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Of the four criteria, the first two are crucial; if both are met, a substance under investigation is considered essential for that particular animal species. Satisfying the third and fourth criteria requires additional effort and time and is a goal of nutrition research. That goal is consistent with the continuous refinement of knowledge concerning the molecular mechanisms for nutrients that have produced substantial public-health benefits for decades (e.g., selenium and zinc) or even centuries (e.g., ascorbic acid).

Evidence For Essentiality

As stated above, two criteria must be met before a substance under investigation can be considered essential in a particular animal species: (1) that it is present in all organisms for which it is essential, and (2) that reduction of exposure below a certain limit results consistently and reproducibly in a reduction of physiologically important functions. It is universally accepted that arsenic is present in living matter. The remainder of this section, therefore, examines the evidence in support of the second criterion. That discussion is followed by consideration of information of secondary importance pertaining to the hypothesis of essentiality. This includes information on biochemical mechanism of action, similarities with selenium, and dose-response relationships.

Physiological Importance

A function is considered physiologically important when its impairment interferes with the normal development or survival of the individual (or the whole species). Data are presented below that examine the physiological importance of arsenic in goats, minipigs, rats, and chicks and in particular how semisynthetic diets with low arsenic concentrations affect reproduction and growth of young animals.

Essentiality in Humans

Arsenic has not been tested for essentiality in humans nor has it been found to be required for any essential biochemical processes.

Goats and Minipigs

Goats and minipigs fed semisynthetic diets low in arsenic (arsenic at less



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