Elements, essential functionally beneficial

Selenium is an essential element and is beneficial at low concentrations, serving as an antioxidant. Lack of selenium affects thyroid function, and selenium deficiencies have been linked to Keshan Disease (34). Selenium at high levels, however, is toxic. Hydrogen selenide (which is used in semiconductor manufacturing) is extremely toxic, affecting the mucous membranes and respiratory system. However, the toxicity of most organ oselenium compounds used as donor compounds for organic semiconductors is not weU studied. 

Elements that do not meet all these criteria, even if there is evidence of positive effects of their presence in a nutritionaUy balanced diet, are not essential. They are referred to as functionally beneficial. Some elements can be found in characteristic quantities in the bodies of all organisms (such as alkali elements, P, S, Cl, Fe, Zn, Cu and Mn) and these are termed invariable elements. Other elements, known as variable elements, occur in higher concentrations only in some organisms. For example, V is present in blood cells of some tunicates, also known as urochordates or in fruiting bodies of some fungi (see Section 6.3.14.2), while in most other organisms it belongs to the trace or ultra trace elements. 

The quality of the experimental evidence for nutritional essentiality varies widely for the ultratrace elements. The evidence for the essentiality of three elements, iodine, molybdenum and selenium, is substantial and noncontroversial specific biochemical functions have been defined for these elements. The nutritional importance of iodine and selenium are such that they have separate entries in this encyclopedia. Molybdenum, however, is given very little nutritional attention, apparently because a deficiency of this element has not been unequivocally identified in humans other than individuals nourished by total parenteral nutrition or with genetic defects causing disturbances in metabolic pathways involving this element. Specific biochemical functions have not been defined for the other 15 ultratrace elements listed above. Thus, their essentiality is based on circumstantial evidence, which most often is that a dietary deprivation in an animal model results in a suboptimal biological function that is preventable or reversible by an intake of physiological amounts of the element in question. Often the circumstantial evidence includes an identified essential function in a lower form of life, and biochemical actions consistent with a biological role or beneficial action in humans. The circumstantial evidence for essentiality is substantial for arsenic, boron, chromium, nickel, silicon, and vanadium. The evidence for essentiality for the... 

As rocks are transformed to soil so a proportion of each element is usually converted to a form which plant roots can absorb. Consequently, most of the elements in the Periodic Table would probably be detected in any plant sample if sufficiently sensitive analytical methods were used. Uptake of an element is no evidence that it plays any role in the development of the plant since a root has no power to reject any soluble element entirely. One must therefore differentiate between those elements which are needed, the essential nutrients, and the rest. The conventional criteria by which the presence of an element is regarded as essential rather than adventitious are these the plant cannot complete its life cycle in the absence of the element the action of the element must be specific in that no other element can wholly substitute for it nor is the element simply involved in beneficially altering the plants root environment the element must be shown to be a constituent of an essential metabolite, or required for the proper functioning of an essential enzyme system or be uniquely involved in maintaining the overall ionic composition of tissue. Table 1-2 lists the elements which are generally accepted as being essential for plants. Major or macronutrients are separated from micronutrients, the latter being present in tissue concentration < 0.1 %. 

A wide variety of elements found in earth materials can be associated with specific health problems (WHO, 1996 Taylor and Williams, 1998 Goyer and Clarkson, 2001). Essential/ beneficial elements (Table 2) are those that are required for the proper physiologic function of the body, and so their associated health problems may result from either deficiencies or excesses. A variety of elements, primarily metals or metalloids, have no known natural physiological benefits but are considered toxic in excess exposure. 

Boron is an essential trace element for plants and is beneficial for animals and humans. Dietary boron obviously plays a role in immune functions. Among the best-known natural boron-containing compounds are polyketide antibiotics such as boromycin, aplasmomycins, borophycin and tatrolons. Attempts are underway to incorporate boron into different biologically active molecules, particularly for medicinal application, e.g. for boron neutron capture therapy of brain tumors. Some boron-containing biomolecules may apparently act as signaling molecules that interact with cell surfaces. 

The average amounts of some essential and non-essential elements in the human body are shown in Table 2.1. This shows that the weights of some of the essential elements in the body vary by six or more orders of magnitude. Oxygen, at 45 kg, is by far the most abundant element with the majority being present as one simple inorganic compound water. Some of the elements listed in Table 2.1, and marked with an asterisk, have no known beneficial function and are present in the body simply because they are present in rocks and soils and find their way into water and foodstuffs and from thence into the human body many of these elements are present in only minute quantities. 

Asher CJ (1991) Beneficial elements, functioned nutrients and possible new essential elements. In Morvedt JJ, Cox FR, Shuman LM and Welch RM, eds.. Micronutrients in Agriculture. 2nd ed., pp. 703-723. Soil Sci Soc Amer Book Series No. 4, Madison, Wl, USA. 

Zinc chelators also serve a beneficial function, not only by minimizing abiotic scavenging of zinc in surface waters, but also by preventing the extremely efficient uptake systems of eukaryotic phytoplankton from completely depleting this essential micronutrient element from surface ocean waters. 

Table 2-1 gives a survey of metals essential for life and summarizes the amounts of metals in the human body. The question is, what makes the function of these around twenty-five elements so valuable in making life. Some metals are necessary in gram quantities. Other trace elements such as Mn, Mo, Co, V, W, Ni and Cr are essential beneficial nutrients at low levels but metabolic poisons at high levels. Some metal ions such as Pb, Cd are called detrimental metal ions because they are toxic and impair the regular course of life functions at all concentrations. 

Another term, beneficial elements, has been proposed to describe some elements that appear to have specific functions in plant nutrition but do not meet all of the three criteria required for essentiality as described above. Two such elements (cobalt and silicon) will be discussed here. 

Asher, C. J. 1991. Beneficial Hements, Functional Nutrients and Possible New Essential Elements, IN... 

In terms of functional food, some beneficial mushroom characteristics can be enhanced by the addition of essential elements. Selenium-enriched button mushrooms, shiitake, or oyster mushrooms naturally are preventive to a spectrum of diseases. Selenium is vital for normal functioning of the body and for protection of DNA, proteins, and lip-... 

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