Hormesis is a little-known term with huge implications. It refers to a fascinating phenomenon: a favorable biological reaction to low doses of chemical toxins, radiation or some other form of stress that is damaging, even fatal, in higher doses.
It was first scientifically noted by German pharmacologist Hugo Schulz in 1887, who found that disinfectants -- which, in large doses, kill yeast -- actually stimulate yeast growth when administered in small doses. Of course, many had observed it anecdotally, and poetically, before that. German philosopher Friedrich Nietzsche famously observed, "What does not destroy me makes me stronger," which gets the gist, but overstates a bit -- a more precise phrasing might be, "What stresses me within certain parameters makes me better adapted."
The mechanism of hormesis appears to be overcompensation to re-establish homeostasis -- which is a technical way of saying that an organism, or group of them, responds to small stresses by becoming more robust, or numerous, to adapt to a challenging environment.
The hormetic response with which most of us are familiar comes from exercise. Lifting weights, for example, does not immediately make you stronger -- it actually weakens the body in the short term and releases a cascade of destructive molecules (free radicals) that can injure tissues. A 2005 study by Hungarian researchers suggested that the body responds to this situation by producing more antioxidants, initiating DNA repair and generally slowing the aging process. The result over the next few hours or days is stronger muscles and generally, a healthier, more resilient body.
To put this in an evolutionary context, the exerciser's body is essentially saying: "This person is in an environment that requires strenuous exertion. I'll respond to the damage the exertion causes by overcompensating via creating extra muscle tissue, making her stronger and better able to survive."
Similarly, many substances that are universally, and quite correctly, regarded as "healthy" are, in fact, toxins. Dietary phytochemicals -- the compounds that give fruits and vegetables their bright colors -- are toxic chemicals that plants have evolved as a defense against fungal and insect pests. These are likely mildly toxic to human beings as well, but in the concentrations found in common foods, probably fall within the "stimulating" range. Result: lowered risk of cancers, cardiovascular disease and neurodegenerative disorders.
Hormesis may also help to explain the conundrum of "healthy drinking." Ethyl alcohol is indeed a toxin, with a long, sad history of causing irreversible tissue damage and death at high doses. Used responsibly, however, it has been shown in plasma samples to boost antioxidant activity. This may help explain why many studies have found modest cardiovascular benefit from moderate consumption, such as one alcoholic drink daily. However, the effect is small enough -- and the risk of abstainers becoming alcoholics large enough -- that I do not believe non-drinkers should start consuming alcohol in pursuit of health.
In a larger sense, hormesis may help explain why people who lead strenuous lives with plenty of moderate physical challenges may be healthier and live longer than those in more comfortable circumstances. A 2008 paper titled "Hormesis in Aging" by researchers from the Laboratory of Cellular Aging, Department of Molecular Biology, University of Aarhus in Denmark concluded that "single or multiple exposure to low doses of otherwise harmful agents, such as irradiation, food limitation, heat stress, hypergravity, reactive oxygen species and other free radicals have a variety of anti-aging and longevity-extending hormetic effects."
All of which suggests that one of the best routes to health is to make yourself a little uncomfortable now and then. The most profitable discomforts are likely those with which human beings have a long evolutionary history such as physical exertion, getting hungry, regularly tipping back a modest measure of alcohol, short-term exposure to cold or heat, and so on. Conversely, novel stressors -- such as the stew of noxious synthetic chemicals in the modern environment with which we have no evolutionary history -- are best regarded as guilty until proven innocent.
Which brings up a word of caution: Throughout history, irresponsible politicians and commentators have cited the hormetic effect to justify reducing restrictions on pollution -- claiming that a little poison or radiation in the water, air or food supply is good for us. This is dangerous nonsense. Hormesis appears to be of value only when dosages are very carefully controlled, which does not describe releasing random mixtures of toxins, especially synthetic ones, into general circulation. There's still a great deal we don't understand about hormesis. Until we do, the smartest policy for governments and industry is to keep the public's exposure to environmental toxins as low as possible.
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Andrew Weil, M.D., is the founder and director of the Arizona Center for Integrative Medicine and the editorial director of www.DrWeil.com. Become a fan on Facebook, follow Dr. Weil on Twitter, and check out his Daily Health Tips Blog.