A major mental illness like clinical depression will send biochemical shock waves through the body. But the intimate relationship of body to mind isn't limited to serious disease. Researchers have come to understand that what lies below the neck can also be harmed by less acute kinds of brain disturbances. The chronic stress that millions of people feel from simply trying to deal with the pressures of modern life can unleash a flood of hormones that are useful in the short term but subtly toxic if they persist. Thus it shouldn't come as a surprise that stress-reduction strategies that take pressure off the mind--meditation, yoga, relaxation exercises and such--can take the heat off the body as well.
Humanity's physical reaction to stress, known as the "fight or flight" response, probably evolved to help our primitive ancestors deal with a treacherous world. When confronted with imminent danger--a saber-toothed tiger, say, or a club-wielding enemy Homo erectus--the body had to be instantly ready either to defend itself or to run like hell.
So the terrified brain would signal the adrenal glands, located on top of the kidneys, to release hormones, including adrenaline (its more technical name: epinephrine) and glucocorticoids (see chart), and the nerve cells to release norepinephrine. These powerful chemicals made the senses sharper, the muscles tighter, the heart pound faster, the bloodstream fill with sugars for ready energy. Then, when the danger passed, the response would turn off.
In the modern world, stress usually takes other forms. But the fight-or-flight response hasn't changed. Sometimes it's still useful: a demanding job can lead to a sense of pride; a bout of precurtain jitters can motivate a spectacular performance. But many modern stresses are continuing, not acute, and arise in situations we can neither fight nor flee: an unreasonable boss, a harrowing commute, a stormy relationship, a plummeting stock market, a general sense that life is out of control.
While some stress hormones can't stay elevated indefinitely, glucocorticoids can and do. Cortisol in particular can weaken the immune system, potentially making cancer and infectious diseases worse. Measuring the influence of stress, though, is tough. Some studies have shown no effect at all. Others offer intriguing clues. Dr. David Spiegel, director of Stanford's Psychosocial Treatment Laboratory, cites a study of psoriasis patients in which half practiced meditation and half didn't; the first group healed faster. Other studies show that patients who are part of a rich social network have lower cortisol levels than loners, that people who pray regularly tend to live longer and that breast-cancer patients who have an optimistic attitude or an ability to express anger about their disease tend to live somewhat longer than those who don't.