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.
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. He has authored about 150 research papers, chapters and reviews, and edited three books on the toxicology of metals.