The metal Chromium - specifically Chromium(III), the trivalent ion - has been suggested by some researchers to be an essential micronutrient. Other groups claim that it isn't essential, but that its presence can be of benefit.

It's commonly found (in trace amounts) in the bodies of birds and mammals, including humans.

The biologically beneficial effects of Chromium(III) continue to be debated. Some experts believe that they reflect pharmacological rather than nutritional responses, while others suggest that they are side effects of a toxic metal. The discussion is marred by elements of negativity and occasionally becomes acrimonious. Chromium is accepted by the U.S. National Institutes of Health as a trace element for its roles in the action of insulin, a hormone critical to the metabolism and storage of carbohydrate, fat and protein. The precise mechanism of its actions in the body, however, have not been fully defined, leaving in question whether chromium is essential for healthy people.

Source : Wikipedia

The metal is (believed to be) transported around the body by a compound (in the form of a peptide protein) called Chromodulin. (also known as low-molecular-weight chromium-binding substance, or LMWCr). The way in which it does so, however, is unknown, as is its physical structure.

Further reading : Biological Trace Element Research volume 166, pages7–12(2015) and The Protein Journal volume 27, pages371–375(2008)

Also see : Ultratrace elementsplugin-autotooltip__plain plugin-autotooltip_bigUltratrace elements

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).