There is more than enough evidence that physical exercise is good for the brain, bringing benefits like lower cholesterol and blood pressure, but here’s more: it can increase the size of your hippocampus, the structure responsible for the formation and storage of new memories as well as for spatial navigation--finding your way around.

In a paper to be published in the journal Hippocampus, scientists report that elderly people who are physically fit generally have a larger hippocampus and better spatial memory than peers who are less fit. Previous studies have shown that challenging the hippocampus—exercising its spatial skills and its memory abilities—can increase its volume, too. London cabbies have bigger ones than your average Londoner, and experienced cabbies have larger ones than newbies, suggesting that making the hippocampus find its way through London’s labyrinth can boost its size. And exercising its memory-making skills seems to do the same thing: a study of German medical students found that the hippocampus got larger as they studied for finals. This is the first study, however, to show that plain old physical exercise, which does not engage the hippocampus to any real extent, can also give it a boost.

The scientists, led by Art Kramer of the University of Illinois and Kirk Erickson of the University of Pittsburgh, studied 165 adults, ages 59 to 81. They measured the volunteers’ hippocampus volume and gave them a test of spatial memory (recalling where three black dots on a computer screen had been a few seconds after they disappeared). Finally, they measured aerobic fitness by VO₂ max. They found what they call a “triple association”: higher fitness levels were associated with a larger hippocampus (taking into account age, sex and years of education), and a larger hippocampus due to higher fitness levels was correlated with better spatial memory. “The higher fit people have a bigger hippocampus, and the people that have more tissue in the hippocampus have a better spatial memory,” said Kramer

Depression, stress, hypertension, chronic heavy drinking and getting old all shrink the hippocampus, studies in humans as well as lab animals show. And in rodents, voluntarily running in an exercise wheel increases the volume of the hippocampus as well as the rate of neurogenesis, the production of new neurons. (It doesn’t work if the mice or rats are forced to run in the wheel—spouses hectoring your partner to exercise, take note.) Or as Erickson put it, the finding “supports the notion that your lifestyle choices and behaviors may influence brain shrinkage in old age. Basically, if you stay fit, you retain key regions of your brain involved in learning and memory.”

Importantly, however, there was no evidence that aerobic fitness slowed the rate at which the hippocampus shrinks in old age. Instead, it seems that fitness lets you enter your later adulthood with a larger hippocampus, giving yourself more of a cushion against the (probably inevitable) shrinkage. Targeted studies will need to be done to determine whether becoming more physically fit after age 60 or so can halt and even reverse the shrinking of the hippocampus.

Let me also mention an upcoming study showing that one commercial brain-training program, Brain Fitness from Posit Science, improved the ability to remember what you heard (auditory memory) and to focus attention. Funded by Posit (potential conflict-of-interest alert), the study of 487 healthy adults over 65 was conducted by the Mayo Clinic and University of Southern California and will appear in the April edition of The Journal of the American Geriatrics Society.

I blogged about this study when preliminary results came out in November, but now the scientists have dotted all the i’s and crossed all the t’s. Volunteers using Brain Fitness for 40 hours over the course of eight weeks improved their memory and attention by about 10 years--that is, 65-year-olds had the brains of 55-year-olds--and the benefits carried over from the lab into real life: people reported noticeable improvements in remembering names they heard spoken and understanding conversations in noisy settings.

Equally important is that the study did not compare Brain Fitness to doing nothing. Half the volunteers did the six Brain Fitness exercises, which involve listening for finer and finer auditory distinctions, and half watched an educational DVD. Only the first group showed the notable improvements, lending support to Posit’s belief that only exercises based on neuroplasticity (the brain’s power to alter its structure and function in response to certain inputs) can produce lasting mental benefits. Importantly, the volunteers improved on mental skills that the exercises did not specifically target, namely memory and attention.

So both aerobic fitness and brain training can reduce your mental age. What are you doing glued to a computer screen?