What is it about physical exercise that produces relaxation and well-being in us and in dogs? The answer is that it causes changes in brain chemistry. Most of us have heard of the expression "runner's high," which was once solely attributed to endorphin release in the brain (endorphins are nature's own morphine-like substances). According to the opioid theory, a good dose of exercise produces an opium-like high that dispels depression and makes the world seem a much rosier place. This concept has merit but may not be the whole story. Princeton University professor Barry Jacobs reports that exercise also releases serotonin, and that this brain chemical contributes to exercise-mediated well-being. Jacobs's take on serotonin is different from the one we usually hear. Serotonin's beneficial effects on mood are well known, but Jacobs notes serotonin's overwhelming importance in modulating the activity of large muscle groups that keep us ambulatory and upright. When we run or engage in fatiguing physical activity, our serotonin system kicks into overdrive. Enhanced mood stabilization is secondary to this process and may have evolved as part of nature's intrinsic reward system. Whereas physical activity increases activity in serotonin nerves, a sudden call for mental activity shuts them down, leading to decreased physical activity. A call for high mental activity, as occurs in stress, might contribute to the development of depression and languor by decreasing activity in serotonin nerves. In that case, augmenting exercise would seem a natural way out of the loop – and so it appears to be.
A year or two ago, I was talking to a friend of mine, Dr. John Ratey, a clinical psychiatrist and assistant professor at Harvard Medical School, about his book A User's Guide to the Brain, the bottom line of which, according to Ratey, is that "exercise increases serotonin so dramatically that its action on mood is even more powerful than the antidepressant Prozac." Who wouldn't rather exercise than take a pill to feel better, I thought, as I contemplated the beneficial effects of exercise in dogs and the way I feel when leaving the gym. It's interesting that depression in people is associated with decreased serotonin in the brain and (sometimes) compensatory overeating, while both exercise and Prozac increase serotonin, stabilizing mood and, at least in the short term, decreasing appetite.
To some, whether the beneficial effects of exercise are mediated by the release of endorphins or serotonin is moot. The net effect – mood stabilization, peace of mind, serenity – are the Holy Grail of behavioral medicine. Exercise is a natural way of helping us and our dogs deal with the pressures of modern life; in addition to making us happier, it also makes us smarter (by promoting the maturation of new brain cells) and helps us sleep more soundly.
Aside from psychological benefits, exercise has important physical benefits for dogs, as it does for humans. The quality as well as the length of life is important for us and for our dogs. The Nutrition Center at Tufts University has coined the term "health span" – as opposed to "life span" – to describe the long, healthy existence for which we all strive and want also for our dogs. Exercise is an important component of a lifestyle offering the best chance of achieving a long and happy life.