Ultratrace elements are chemical elements, derived from diet, which appear to play significant - and sometimes essential - biological roles in animals, including humans.
They typically comprise less than one microgram per gram of a given organism (i.e. less than 0.0001% by weight).
Deficiencies can cause physiological problems, sometimes severe in nature. For example, chromium deprived humans have deficient insulin binding to red blood cells.
In many cases, the reasons for their essential necessity is unknown.
“The term ultratrace element has been defined as an element with an established, estimated, or suspected requirement generally indicated by μg/day for humans. Between 1970 and 1984, it was suggested that 11 elements should be added to the list of ultratrace elements that included chromium, molybdenum, and selenium; these elements were arsenic, boron, bromine, cadmium, fluorine, lead, lithium, nickel, silicon, tin, and vanadium.
Since 1984, it has been suggested that three more elements, aluminum, germanium, and rubidium, should be added to the list, and circumstantial evidence has continued to accumulate which indicates that several of the ultratrace elements in addition to iodine and selenium, particularly arsenic, boron, chromium, nickel, silicon, and vanadium, are more important in nutrition than currently acknowledged.”
Source: Ultratrace elements in nutrition: current knowledge and speculation United States Department of Agriculture
Also see : Chromium as a micronutrient