Gerber, Linda M., Dept of Public Health, Cornell University Medical College, New York, N.Y. 10021 USA
Williams, George C., Department of Ecology and Evolution , State University of New York, Stony Brook, N.Y. 11794 USA
Gray, Sandra J., Department of Anthropology, University of Kansas, Lawrence, Kansas 66045 USA

ABSTRACT:
http://www.ncbi.nlm.nih.gov/htbin-post/Entrez/query?uid=10513405&form=6&db=m&Dopt=b

Brief extracts from the paper:

"In human nutrition, essential nutrients are identified from specific pathologies, such as scurvy and pellagra, that arise from their abnormally low intake.

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"Protein and vitamins A and D clearly illustrate the plurality of the physiological effects among major nutrients and vitamins. Vitamin A is toxic at concentrations above any likely to be reached by eating foods (Kolb 1994) but it can harm a fetus at a small fraction of the intake that would harm an adult. Vitamin D at abnormally high concentrations can inflict heart and kidney damage (Speth 1991; Calabrese 1996).

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"In his study of 16 widely recognized toxic elements, Katz (1995) found only four (arsenic, lead, mercury and cadmium) that were not essential to humans at trace concentrations. This need not mean that none of these four is slightly beneficial at dosages well below those shown to cause damage. Mercury is known to facilitate recovery from DNA damage in human buccal cell cultures at one uM per liter. (Liu et al 1997) That such benefits from mercury may be widespread is suggested by increased reproductive rates in minute marine polychaetes. (Ctenodrilus serratus and Ophyrotrocha diadema)

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"Experimental animal studies have shown beneficial effects of low dose exposure to both chemical toxins and radiation, including weight gain, tumor inhibition and lengthened reproductive life. (Abelson 1994; Luckey 1991; Calabrese 1996)

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"Given these mixed roles in human nutrition and health, it is clear that minerals and organic compounds in the human environment can be categorized as beneficial or harmful only when dosages and other conditions are specified.

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"We assume that anything that has persisted in the diet over thousands of years should cause natural selection for changes that reduce the harm that frequent exposures are likely to produce, minimizing the rate at which increasing dosage will cause increased harm, maximize the attainable benefit from the substance, minimize the dosage needed for such benefit, maximize the excess of benefit over cost for commonly experienced dosages, maximize the ability to survive and reproduce when the substance is in short supply, and, especially, optimize the tradeoffs among these potentially conflicting values.

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"The demonstration of hormesis is more convincing, whether for measures of biological fitness or of less easily defined human well-being, if based on diverse effects of a given dosage (Parsons 1992:194). This ideal is met by some of the works cited by Luckey (1991) on radiation exposure, which has many formal similarities to dietary effects. Luckey reviewed data from many sources that showed, for experiments on rodents and a great diversity of other organisms, that ionizing radiation of moderately greater than average background levels increased many kinds of adaptive performance: growth, fertility, resistance to pathogens and neoplasms and longevity.

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"As noted above, many substances, both essential and of limited benefit, may show this phenomenon of hormesis, with positive effects at low dosages and negative effects at high dosages.

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"Ideally, the regulatory agencies should not only seek to protect us from toxic effects of pollutants, food additives and radiation, but should also work to optimize low-dose exposures.

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"Hormesis is generally recognized when measures of adaptive performance are higher at low-dose exposure to a demonstrably harmful substance than they are at zero exposure. Is radium hormetic? Its concentration in natural seawater is about 6x10(-17)… a gram of seawater would contain more than 10,000 atoms of radium…A zero dosage of any substance can only be approximated and exposures below normal background levels are difficult to provide. We would assume that hormesis is provisionally demonstrated when any dosages above the lowest (the pragmatic zero) result in significantly higher performances. If the slope of the toxicity curve decreases as zero dosage is approached, we would suspect hormesis, even if a peak value above zero is not demonstrable. We thus tentatively support Calabrese and Baldwin’s (1998a) suggestion that hormesis be used as the default assumption.

"There can be no doubt about the reality of hormesis. As argued above, failure to consider possible benefits may make some apparent examples dubious, but not a significant proportion of the enormous number that have been reported. Positive effects of low doses of toxins are usually found when they are looked for carefully, and it is unrealistic to assume that experimenters usually just happen to look for effects that prove positive, rather than those on which net costs are imposed.

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"We suggest that hormesis may be almost universal for substances normally present throughout geologic time, such as mercury.

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"It is also likely that our evolved dietary inclinations and digestive-tract physiology especially the monitoring and processing of blood that drains from the gut lining to the liver via a hepatic portal vein, would usually make a dietary dosage of any substance safer than its inhalation. There are no comparable mechanisms that process toxins that reach the lungs from smoking or polluted air. We suggest that inhaling pollutants could be a more frequent source of harm than eating foods polluted by pesticides or other toxins."