Glossary
Hormesis.
Direct answer
Hormesis describes a biphasic response to a stimulus: a low intensity triggers adaptation, while a high intensity of the same stimulus causes harm. The term names the shape of the relationship between stimulus intensity and effect, not the usefulness of any particular substance or practice.
Hormesis is often read as a licence: a little of something harmful must be good. That is a shortcut. The curve is described only for a given stimulus, a given tissue and a given time frame, and the recovery phase belongs to it. Without recovery a stimulus produces accumulation of load rather than adaptation.
Exercise is the clearest example, in horses as in humans. A training stimulus temporarily generates reactive oxygen species and small tissue injuries, to which the body answers by strengthening protection and repair. In the horse, workload, going and rest interval are all decided by a person, which shifts responsibility for the curve.
The evidence letter
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