Trace and Ultratrace Elements

Trace elements are elements of the periodic table that occur in the body in microgram per gram (μg g −1 ) amounts or less. Essential trace elements are usually required in milligrams per day (mg day −1 ) amounts. Ultratrace elements is a term sometimes used for elements required in micrograms per day (μg day −1 ) amounts; the term also has been applied to elements found to have beneficial bioactivity in humans, but not generally accepted as essential because they lack a defined biochemical function. This article describes the biological roles, homeostatic regulation, nutritional importance, requirements, and toxicity of the trace elements copper, manganese, and the ultratrace elements iodine, molybdenum, chromium, selenium, boron, and silicon for humans.