Hormesis

Hormesis is the term that describes any process in a cell or organism that exhibits a biphasic response to exposure to increasing amounts of a substance or condition. Within the hormetic zone there is generally a favorable biological responses to low exposures to toxins and other stressors. Hormesis comes from Greek hórmēsis "rapid motion, eagerness", itself from ancient Greek hormáein "to set in motion, impel, urge on". A pollutant or toxin showing hormesis thus has the opposite effect in small doses as in large doses. Hormetics is the term proposed for the study and science of hormesis. A related concept is Mithridatism, which refers to the willful exposure to toxins in an attempt to develop immunity against them.

In toxicology, hormesis is a dose response phenomenon characterized by a low dose stimulation, high dose inhibition, resulting in either a J-shaped or an inverted U-shaped dose response. Such environmental factors that would seem to produce positive responses have also been termed "eustress". The hormesis model of dose response is vigorously debated. The notion that hormesis is important for chemical risks regulations is not widely accepted.

The biochemical mechanisms by which hormesis works are not well understood. It is conjectured that low doses of toxins or other stressors might activate the repair mechanisms of the body. The repair process fixes not only the damage caused by the toxin, but also other low-level damage that might have accumulated before without having triggered the repair mechanism.

German pharmacologist Hugo Schulz first described such a phenomenon in 1888 following his own observations that the growth of yeast could be stimulated by small doses of poisons. This was coupled with the work of German physician Rudolph Arndt, who studied animals given low doses of drugs, eventually giving rise to the Arndt-Schulz rule. Arndt's advocacy of homeopathy contributed to the rule's diminished credibility in the 1920s and 1930s. The term "hormesis" was coined and used for the first time in a scientific paper by Chester M. Southam and J. Ehrlich in 1943 in the journal: Phytopathology, volume 33, pp. 517–541. Recently, Edward Calabrese has revived the concept of hormesis. The evidence for and importance of hormesis in physiology and health was advanced by Mark Mattson, who elucidated cellular and molecular mechanisms by which the nervous system responds adaptively to mild bioenergetic stresses such as fasting and exercise. Cells respond to such challenges by increasing their production of neurotrophic factors, DNA repair proteins and antioxidant enzymes. Mattson also proposed that the reason that vegetables, fruits, tea and coffee can improve brain health is that they contain 'noxious' chemicals that are produced by the plants to protect themselves from being eaten by insects and other organisms. Such phytochemicals trigger hormetic responses in brain cells which can improve brain function and may increase the resistance of neurons to injury and age-related neurodegenerative disorders such as Alzheimer's disease and Parkinson's disease.

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