A sound mind in a sound body is a short, but full description of a happy state in this world," wrote the British philosopher John Locke. Three hundred years later, research shows that we should begin thinking of body and mind health as conceptually identical. The two are linked at the deepest levels.
For several decades we've known about one effect of exercise on the brain, the "endorphin high" that makes us feel good during and right after exercise. Recently, scientists have uncovered some longer-lasting effects of exercise on the brain. Regular exercise improves your mood, decreases anxiety, improves sleep, improves resilience in the face of stress and raises self-esteem. All these benefits don't come because you notice what you've lost around your waist. Rather, they come from exercise-induced alterations inside your head.
With exercise, several biological changes occur that make your nerve cells more robust. The blood and energy supply to the brain improves. The genes in nerve cells signal the production of proteins called neurotrophic factors or growth factors. These substances induce nerve cells to grow, branch and make connections with one another (neuroplasticity) and—in some brain areas—give rise to new nerve cells (neurogenesis). These important biological processes, which are essential to adaptation and learning, tend to slow down with age and also in response to stress, after brain injury and in depression. Exercise can speed the process back up again, making it a respectable, though partial, antidote to stress and aging.
Exercise is a pretty good antidepressant, too—equal to drugs or psychotherapy in some studies. Exercise and antidepressant medications also appear to be biologically equivalent. Consider the hippocampus, a seahorse-shaped region in the temporal lobe of the brain. It is involved in regulating mood and storing memories. When neuroplasticity and neurogenesis are sluggish, the hippocampus gets smaller. Neuroscientists see this in brain scans of people with depression. Antidepressants and electroconvulsive therapy appear to spur nerve growth in this region. Exercise probably relieves—and likely also prevents—depression through the same mechanism.
Using an animal model of depression well known among neuroscientists, researchers at the Karolinska Institute in Sweden demonstrated these changes in the brain in a series of experiments. They saw that "depressed" rats grow more nerve cells in the hippocampus in response to exercise. Then, by measuring the levels of mRNA, an indicator of gene activity, they found that exercise caused an increase in the production of neuropeptide Y (NPY) in the hippocampus. Also working with rats, a group at California State University found that exercise induced an increase in brain-derived neurotrophic factor (BDNF) in the same region. These two proteins—NPY and BDNF—are important prerequisites for nerve growth and survival.
Just knowing what exercise can do for your brain won't guarantee that you'll jump off your couch and start jogging down the road. You still need motivation, but where does it come from? Even that is under genetic control.
Recent research finds that the brain governs how much activity your body is ready for, in part using signals from your muscles as a guide. The theory goes that the brain needs to be in charge, to regulate energy output and preserve the integrity of the whole body. In this model, the brain decides when you need to be active and when you don't. How good a job your brain does at managing this is genetically determined. And, just as with weight control, the brain isn't always right: just as the brains of overweight people often make them hungry, the brains of some inactive people often encourage even more inactivity.
Indeed, some researchers are beginning to wonder if genes that make a person vulnerable to depression also make exercise less pleasant—or less reinforcing. Exercise has been demonstrated so effective at reducing health risks with so few adverse effects that it should be an easy sell. Yet no one has yet found the most effective way to promote a healthy lifestyle. Information about the positive effects of exercise does not seem to be enough. To improve motivation for exercise, it would help us to better understand how our genes control our experience of exercise.
At the University of Colorado at Boulder, investigators have developed a model of exercise behavior to guide their research. They make the reasonable assumption that genes influence both our physiological responses to exercise (for example, our heart rate or body temperature) and our subjective experience of it (how our moods change or how tired we feel). They conducted a study in which participants were assigned to periods of both exercise and inactivity. The subjects' responses were recorded. The researchers, knowing that there are two forms of a gene involved in BDNF function, also determined the genotype of each subject. The results suggested that the degree of mood improvement and the perception of exertion was partly determined by the form of the participant's BDNF gene.
These are very early returns, so in the meantime some practical advice may help. You don't have to invest in fancy equipment or a health club to get the benefits of exercise. Vigorous, sweat-inducing effort is a good thing, but psychological well-being or an improvement in depressive symptoms does not seem to depend upon the intensity or duration of the workout. More important is your ability to sustain an exercise routine. The research indicates that if you can stick with any program for at least two months, you're giving yourself the best chance to feel better. Fortunately, there does appear to be a priming effect—after a while it gets easier to continue, and some people end up describing their exercise as a habit they enjoy continuing.
The type of exercise also doesn't seem to matter much—aerobic exercise and strength training or a combination are equally effective. Thirty minutes per day of moderate exercise—a brisk walk, for example—can help. Start even smaller if you want to. Build 10 minutes of walking into your daily commute by parking a little farther from the office. Take stairs instead of an elevator. There is some evidence that exercise is more pleasant if you stay hydrated. So if you are going to exercise for any extended period, drink water before you start and again every so often during the workout. And if your exercise program is boring, keep yourself stimulated during exercise: listen to music or an audiobook.
You may have concluded that you are one of those people with I-don't-feel-much-like-exercising genes. But if you find the science convincing, begin to picture healthy nerve cells plumping and sprouting. Does that motivate you? If not, let's hope that scientists will soon find that gentle nudge—new information, a form of psychotherapy or a medication—that will help you feel like it and keep you moving. Whoever finds the starter motor, the genetic wellspring of motivation, will have found a key to good health.