Over twenty dietary elements are necessary for mammals, and several more for various other types of life. The total number of chemical elements that are absolutely needed is not known for any organism. Ultratrace amounts of some elements (e.g., boron, chromium) are known to clearly have a role but the exact biochemical nature is unknown, and others (e.g. arsenic, silicon) are suspected to have a role in health, but without proof.
Many elements have been suggested as essential, but such claims have usually not been confirmed. Definitive evidence for efficacy comes from the characterization of a biomolecule containing the element with an identifiable and testable function. One problem with identifying efficacy is that some elements are innocuous at low concentrations and are pervasive (examples: silicon and nickel in solid and dust), so proof of efficacy is lacking because deficiencies are difficult to reproduce.
|Arsenic||Essential in rat, hamster, goat and chicken models, but no biochemical mechanism known in humans.|
|Nickel||There have been occasional studies asserting the essentiality of nickel, but it currently has no RDA.|
|Chromium||Chromium has been described as nonessential to mammals. Some role in sugar metabolism in humans has been invoked, but evidence is lacking, despite a market for the supplement chromium picolinate.|
|Fluorine||Fluorine (as fluoride) is not generally considered an essential element because humans do not require it for growth or to sustain life. However, if one considers the prevention of dental cavities an important criterion in determining essentiality, then fluoride might well be considered an essential trace element. However, recent research indicates that the primary action of fluoride occurs topically (at the surface).|
|Boron||Boron is an essential plant nutrient, required primarily for maintaining the integrity of cell walls. In animals, supplemental boron has been shown to reduce calcium excretion and activate vitamin D. However, whether these effects were conventionally nutritional, or medicinal, could not be determined.|
|Lithium||It is not known whether lithium has a physiological role in any species, but nutritional studies in mammals have indicated its importance to health, leading to a suggestion that it be classed as an essential trace element with an RDA of 1 mg/day. Observational studies in Japan, reported in 2011, suggested that naturally occurring lithium in drinking water may increase human lifespan.|
|Strontium||Strontium has been found to be involved in the utilization of calcium in the body. It has promoting action on calcium uptake into bone at moderate dietary strontium levels, but a rachitogenic (rickets-producing) action at higher dietary levels.|
|Other||Silicon and vanadium have established, albeit specialized, biochemical roles as structural or functional cofactors in other organisms, and are possibly, even probably, used by mammals (including humans). By contrast, tungsten and cadmium have specialized biochemical uses in certain lower organisms, but these elements appear not to be utilized by humans.|