The basic theory is outlined in a text from 200 B.C., attributed to the Yellow Emperor (a mythical ruler). In brief, it recognizes in people and in nature a vital energy or life force known as qi (pronounced chee). Qi is the source of movements ranging from voluntary muscle action to blood flow; it protects the body from external influences, and it generates warmth. Qi flows through the body and to the organs by way of an extensive system of channels known as meridians. If the flow of the force is disturbed, the theory goes, the resulting deficiency, excess, or stagnation of qi causes bodily malfunction and thus illness. Acupuncture, in which needles are inserted into specific points along the meridians and manipulated, is said to restore the proper flow of qi and thereby return the body to health.

Practitioners recognize some 1,500 acupoints, most of which have no obvious relationship to their intended targets. For example, a point on the second toe is used to treat headaches and toothaches, while a point near the elbow enhances the immune system. Unlike Western medicine, in which many complex functions are understood to be controlled by interactions in the brain, traditional Chinese medicine holds that there is little connection between the brain and various organs and that stimulating an acupoint sends a message directly to the targeted organ.

What those researchers have been able to measure is a flow of acupuncture-induced endorphins. According to neuroscientist Bruce Pomeranz, of the University of Toronto, numerous studies over the past 20 years have shown that inserting needles into acupoints stimulates nerves in the underlying muscles. That stimulation, researchers believe, sends impulses up the spinal cord to a relatively primitive part of the brain known as the limbic system, as well as to the midbrain and the pituitary gland. Somehow that signaling leads to the release of endorphins and monoamines, chemicals that block pain signals in the spinal cord and the brain. The result: A well-documented generalized "acupuncture analgesia."

As odd as it seemed, sticking a needle into someone's foot had the very same effect as shining a light in someone's eyes. And this was not the generalized analgesic effect, produced by the primitive limbic system, that was seen in the pain studies; this was a function-specific response occurring in the brain's cortex, the area responsible for such sophisticated functions as speech and hearing, memory and intellect. Moreover, the magnitude of brain activity seen on acupuncture stimulation was nearly as strong as that elicited by the flash of light.

Despite the absence of clear-cut explanations, acupuncture's clinical results are attracting interest from mainstream medicine. A panel of independent experts convened last year by the National Institutes of Health concluded that acupuncture is indeed effective in treating nausea due to anesthesia and chemotherapy drugs.