If it's a problem requiring the old gray matter that you're confronting, being in good physical shape may help ? especially if your gray matter is, in fact, old. Or your white matter, for that matter.

Six months of aerobic exercise ? a walking program ? beefed up gray and white matter in the brains of formerly inactive senior citizens in a recent University of Illinois study, at a time of life when folks were once thought to suffer an inevitable decline in brain volume and density.

"More and more of these studies are suggesting things as simple as walking ... tend to be neuroprotective," said Art Kramer, a University of Illinois psychology professor who studies the effects of exercise on the brain.

Kramer also pointed to a body of literature correlating the amount of brain tissue older people retain and their performance on mentally rigorous tasks.

"For many different kinds of cognitive ability, more is better," he said.

Kramer, UI Professor Edward McAuley and colleagues have used Magnetic Resonance Imaging previously to look at changes in the brain when older people exercise.

But where prior studies focused more on brain function, tracking blood flow for instance, this one examined changes in the brain volume and density of individuals, particularly of the gray and white matter in the brain.

Participants in the study, 59 largely sedentary men and women 60 to 79 years old, had their brains imaged before embarking on an exercise program and after six months of exercising under the direction of the UI researchers.

Half the group in the program, which included sessions three days a week for 45 minutes to an hour, did nonaerobic stretching and toning exercises; the other half walked.

Aerobic exercises, like walking, are generally those that maintain an elevated heart rate over an extended period, causing the body to use oxygen in fueling the muscles.

At the end of the six months, the MRI scans showed significant increases in both gray and white matter in the brains of the walkers, but not in the other exercisers. The results are being published this month in the Journal of Gerontology: Medical Sciences. Former UI postdoctoral researcher Stanley Colcombe, now a professor at the University of Wales, Bangor, is the lead author. The study was supported by the National Institute on Aging and the Institute for the Study of Aging.

In animal studies, increased gray matter has been shown to indicate new neurons, signal-firing cells, in the brain or new synapses, connections between those cells, as well as improved conditions for blood flow, Kramer said.

More white matter seems to indicate improved connections between the two major segments, the hemispheres, of the brain and conditions that allow those links to function faster.

Kramer said the researchers don't yet know exactly why aerobic exercise appears to make the brain as well as the body more buff, but they have some ideas.

At a molecular level, exercise may increase production of certain proteins, called "growth factors," that set off gene activity to protect and improve brain structure.

The researchers have a second study, with UI postdoctoral researcher Kirk Erickson as the lead author, that shows older women on hormone replacement therapy ? to replace lost estrogen after menopause ? exhibiting similar benefits in brain structure, and in performance on cognitive tests, when they're physically fit.

In that case, according to the report in the journal Neurobiology of Aging, fitness seems to play a role in overcoming negative effects from more than 16 years of hormone replacement therapy, which has been linked to a decline in brain volume.

Eventually, research of this sort could lead to exercise programs tailored to make certain things happen in our brains at a molecular level, in much the same way future medications may be geared to prompting specific gene activity, Kramer said. That could be a boon in treating diseases stemming from deterioration of the brain with aging, such as Alzheimer's disease.

Kramer said a few epidemiological and animal studies already indicate that exercise can benefit Alzheimer's sufferers.