Mental and physical exercise can keep the brain fit and help it recover capacities lost to disease and trauma
Can the brain heal and preserve itself—or even improve its functioning—as we get older? For some time, many scientists have tended to think of our brains as machines, most commonly as computers, destined to break down over time under the strain of age and use. In recent years, however, research in neuroscience has begun to show the inadequacy of this metaphor for describing the physiology of the brain. It turns out that our brains, like our bodies in general, are far more likely to waste away from under-use than to wear down from overuse.
As people reach middle age, exercising the brain and the body to which it is attached—keeping both active—becomes more important. It is one of the few reliable ways to offset the natural wasting process and the damaging influence of our unnaturally sedentary modern lives. It also points to new possibilities for the brain to heal itself in the face of disease and trauma.
For decades, physicians and scientists generally believed that the prognosis for most brain problems was grim. The standard view was that the brain had evolved to be so complex and specialized that we had to pay a price for its sophistication: It couldn’t repair or restore itself with replacement parts, as was possible with other organs, such as the skin, liver and blood.
That view fit with, and partly stemmed from, an image that had prevailed since the days of the great French philosopher and scientist René Descartes, who described the brain as a glorious machine with discrete parts. Descartes’s heirs argued that each of these parts performed a single mental function in a single location. If a part was damaged—by a genetic fault, or stroke, or injury or disease—it was assumed that the body had no resources of its own to deal with the problem: After all, machines cannot repair themselves or spontaneously grow new parts.
Once the electrical nature of the brain was delineated in the 19th century, scientists began speaking of it as a grander sort of machine, an electrical one, with “circuits”—a metaphor still very much with us. They came to see its circuits as analogous to those of electronic gadgets—unchangeable or “hard-wired.”
As the machine metaphor evolved, neuroscientists took to describing the brain as a computer. This “master analogy,” as computer scientist David Gelernter calls it (in criticizing this view), encourages us to see thought as “software” and the brain’s structure as “hardware.”
The unhappy practical implication of this view, for anyone wishing to maintain his or her brain, is clear: Hardware inevitably degenerates with time and use. The rule for a machine is, “Use it and lose it.” Many clinicians under the sway of this analogy saw patients’ attempts to resist their brains’ decline through activity and mental exercise as a harmless waste of time.
Fortunately, a growing body of research suggests that this older view is wrong. It seems that a more accurate rule for our brains is “Use it or lose it.”
In the late 1970s, research by Mark Rosenzweig of the University of California at Berkeley and Michael Merzenich of the University of California at San Francisco and others began to show that the brain’s circuitry changes microscopically with experience and activity. Dr. Rosenzweig and colleagues found that, with environmental stimulation, the brains of animals grew in key areas.
Dr. Merzenich discovered that if an animal stopped using a body part, the brain area that processed sensory input from that part weakened or was taken over to perform another function. These findings have since been replicated many times.
The mainstream view in neuroscience and medicine today is that the living brain is actually “neuroplastic”—meaning that its “circuits” are constantly changing in response to what we actually do out in the world. As we think, perceive, form memories or learn new skills, the connections between brain cells also change and strengthen. Far from being hard-wired, the brain has circuits that very rapidly form, unform and reform.
This capacity is the foundation for the brain’s distinctive way of healing. If an area is damaged, new neurons can often take over old tasks. Nor are we just our neurons. Our memories and experiences are also encoded in the patterns of electrical energy produced by our brain cells, like a musical score. As with an orchestra, when one member of the string section is sick, the show can still go on if a replacement has access to the musical score.
This new “plastic” understanding has major practical implications for how we treat brain problems and maintain brain health. And it has led to some surprising discoveries.
Consider dementia, which in some form affects some 15% of people in the U.S. over age 70 and advances rapidly as we age. A brain with Alzheimer’s, the most common form of dementia, turns out, by various measures, to be a brain that is losing its overall plasticity. It shrinks and loses connections. But a growing body of research has found that exercise, both mental and physical, can lower the risk of experiencing dementia.
Last year, Peter Elwood and a team from the Cochrane Institute of Primary Care and Public Health at Cardiff University in the U.K. released results from the most detailed study ever done on the effect of lifestyle and exercise on the risk of getting dementia. The researchers followed 2,235 men—almost all the male inhabitants of Caerphilly, Wales, with initial ages between 45 and 59 years old—for 30 years.
They found that men who consistently did a few things reduced their risk for cognitive decline and dementia by a staggering 60%. These activities included eating a healthy diet (at least three to four servings of fruits and vegetables a day); maintaining a normal weight, with a body-mass index of 18 and 25; limiting alcohol to about a glass of wine a day; and not smoking.
But the activity with the biggest impact on risk was walking at least 2 miles a day, biking 10 miles a day or engaging in some other regular, vigorous physical exercise. All five of these factors have been found in other studies to promote the general health of two types of cells in the brain: neurons and glial cells (which interact with and protect neurons).
Imagine if there were a drug that could reduce the risk of dementia by 60%. It would be the most talked-about drug in history, but this astonishing finding has been fairly quietly received.
One reason is that many people assume that Alzheimer’s disease is “all in your genes.” But as neurologist and dementia researcher Tiffany Chow of the Rotman Research Institute and the University of Toronto points out, environmental factors “interact with…genetic makeup to eventually allow or deny dementia a foothold.” Even having multiple copies of the genetic materials associated with risk, Dr. Chow points out, “is not sufficient to produce Alzheimer’s disease.” For the majority of people, how they live matters.
The research in Wales followed at least 10 other studies showing that exercise in midlife was associated or correlated with lower rates of dementia—and that a lack of regular exercise corresponded with higher rates of dementia.
Another recent study—a randomized, controlled trial by Kirk Erickson of the University of Pittsburgh and colleagues published in the Proceedings of the National Academy of Sciences—shows that those without dementia who did aerobic exercise for a year showed significant hippocampal enlargement. The hippocampus is the brain region that turns short-term memories into long-term ones, and it is often the first to degenerate in Alzheimer’s cases and with age in general. Earlier studies showed aerobic exercise increased the brain’s gray and white matter in the frontal lobes, areas involved in planning and goal-directed activity.
How does this form of healing work? Exercise triggers the growth of new brain cells in the hippocampus. It also triggers the release of “neurotrophic growth factors”—a kind of brain fertilizer, helping the brain to grow, maintain new connections and stay healthy.
Frederick Gage of the Salk Institute, co-discoverer of stem cells in the brain, has suggested that new cells arise from long walks because, in an evolutionary sense, our bodies associate the exertion with moving from an existing territory, which had perhaps become depleted of food or too dangerous, to a new, unexplored territory whose details must be learned. In anticipation, the brain releases new cells and growth factors, which create a more plastic state and make possible new neural connections. This may be one of the mechanisms by which exercise helps to protect against diseases such as Alzheimer’s.