive multi-mineral supplement is also highly recommended.