COMMENT:TOO SHORT A YARDSTICK

The original recommendations of the US National Research Council in l94l, which first proposed the Recommended Daily Allowances of nutrients, were aimed at “the building up of our people to a level of health and vigour never before attained or dream

Five decades on, during a relatively affluent peacetime,at least 50 per cent of the British and American population fail to meet the official dietary guidelines. The DHSS Report “The Diets of British Schoolchildren” revealed that, in certain age/sex subgroups, as many as 95 per cent of those studied did not take in sufficient iron, and 96 per cent failed to meet the (US) RDA for vitamin B6.

The figures for RDAs are essentially derived from two sets of data: the quantity of the nutrient required to maintain metabolic balance in the long term, and the quantity required to prevent the development of the specific deficiency syndrome.

The use of nutritional balance as a measure of nutritional adequacy has a number of problems. Some studies of B6 levels have shown that while a saturation can be achieved normally after about three weeks of taking a supplement, there appears to be increased enzyme synthesis the more B6 is available. When examined the other way round, this proves the theory, often pointed to in third world nutrition, that the body adapts to a reduced supply of nutrients by restricting turnover of the relevant molecules. To use balance as the yardstick, therefore, may mean starting with a short yard.

Using the prevention of specific deficiency diseases as the yardstick displays similarly flawed logic. With vitamin C, for example, it’s known that approximately l0 mg daily can prevent or cure overt scurvy in adults. But neither the UK nor US RDA (30 mg and 60 mg for men, respectively) takes into account the possibility that ascorbic acid may be necessary for functions other than the mere prevention of the disease, including the neuro-psychological. Kubala & Katz (Journal of Genetic Psychology l960) found that the intelligence of schoolchildren is positively correlated with vitamin C status well above that of RDA levels. Cheraskin & Ringsdorf, who studied over l000 doctors and their wives, on the other hand, found that health problems diminished progressively up to an intake level of 500 mg of vitamin C daily. Other evidence suggests that an intake as high as l g daily is advisable, for, say, those with identifiable cardiovascular risk factors.

The picture becomes more complex, and nutrient intake requirements pushed up even further, when you consider those factors which can impair absorption or enhance the breakdown or excretion of nutrients. Besides the increased vitamin C requirements in infection and stress, it is reasonably well established that smoking increases requirements.

Caffeine impairs levels of calcium, magnesium, sodium, potassium, phosphorus and iron. Potassium losing diuretics also cause loss of magnesium. The ways in which oral contraceptives interfere with nutritional status are too numerous to list.

What this all points to is that we are biochemical individuals. One set of researchers found out the the individual requirements of vitamin C in guinea pigs ranged at least twenty fold; the range of human individual requirements can be assumed to be at least as great. For vitamin B6 and thiamine, the established range seems less, around fourfold. But even a fourfold variation means that there will always be some individuals who are inadequately nourished whatever the target intake of the population as a whole.

It seems, therefore, that the human factor invariably defeats the attempts of nutritionists to generalize. RDAs may be valued servants of the food industry and central planning, in peace as in wartime, but the individual is ill served by them. Thus far, a system for individuals has yet to be developed.

Dr. Damien Downing

Excerpted from The Journal for Nutritional Medicine.