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. 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. 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.