The incidence of trace element deficiency diseases.

On present evidence, 26 of the 90 naturally occurring elements are known to be or are claimed to be essential for animal life. These consist of 11 major elements (carbon, hydrogen, oxygen, nitrogen, sulphur, calcium, phosphorus, potassium, sodium, chlorine and magnesium) and 15 elements generally accepted as trace elements (iron, iodine, copper, manganese, zinc, cobalt, molybdenum, selenium, chromium, nickel, tin, silicon, vanadium, arsenic and fluorine). Evidence for the essentiality of the last six of these, often referred to as the ‘ newer ’ trace ele­ ments, rests largely on their effects on growth or reproduction in laboratory species maintained on highly purified diets under strictly controlled environmental conditions in which atmo­ spheric contamination is minimized. Specific physiological functions, or involvement in vital enzyme systems, have not yet been identified for any of them except silicon, and it seems likely that the apparent essentiality of fluorine is due to a pharmacological effect in improving iron utilization in a diet only marginally deficient in iron (Tao & Suttie 1976). It seems unlikely that these ‘newer’ trace elements will be found to have much practical significance in the nutrition of man or domestic animals. Naturally occurring deficiencies have not been reported for nickel, tin, silicon, vanadium, arsenic or fluorine, although the well established beneficial effects of supplementary fluoride in reducing the incidence and severity of dental caries in man could be interpreted as evidence of a dietary deficiency of that element. Such reasoning does not appear valid to me because animals can remain free from dental caries with no fluoride supplementation, and human communities exist, for instance in parts of Papua-New Guinea, where caries incidence is nil and the fluoride intakes are no higher than in other areas where the disease is rampant. It is nevertheless dangerous to assume that the newer trace elements will never be found to be of practical dietary significance, because we know so little of their functions and minimum requirements and of the factors that influence their movements through the food chain from the soil to animals and man. It is salutary to recall that the essential trace elements, selenium and chromium, were originally believed to be only of scientific interest. It is now known that a naturally occurring deficiency of selenium affecting the growth and health of domestic livestock is widespread throughout the world and that chromium deficiency is not uncommon in children and adults consuming diets high in refined carbohydrates.