Are fit people smarter than couch potatoes? A new study hints that if they aren't smarter right now, they will be in 20 years. It's hard to maintain your mental bandwidth when your brain shrinks, and that is what happened to people who didn't keep fit, a recent study finds.

The study is part of the Framingham Heart Study, which began in 1948 and continues to this day. Researchers estimated people's fitness by measuring how much oxygen their bodies were capable of utilizing while exercising on a treadmill. The first measurement was done when people were about 40 years old. People who had heart disease at that time were excluded from the study.

People whose blood pressure and heart rate went up at a higher rate during exercise were more likely to have smaller brain volumes two decades later.

By measuring peak oxygen consumption and then factoring in how long people could exercise on the treadmill before their heart rate rose too high and also their body weight, the researchers were able to come up with a numerical unit that estimated a person's overall fitness.

Twenty years later, the test was repeated and MRI brain scans were also taken.

For every eight units lower a person's fitness score was on their original treadmill test, their brain volume twenty years later was considerably smaller, having shrunk by an amount normally seen in two years of accelerated brain aging. When people who had developed heart disease or had started taking beta blockers were excluded from the calculations, brain shrinkage, while still sizeable, was cut in half. Then, the observed shrinkage was equal to about one year of accelerated brain aging for every eight units of poorer physical performance on the treadmill 20 years earlier.

The study also found that people whose blood pressure and heart rate went up at a higher rate during exercise were more likely to have smaller brain volumes two decades later. People who are in poor physical condition must work harder than people who are more fit when they exercise, and this shows up in larger increases in heart rate and blood pressure.

This study shows us the relationship between fitness and brain volume; it can't prove that a lack of exercise causes the decrease in brain volume 20 years later. But it joins a list of several other studies that hint that what's bad for the rest of the body is also bad for the brain. Such studies have linked heavy TV viewing, having a large waist size, as well as a poor diet to declines in brain size or mental abilities.

The finding that brain shrinkage was much larger in people who had developed heart disease hints that keeping fit is especially important for them.

An article on the study appears in Neurology.