These inorganic micronutrients invariably have a catalytic function and are found in the metabolic pathways controlling the assimilation and utilization of other nutrients, in the synthesis of new tissues and in the use of energy. Deficiencies are therefore more likely to appear in tissues that have a high rate of metabolism or rapid turnover, such as the lining of the intestine, the skin, the immune system, the liver, the parts of the body that contribute to growth in children, and the developing foetus.

Deficiencies of chromium, molybdenum, and possibly manganese have been observed in experimental studies, but not as yet in the general population. Deficiencies of iron, iodine, zinc, copper and selenium have been observed in the human population.

It has been estimated that 600 million people suffer from iron deficiency and 200 million from iodine deficiency. These deficiencies have an impact on neurological development and are therefore of considerable socio-economic importance. A number of international programmes have been established to eliminate these problems; iodine supplementation has been particularly successful, and selenium supplementation has virtually eliminated a particular form of fatal heart disease that occurred extensively in China. Deficiencies of this sort are most evident in infants, children and pregnant women.

The geographical variation in the types of chronic malnutrition which occur in the world are thought to be the result of deficiencies of various micronutrients. Traditionally, these types of malnutrition have been associated with inadequate intakes of protein and of energy (calories). However, the best sources of protein are also the best sources of essential micronutrients such as iron, zinc and copper. Thus, the images of malnutrition shown with famine relief publicity are really syndromes of multiple nutritional deficiencies. Often these are due not only to inadequate diets, but also to infections and gastrointestinal upsets causing the body to lose many of its essential nutrients. It is therefore not surprising that many children being rehabilitated from malnutrition fare much better when they are given zinc and copper supplements as part of their treatment.

Mild deficiency of zinc stunts growth and, to a certain extent, maturation in children, especially boys. A variety of supplementation trials now suggest that zinc deficiency exists in disadvantaged populations, irrespective of whether they are in the developed or developing world. Copper deficiency has been described in malnutrition syndromes and in babies who have been fed inappropriately. Some scientists argue that copper deficiency might contribute to the causes of heart disease and osteoporosis in adults, but this is a contentious issue.

While it may be tempting to initiate multi-element supplementation programmes to avoid any risk of deficiency of these nutrients, such a practice can actually create problems - i.e. the physico- chemical similarities of iron, zinc and copper can cause the micronutrients to interfere with each other's absorption and mutually induce further deficiencies.

One major problem is that we do not really know how much of these micronutrients are needed in our diet. There is no good way to detect deficiencies of the micronutrients, other than being vigilant for features of deficiency under circumstances associated with such problems.