COMMENTARY — Pass the molybdenum, please

Metals generally regarded as essential for human health in trace amounts include iron, zinc, copper, manganese, chromium, molybdenum and selenium.

These metals are essential because they form an integral part of one or more enzymes involved in a metabolic or biochemical process. Such elements act 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, where they then become part of the food chain, ultimately to be ingested by humans. (There are, of course, other metallic elements that are essential to human health, such as calcium and sodium.

However, nutritionists do not regard these as metals, per se.) A characteristic associated with essential metals is that the body provides homeostatic mechanisms (internal stabilizers), which 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 hemoglobin, myoglobin and a number of enzymes. Iron is stored in body tissue to supply body needs. However, deficiency may occur from inadequate dietary intake or blood loss, which results in anemia and loss of well-being. Deficiency in infants and young children increases susceptibility to infection and impairment of growth.

Zinc is a constituent of more than 300 enzymes involved in numerous body functions, including gene expression. Zinc deficiency impairs cell growth and the repair of injured tissues. Meat, eggs and seafood are good dietary sources of zinc, whereas the metal in vegetable sources, particularly in cereal grains, is less readily used by the body as those from meat sources.

Copper is also essential for numerous enzymes, and is a constituent of hair and elastic tissue contained in skin, bone and other 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. Abnormalities observed in copper-deficient animals include anemia, 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. Deficiency in animals results in poor reproductive performance, bone and cartilage abnormalities, and defects in glucose metabolism.

Chromium in its 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, and is closely related to the antioxidant activities of vitamin E.

— From the newsletter of the International Council on Metals and the Environment.