"What doesn't kill you makes you stronger" is a phrase that contains more than a grain of truth. It describes the theory of hormesis -- the process whereby organisms exposed to low levels of stress or toxins become more resistant to tougher challenges.
The theory of hormesis has been around for decades, but has long been met with skepticism or downright suspicion. In recent years, however, biologists have pieced together a clear molecular explanation of how it works, and hormesis has finally been accepted as a fundamental principle of biology and biomedicine.
As an example, exposing mice to small doses of gamma ray radiation shortly before irradiating them with very high levels of gamma rays actually decreases the likelihood of cancer. A similar effect occurs when dioxin is given to rats.
The biochemical mechanisms by which hormesis works are not well understood. It is thought that a low dose of a toxin can trigger certain repair mechanisms in the body, and these mechanisms, having been initiated, are efficient enough that they not only neutralize the toxin's effect, but can even repair other defects not caused by the toxin.
One of the areas where the concept of hormesis has been explored extensively is aging. It is thought that exposing cells to mild stress could result in the adaptive or hormetic response that has anti-aging effects. Some of the mild stresses that might work for this include heat shock, irradiation, pro-oxidants, hypergravity, food restriction, and even exercise.
* New Scientist August 6, 2008