In the experiment, preliminary results of which were presented last month at the annual meeting of the Society for Neuroscience in Chicago, scientists allowed one group of rats to run. Another set of rodents was not allowed to exercise. Then all of the rats swam in cold water, which they don’t like to do. Afterward, the scientists examined the animals’ brains. They found that the stress of the swimming activated neurons in all of the brains. (The researchers could tell which neurons were activated because the cells expressed specific genes in response to the stress.) But the youngest brain cells in the running rats, the cells that the scientists assumed were created by running, were less likely to express the genes. They generally remained quiet. The “cells born from running,” the researchers concluded, appeared to have been “specifically buffered from exposure to a stressful experience.” The rats had created, through running, a brain that seemed biochemically, molecularly, calm.
The stress-reducing changes wrought by exercise on the brain don’t happen overnight, however, as virtually every researcher agrees. In the University of Colorado experiments, for instance, rats that ran for only three weeks did not show much reduction in stress-induced anxiety, but those that ran for at least six weeks did. “Something happened between three and six weeks,” says Benjamin Greenwood, a research associate in the Department of Integrative Physiology at the University of Colorado, who helped conduct the experiments. Dr. Greenwood added that it was “not clear how that translates” into an exercise prescription for humans. We may require more weeks of working out, or maybe less. And no one has yet studied how intense the exercise needs to be. But the lesson, Dr. Greenwood says, is “don’t quit.” Keep running or cycling or swimming. (Animal experiments have focused exclusively on aerobic, endurance-type activities.) You may not feel a magical reduction of stress after your first jog, if you haven’t been exercising. But the molecular biochemical changes will begin, Dr. Greenwood says. And eventually, he says, they become “profound.”