a1 CEBAS (CSIC), Campus de Espinardo, Apartado de Correos 4195, MURCIA, 30100, Spain
Hormesis defines an effect where exposure to a low dose of a toxic agent results in a beneficial response. It has been described in organisms exposed to low-dose radiation, heat stress, and chemicals. The effect is characterised by a J-shaped dose–response as opposed to a linear dose–response. Confirmation of the general phenomena of hormesis has proved difficult due to the lack of appropriate methodology and the absence of well-defined mechanisms to support the experimental observations. In the nutritional field there are few reports of its existence. The clearest illustration of the effect is seen in animals that are energy restricted when there is a clear benefit in the reduction of age-related disease, and an extension of maximum lifespan. DNA microarray experiments have shown that there is a down regulation of the stress-response genes that are up regulated through the ageing process. Electrophilic phytochemicals, that have been shown to have beneficial health effects at low doses, up regulate the antioxidant–electrophile response element. This probably occurs through an alteration in the redox state of the target cells which causes activation of protein kinases, the activation of the Nrf2 transcription factor and the up regulation of the phase II enzymes, similar to responses that occur under mild chemical stress. This situation might enable organisms to adapt to stress such that the effects of a subsequent exposure to a harmful challenge are reduced. There may be a permanent alteration in cellular homeostasis, or redox state, if the low level exposure is maintained. It remains to be proven if such a situation occurs in response to chronic low-dose exposure to dietary phytochemicals such that the target cells are better able to respond to a subsequent stress challenge.