Metals generally regarded as essential for human health in trace amounts include iron, zinc, copper, manganese, chromium, molybdenum and selenium. They are essential because they form an integral part of one or more enzymes involved in a metabolic or biochemical process. The primary role of such elements is as a catalyst, and only trace amounts are necessary for cellular function. These metals are widely found in nature, particularly in various mineral deposits and soils, meaning that they are available to be taken up by plants and animals that serve as food sources for humans. (Of course, there are many other metals – for example, calcium and sodium – that are essential for human health but which are not viewed by nutritionists as metals.)
Criteria for essentiality for human health are that withdrawal or absence of the metal from the diet produces either functional or structural abnormalities, and that the abnormalities are related to, or are a consequence of, specific biochemical changes that can be reversed by the presence of the essential metal. To establish such criteria requires an understanding of the metal as well as sensitive instrumentation to measure the metal and its biological effect. A characteristic associated with essential metals is that the body provides homeostatic mechanisms that increase or decrease uptake and excretion as needed to maintain the necessary levels in the body. Adequate amounts are particularly important during pregnancy, and for infants and children during periods of rapid growth.
Iron is an essential constituent of haemoglobin, myoglobin and a number of enzymes. Iron is stored in body tissues to supply body needs. However, deficiency may occur from inadequate dietary intake or blood loss which results in anaemia and loss of well-being. Deficiency in infants and young children increases susceptibility to infection and impairment of growth.
Zinc is a constituent of over 300 enzymes involved in numerous body functions, including enzymes involved in gene expression. Deficiency impairs cell growth and repair of tissue injury. Meat, liver, eggs and seafood are good dietary sources of zinc, whereas zinc in vegetable sources, particularly in cereal grains, is less bioavailable than from meat sources.