Beginning early in the twentieth century, scientists were able to qualitatively detect small amounts of several elements in living organisms. In reports, these elements were often described as being present in “traces” or “trace amounts,” and within a short time, they became known as the trace elements. Today, trace elements are understood to be those elements of the periodic table that occur in the body in micrograms per gram (μg/g) of body weight or less. They may be essential – that is, they may be indispensable for growth, health, and completion of the life cycle – or they may be nonessential: fortuitous reminders of our geochemical origins or indicators of environmental exposure. Some of the nonessential trace elements can be beneficial to health through pharmacologic action, but all are toxic when consumed in excess.

An essential element is defined by many in the scientific community as an element whose dietary deficiency consistently results in a suboptimal function that is preventable or reversible by physiological amounts of the element. However, other experts accept an element as essential only if it has a defined biochemical function. Thus, there is no universally accepted list of trace elements that are considered essential.

The essential trace elements are usually required by humans in amounts indicated by milligrams per day (mg/d). In 1980, the term “ultratrace element” began to appear in the literature; this was defined as an element with a daily dietary requirement of some 50 nanograms per gram (ng/g) for animals (Nielsen 1980). For humans, however, the term often is used to indicate elements with an established, estimated, or suspected requirement of less than 1.0 mg/d (generally indicated as micrograms per day, or mg/d) (Nielsen 1994b).