Anyone who exercises knows intuitively that being fit not only makes you feel better, it sharpens your mind.

Lots of studies have indicated a correlation between fitness and mental acuity. But the question has often lingered: is this just because people who are lucky enough to be primed genetically for fitness are also primed for mental acuity?

An ingenious new study of 162 pairs of middle-aged women in the UK demonstrates that physical fitness does cause mental fitness, as measured both in intelligence tests and brain scans.

The scientists looked for twins who, 10 years previously, had completed extensive computerized examinations of their memory and thinking abilities, as well as assessments of their metabolic health and leg-muscle power, which measure muscles’ force and speed.

The scientists focused on the twins’ muscles rather than their exercise habits largely because the power measures were objective, unlike people’s notoriously unreliable recollections of how much they have worked out. (There was a correlation, though, between more self-reported exercise and sturdier legs.)

The scientists then asked the twins to visit a laboratory and repeat the cognitive tests.

Twenty of the identical twin pairs also completed brain-imaging scans. Then the researchers compared leg power 10 years earlier with changes in brain function over the same time period. They found that of the 324 twins, those who had had the sturdiest legs a decade ago showed the least fall-off in thinking skills, even when the scientists controlled for such factors as fatty diets, high blood pressure and shaky blood-sugar control.

The differences in thinking skills were particularly striking within twin pairs. If one twin had been more powerful than the other 10 years before, she tended to be a much better thinker now.

In fact, on average, a muscularly powerful twin now performed about 18 percent better on memory and other cognitive tests than her weaker sister.

Similarly, in the brain imaging of the identical twins, if one genetically identical twin had had sturdier legs than the other at the start of the study, she now displayed significantly more brain volume and fewer “empty spaces in the brain” than her weaker sister, Dr. Steves said.

Here’s a link to more information about the positive influence of fitness on intelligence and brain activity.