It’s well established that exercise substantially changes the human brain, affecting both thinking and emotions. But a sophisticated, multifaceted new study suggests that the effects may be more nuanced than many scientists previously believed. Whether you gain all of the potential cognitive and mood benefits from exercise may depend on when and how often you work out, as well as on the genetic makeup of your brain.

For the experiment, published last month in Neuroscience, researchers in the department of psychology and neuroscience at Dartmouth College in Hanover, N.H., recruited 54 adults, ages 18 to 36, from the college and the surrounding community. The volunteers were healthy but generally sedentary; none exercised regularly.

During their first visit to the lab, they completed a series of questionnaires about their health and mood, including how anxious they were both at that moment and in general.

They also gave blood for genetic testing. Earlier studies had shown that exercise can increase levels of a protein called brain-derived neurotropic factor, or BDNF, which is thought to play a role in the positive effects of exercise on thinking. But some people produce less BDNF after exercise than others because they have a variation in the gene that controls BDNF production, though it’s unknown whether they derive less cognitive benefit from exercise as a result. So the scientists wanted to determine each volunteer’s BDNF gene status.

Then the group submitted to a memory test, consisting of pictures of objects flashed across a computer screen. Soon after, another set of pictures appeared, and the volunteers were asked to note, with keystrokes, whether they’d seen each particular image before.

This task involves a different part of the brain from the one most often focused on in studies of exercise and memory, says David Bucci, an associate professor of psychology and brain science at Dartmouth, who oversaw the study. Other experiments typically examine the effect of exercise on the hippocampus, the brain’s primary memory center, he says, but the object-recognition task involves activity in the perirhinal cortex, a portion of the brain essential to remembering particular things or objects and whether they happen to be new in your experience. Without a healthy perirhinal cortex, you might recall where you’ve put your car keys (a hippocampal memory task), but not what car keys are.

Finally, after completing the tests, the volunteers were randomly assigned to exercise or not during the next four weeks. Half began a supervised program of walking or jogging four times a week for at least 30 minutes. The other half remained sedentary.

After a month, the volunteers returned to the lab for retesting. But first, some exercised. Half of the exercising group walked or jogged before the testing; half did not. Ditto for the sedentary group: Half exercised that day for the first time since the start of the study; the rest did not.

The earlier tests of memory and mood were repeated.

The results were, in certain aspects, a surprise. As expected, many of the volunteers who’d been exercising for the past month significantly improved their scores on the memory and mood tests. But not all of them did. In general, those volunteers who had exercised for the past month and who worked out on the day of retesting performed the best on the memory exam. They also tended to report less anxiety than other volunteers.

Those who had exercised during the preceding month but not on the day of testing generally did better on the memory test than those who had been sedentary, but did not perform nearly as well as those who had worked out that morning.

Interestingly, while exercising before the test didn’t improve the memory scores of those who’d remained sedentary for the past month, it did increase their self-reported anxiety levels. They were more jittery than they had been on the first lab visit.

Perhaps most intriguing, though, was what the researchers discovered when they compared the volunteers’ BDNF gene variants and their scores on the memory test. They found that those with the variant that blunts BDNF production after exercise - a fairly common variation, existing in about 30 percent of people of European Caucasian heritage - did not improve their memories, even if they exercised regularly. (No consumer test exists to check for the variant.)

What all of this means for people who are hoping that exercise will improve their minds is unclear, Dr. Bucci says, but it does suggest that the interplay of physical activity and brainpower is more complex than we have perhaps yet acknowledged.

Some people’s ability to recall objects, for instance, “may respond less robustly” to exercise than other people’s, he says, if their genetic makeup doesn’t promote the release of BDNF.

But the overall message of this study and of ongoing research in his lab, Dr. Bucci adds, is that exercise generally enhances the ability to remember. The people who did improve their memory test scores, he points out, were invariably those who’d exercised throughout the previous month and again the morning of the testing, suggesting a powerful cumulative effect from the exercise sessions, he says.

More generally, Dr. Bucci says, there are many types of memory involving many different areas within the brain, and few seem unaffected by regular, moderate exercise, although the effects may be inconsistent from person to person.

“The current data strongly suggests that people should be physically active” if they wish to enjoy a sturdy, unporous memory in the long term, Dr. Bucci says. Walk or jog regularly, in other words, and most of us can expect to continue recognizing our keys as keys.

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By GRETCHEN REYNOLDS