Energy, stress and the invalid linear no-threshold premise: a generalization illustrated by ionizing radiation.

The linear no-threshold (LNT) premise for environmental agents is assessed in the context of the habitats of organisms where exposure to a multiplicity of environmental agents occurs. Adaptation towards high energy efficiency or fitness to counter the metabolic consequences of the stresses from environmental agents is expected over time. This evolutionary process leads to non-linear continua for energy efficiency across environments whereby maximum efficiency should occur at around background exposures; this therefore is a description of hormesis in energy terms. Consequently the LNT premise is invalid for all environmental agents including ionizing radiation. However, published longevity and survival data, being measures of fitness or energy efficiency, indicate that non-linearity and hence radiation hormesis extends to exposures substantially in excess of background radiation. An interpretation is suggested based upon the metabolic and energy reserves required for the simultaneous adaptation to the metabolic consequences of the various environmental agents to which organisms are exposed in their habitats, especially from stresses of climatic origin.