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.
Copper is also essential for numerous enzymes and is a constituent of hair and of elastic tissue contained in skin, bone and other body organs. There are a number of important copper-containing proteins and enzymes, some of which are essential for the proper utilization of iron. Dietary deficiency is rare, but does occur in certain acquired or hereditary disorders that impair intestinal absorption. Several abnormalities have been observed in copper-deficient animals, including anaemia, skeletal defects and degeneration of the nervous system.
Manganese activates several enzyme systems in vitro and is an essential component of two important mitochondrial enzymes. Dietary sources are whole grains, cereal products, fruit and vegetables. Manganese deficiency is practically unheard of in humans, probably because of strong homeostatic mechanisms, but deficiency in animals results in poor reproductive performance, abnormalities of bone and cartilage, and defects in glucose metabolism.
Chromium in its trivalent (natural) form is required for maintaining normal glucose metabolism and as a cofactor for insulin. Molybdenum is required for the function of several enzymes, and its content in food varies with soil conditions. Selenium is a more recent addition to the list of essential metals. It is a component of an important enzyme, glutathione peroxidase, and is closely related to the antioxidant activities of vitamin E.
There are other metals that have been shown to be essential for plants, and lower forms of animal life and even other mammals, but essentiality for humans has not been convincingly demonstrated. Such metals include arsenic, nickel and boron. Arsenic deficiency depresses growth and impairs reproduction in laboratory animals and in chickens. Nickel deficiency results in decreased growth and formation of blood cells in several animal species. Boron deficiency appears to affect calcium and magnesium metabolism and may affect membrane function. Boron is essential for the growth of most plants.
About the author: Dr. Robert A. Goyer is Emeritus Professor of Pathology, University of Western Ontario (Canada) and former Deputy Director of the National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina. He has authored about 150 research papers, chapters and reviews, and edited three books on the toxicology of metals. He has served on numerous task groups for the International Programme on Chemical Safety of the World Health Organization and on various governmental and national research committees in the USA. Add Content...