The functions of thiamine within the body are highly complex biochemically, but what's important to know is that thiamine and its associated enzymes are essential for the body's production of energy from food. As always with the B complex vitamins, however, the proper functioning of thiamine depends on an adequate supply of the other members of the complex, and the performance of the thiamine related enzymes in this case is particularly dependent on the associated vitamins, riboflavin (B2) and niacin (B3).

That said; there is a characteristic disease of severe thiamine deficiency, beriberi, which has been recognised for several thousand years. This disease should never now be seen outside medical text books in affluent Western societies, but alcoholics and heavy drinkers, for whom the absorption of adequate thiamine presents particular problems, frequently show symptoms.

Beriberi is regarded as having "wet" and "dry" forms, the symptoms of the former being principally observed in problems with the cardiovascular system, including severe fluid retention and in severe cases even congestive heart failure. So-called "dry" beriberi is characterised by problems with the nervous system, particularly the peripheral nerves of the limbs, which may lead to pain and weakness in the muscles.

Beriberi may also have serious effects on the brain, partly through increased free radical activity, leading in extreme cases to conditions known as Wernicke's encephalopathy and/or Korsaloff's amnesia or psychosis. Wernicke's is identified by characteristic physical nervous "ticks", especially unusual movements of the eyes, whereas Korsaloff's is the term applied when these symptoms are accompanied by severe amnesia.

If you think these symptoms remind you of the archetypal "street wino" you'd be right, because in advanced societies they're most commonly found in alcoholics and heavy drinkers, supporting the theory that malnutrition is a major contributor to their problems. It makes sense that this should be so. For not only do such people tend to have very inadequate diets, but their damaged livers also struggle to metabolise the few nutrients which they do take in. Alcohol, of course, is also known as a powerful diuretic, and when you consider that thiamine, in common with the other vitamins of the B complex, is highly water soluble, and easily excreted by the body, you have a potent recipe for nutritional disaster.

So how much thiamine do you need to avoid this disaster?

As always, the Recommended Dietary Allowance (RDA) for thiamine (most recently established in 1998) is set at the level designed to prevent deficiencies in normally healthy people. But of course, the prevention of deficiency is not at all the same as thing as ensuring optimum health, and the RDAs for thiamine are therefore set at the very low levels of 1.2 mg for men and 1.1 mg for women. As the slight differential suggests, higher intakes are required in proportion with higher bodyweight, and particularly muscular bodyweight. An increased intake of 1.4 mg is also suggested for pregnant women.

A number of common every day foods provide good sources of thiamine. A serving of fortified breakfast cereals, for example, may provide 0.5 ? 2mg, a single cup of wheatgerm 4 or more mg. A 3 oz serving of pork will contain up to 0.75 mg, lentils, peas, brown or enriched white rice 0.2 mg, and a slice of wholemeal bread 0.1 mg.

These figures would seem to suggest that most people should have little difficulty in achieving their RDA. But the problem is that thiamine is notoriously fragile, and almost any type of processing of these foods, including boiling or even toasting bread may dramatically reduce thiamine content.

So it's perhaps not surprising that research suggests average intakes in Western societies may be as low as 2 mg a day for men and 1.2 mg for women. These figures are worryingly close to the RDAs which, as noted, are in any case set at a level only designed for the avoidance of outright deficiency. Being averages, it likely follows that there must be many people who routinely fall below them, and there are also factors to be considered which may dramatically increase the body's demand for thiamine and therefore the risk of deficiency.

As well as the consumption of alcohol, these include intensive physical exercise, normal growth in adolescence, pregnancy and breast feeding, and feverish illnesses, particularly malaria. As with alcohol, heavy intakes of tea and coffee have been shown to have a severely depleting effect on the body's levels of thiamine, and this is due to so-called "anti-thiamine factors", in addition to the loss of the water soluble vitamin which may be attributed to the diuretic effects of these drinks. There is also evidence that older people may struggle to absorb sufficient quantities of this nutrient even when their diet appears adequate.

There is no recommended upper safe limit for the intake of thiamine, any excess being easily excreted by the body, and no known toxic effects. Supplementing with thiamine is therefore recommended for the vulnerable groups listed above, which in fact comprise a significant proportion of the supposedly well population, and may well be beneficial for all who seek optimal health and maximum energy levels. The close interdependency of the B vitamins, however, means that thiamine should be taken as part of a supplement containing the whole complex. The proper functioning of the vitamin also requires the presence of adequate minerals, particularly magnesium.