Memory loss is the single biggest fear for Americans over the age of 55. And it’s understandable: over 4 million currently suffer from Alzheimer’s disease, and those numbers are expected to quadruple by 2050, according to the Alzheimer's Foundation. That may be why products promising to improve your brain function are flooding the market. Sudoku and crossword puzzles are said to improve memory association skills, though critics believe only when put to task by those puzzles. Ginkgo infused soft drinks line the grocery aisle, ever since the root was suggested to combat dementia (it doesn't). Even celery has been loosely linked to mental acuity. But the truth is there’s not enough hard evidence that any of these things really work.

In fact, there’s only one practice that’s been proven, without question, to preserve your memory: exercise. "Aerobic activities tend to show larger effects than non-aerobic activities," University of Pittsburgh psychologist Kirk Erickson tells Yahoo.

Working up a sweat helps your mind stay fit better than any crossword puzzle--unless you're doing that crossword on a treadmill.

The good news is that you don’t need to run a marathon. Just walking six miles a week can ward off memory disorders caused by aging, according to Erickson's research published this month in the medical journal Neurology. "It appears that if people start exercising their memory may improve and if you continue to exercise, that might delay, or offset, the age-related decline in memory," he explains.

And you don't need to lift any heavy barbells either. Erickson and his team monitored 300 senior adults over a period of 13 years, and found that those who walked between 6 and 9 miles a week—whether to work or with the dog--had half the brain deterioration of those who didn’t. "Exercise seems to enhance some of the more fundamental properties of our brain," Erickson explains. "It increases the growth of new cells and improves cellular processes associated with learning and memory." To put it simply, walking keeps your gray matter from shrinking. And the more matter, the more mind.

Another study published earlier this year suggests exercise can actually help your brain grow. A moderate workout may generate new brain cells. And not just any brain cells, but cells that specifically help to distinguish between memories, so each recollection stands out. It’s the kind of function you rely on every day, says Tim Bussey, one of the authors of the Cambridge University study. "[These cells help with] remembering which car parking space you have used on two different days in the previous week."

But exercise isn't the only way to keep tabs on your parking spot. There are some supplemental practices that doctors recommend in addition to a regular walk-a-thon. Diets rich in Omega fatty acids are instrumental in keeping your brain from aging. Two servings of salmon a week, provides ingredients that support brain tissue and enhance nerve cell function. Balancing fish with the other elements of a Mediterranean diet, like fruits and vegetables, has been found to lower the chances of cognitive decline. When it comes to memory retrieval, self-testing can be beneficial. In other words, pausing between paragraphs of an article and asking yourself to paraphrase the information, or repeat a fact. It can't hurt if that article is written in another language. Bilingualism, says one new study, helps ward off Alzheimer’s for up to four years. But it doesn't prevent the disease altogether. Your best bet: Walk it off.