Researchers have proven over and over that exercise pumps up more than just muscules — it can also improve memory and other brain functions. But a lot of those studies have focused on gray matter, the stuff useful for memory that shrinks in old age. New research says fitness boosts another kind of brain matter, the white kind.

Examining the brains and fitness levels of 24 children ages 9 and 10, scientists at the University of Illinois at Urbana-Champaign said white matter is thicker and denser in the fitter kids. That suggests a greater capacity for memory, attention span and cognitive efficiency. "Previous studies in our lab have reported a relationship between fitness and white-matter integrity in older adults," said psychologist Arthur F. Kramer in a news release. "Therefore, it appears that fitness may have beneficial effects on white matter throughout the lifespan."

Here's the disclaimer part. Exercise isn't necessarily going to make you smarter. You still need to study for your history exam. And you'll still need to make some effort to learn your nephew's new girlfriend's name (was it Lisa? Laura?). In fact, the researchers in the new study, published Tuesday in Frontiers in Human Neuroscience, took things like IQ and learning disabilities out of the equation, testing solely for the impact of fitness on the physical brain.

To measure white matter, they used a technology called diffusion tensor imaging to see how water diffuses into the brain tissue in five key regions. Less diffusion means the "tissues are more fibrous and compact," which is a good thing. White matter is like the interstate trucker of the brain, delivering messages across disparate regions — like from the cerebral cortex to the brain stem or from the left hemisphere to the right. Kids who were more physically fit had better white matter. But it will take more research, they said, to find out exactly why that is.

Surprisingly, it doesn't take much to make improvement, other studies have shown. For example, if you're over 50 and sedentary, just six weeks of regular aerobic workouts can improve memory noticeably. In another study, kids with ADHD instructed to walk briskly for 20 minutes performed better in reading comprehension and math exams. "This study extends our previous work and suggests that white-matter structure may be one additional mechanism by which higher-fit children outperform their lower-fit peers on cognitive tasks and in the classroom," said postdoctoral researcher Laura Chaddock-Heyman, lead author of the new study.

Source: L. Chaddock-Heyman, A. Kramer, C. Hillman, et al. Aerobic fitness is associated with greater white matter integrity in children. Frontiers in Human Neuroscience. 2014.