Exercising the body can benefit the mind
This is part one of a two-part series on lifestyle and brain fitness. Part II: "Eat Smart," is available at http://www.sciencen
ews.org/articles /20060304/ bob8.asp.
Anyone who frequents the local gym has probably noticed a cyclical pattern to attendance. Workout kings and queens exercise religiously throughout the year, but as swimsuit season approaches, a rash of new faces flocks to the facility. Every treadmill is taken, each elliptical machine is engaged, and without fail, there's a waiting line for a weight machine.
Just as physical exercise primes the body, researchers are finding that it also primes the mind. Exercise prompts brain cells to multiply, strengthens their connections, and boosts their resilience against damage and disease.
While exercise may be the path to looking great in a two-piece, everyone knows that it's also healthy for the body. It strengthens the heart and lungs, shores up thinning bones, and wards off a host of evils, including diabetes, heart disease, and stroke.
But what these newly inaugurated gym rats probably don't know is that besides buffing up their bodies for summer, they're also buffing up their brains. New research suggests that physical exercise encourages healthy brains to function at their optimum levels. Fitness prompts nerve cells to multiply, strengthens their connections, and protects them from harm. Benefits seem to extend to brains and nerves that are diseased or damaged. These findings could suggest new treatments for people with Alzheimer's disease, Parkinson's disease, and spinal cord injuries.
Sweating to the oldies
The cliché about a healthy mind residing in a healthy body has ancient roots. The famous quote of the same meaning, "mens sana in corpore sano" came from the Roman writer Juvenal in the early 100s A.D. And a century earlier, the philosopher Seneca was prescribing exercise as a way to achieve both physical and mental health.
But it wasn't until the early 1950s that reports that exercise conveys neurological benefits appeared in the scientific literature. These articles usually described what doctors had witnessed in their own practices, says neurobiologist Fernando Gómez-Pinilla of the University of California, Los Angeles. "This clinical literature described that exercise could be good for many different things," he says. The studies cited benefits ranging from alleviating depression and pain to regaining mobility in paralyzed limbs to maintaining good memory in old age.
However, for scientists who research how nerve cells work at a molecular level, such reports raise a bevy of questions. Gómez-Pinilla and other neurobiologists have aimed to fill this information gap by working with lab animals such as mice and rats—creatures that can be easily manipulated to sort out each one of an experiment's variables and that, unlike people, can be dissected in the end to get an insider's view of the brain.
By the mid-1990s, researchers began to get answers. Preliminary studies indicated that when lab animals exercise, their nerve cells release chemicals called neurotrophic factors. These proteins buffer nerve cells against illness or injury, prompt them to grow and multiply, and strengthen each neuron's connection with other nerve cells.
Out of the variety of neurotrophic factors released during exercise, however, scientists found that one in particular stood out: brain-derived neurotrophic factor, or BDNF. This protein seems to act as a ringleader, both prompting brain benefits on its own and triggering a cascade of other neural health–promoting chemicals to spring into action.
"I think of BDNF as brain fertilizer. It's thrilling to see what it does to cells in culture," says Carl Cotman, a neuroscientist at the University of California, Irvine. Sprinkling a dilute solution of BDNF onto neurons in a lab dish makes the cells "grow like crazy," he adds. The cells sprout branches prolifically and extend them rapidly.
Let's get physical
Knowing what BDNF can do to neurons in the lab, researchers wondered whether the BDNF that exercising animals produce has similar effects on neurons in their brains. If so, could these physical effects translate into behavioral ones, making the animals learn quicker and better?
In 1999, Fred H. Gage of the Salk Institute in La Jolla, Calif., and his colleagues, including Salk's Henriette Van Praag, began exploring these questions. They studied two groups of healthy mice housed individually in cages that were identical except for one detail: One group of mice had running wheels.
"The mice just love [the wheel]. They run on it as soon as you put it in their cages," says Van Praag. "If you let them run as much as they want, they run all night long."
Over the next several weeks, the researchers kept track as the runners voluntarily racked up an average of 4 to 5 kilometers on their wheels every night. The scientists then tested whether the groups differed in how quickly each mouse solved a popular learning test known as the Morris water maze.
Although both groups of mice swam at about the same speed, Gage and his colleagues noticed that the runners learned the location of a platform hidden under the maze's opaque water significantly sooner than their less-fit counterparts did.
Dissections showed that the runners had about twice as many new brain neurons as the sedentary mice did. When the researchers tested individual neurons isolated from both groups, they discovered that neurons taken from the runners showed greater signs of strengthened connections and cellular learning.
In a related study published in 2004, Gage's team teased out the molecular factors responsible for the behavioral effects that come with exercise. The researchers provided a group of rats with running wheels and compared them with rats without access to the wheels. On average, the runners voluntarily racked up an astounding 48 km per day over the next several weeks.
When they dissected the rats' brains, Gage's team found changes similar to those that they'd seen in the previous study's mice: The runners had more new neurons and stronger connectivity, which is evidence of learning, than did the rats that didn't have running wheels. After examining the messenger RNA of both groups, an indicator of gene expression, the researchers found that the running rats had consistently higher activity in the gene that codes for BDNF than the nonrunners did.
Gómez-Pinilla and his colleagues added more evidence that BDNF is a primary source for the behavioral benefits of exercise. Like Gage's group, Gómez-Pinilla' s team worked with rats that were either sedentary or had access to a running wheel. After a week, some members of each group began receiving daily injections of a drug that blocked the action of BDNF. The rest of the animals were injected daily for several days with a chemical called cytochrome-C, which isn't known to cause any physical or behavioral effects.
The researchers then tested all the animals on the Morris water maze. While runners receiving cytochrome-C excelled at the test, runners that received the chemical that blocked BDNF performed only as well as the sedentary mice did. Performance by the nonrunners was about the same, regardless of which injection they received. "If we block the action of BDNF, we block learning and memory," concludes Gómez-Pinilla.