It’s a thought that crosses many middle-aged minds when a word is forgotten or a set of keys misplaced: Is this a fluke, or the first sign of dementia?

“Most of us will experience some cognitive changes with age,” says Molly Wagster, chief of the behavioral and systems neuroscience branch of the National Institute on Aging, who likens the mental change to the slowing of a marathon runner’s times with advancing years.

The ability to call up words is one of the first things to slip. “You might find it more difficult to recall someone’s name or the name of a book you read or favorite movie. Eventually, you’ll remember it, but it takes a little longer,” Wagster says. Such problems are irritating and frustrating, but they’re usually not a sign that your mind is going, Wagster says.

Still, most of us would like to retain our mental acuity as we age. Isn’t there something we can do that’s proven to keep our minds sharp? Right now, the short answer is no.

“There are a lot of things that have some evidence behind them, but it’s hard to find interventions that have convincing evidence behind them,” says Victor Henderson, a neurologist who studies cognitive aging at Stanford University Medical School.

Physical activity seems like the most promising thing you can do to keep your brain at its best, Henderson says. The evidence comes mostly from observational studies rather than the randomized, controlled trials that are considered the gold standard, but it’s consistent: People who engage in aerobic activity — for instance, walking several times per week — show improvements in their cognitive function, particularly in their ability to switch quickly from task to task, Wagster says.

A few small clinical trials and some animal research also support the role of exercise in maintaining cognitive health, but the evidence at this point, Henderson says, is not definitive.

Even so, exercise can help cut your risk of high blood pressure, cardiovascular disease, diabetes and obesity, and these are known risk factors for developing dementia, says Laurie Ryan, program director for Alzheimer’s disease clinical trials at the NIA. Clogged arteries and impaired circulation don’t just up your chance of a heart attack; they also can reduce the blood flow to your brain, and that puts you at risk for dementia.

Diet is another promising way to reduce your dementia risk, Ryan says. A 2009 study of more than 1,300 people found that people who ate a Mediterranean diet — low in saturated fats and red meat and rich in vegetables, fish, whole grains and omega-3-fatty acids — had a lower risk than others of developing mild cognitive impairment. “We can’t say for sure that this will help,” Ryan says, but improving your overall health seems like a reasonable way to promote healthy aging of the brain.

The Internet is packed with ads for supplements, vitamins and other products that promise to keep your memory sharp, but science so far has found little proof to support such claims.

After reviewing studies on interventions to stop or prevent cognitive decline, Raza Naqvi, a geriatric medicine fellow at the University of Toronto, concluded that there was no strong evidence for any of the herbal products or supplements often touted for this purpose. And medicines that once seemed promising for slowing memory loss — estrogen and anti-inflammatory drugs — seemed to do more harm than good, Naqvi says his review found.

A study that appeared this year in the Proceedings of the National Academy of Sciences showed that high doses of a cocktail of vitamins that included folic acid, Vitamin B6 and Vitamin B12 slowed the atrophy of regions of the brain thought to be especially vulnerable to Alzheimer’s disease. But the benefits were seen only in participants who started the study with high blood levels of homocysteine, an amino acid linked to a higher risk of heart disease, which B vitamins are known to lower. This result suggests that the vitamins may not help people with healthy homocysteine levels.

Also, while the study was well designed and well controlled, it was very small, Ryan says. “We’ve seen over and over again that these small studies can look very good, but then they don’t hold up in larger trials,” she says. “I would not recommend that people go out and start taking high levels of B vitamins.”

The approach that came out on top in Naqvi’s review was cognitive training exercises — mental workouts designed to improve memory. “Three really different studies looked at different ways of stimulating the brain, and all three showed that memory was better in the treatment group than in the controls,” Naqvi says. “The challenge is: How do we translate this into things we can do in everyday life?”

Right now, the evidence isn’t sufficiently strong to recommend one form of cognitive activity over another, Henderson says. The computer games that researchers have used in studies may be thoughtfully designed, “but whether they are better than reading a book or having an engaging conversation with someone or learning to play a musical instrument or speak a new language — we’re not at a stage where we can recommend one over the other,” Henderson says.

So, while scientists do the “further studies” that need doing, what’s an aging mind to do?

Skip the nutritional supplements and vitamin pills that you think are helping your brain, say the experts. Instead, eat a healthful diet and engage in social interactions and mentally challenging activities. And do some kind of regular aerobic activity (about 30 minutes, five days per week), Henderson says. These steps won’t guarantee that you won’t forget the name of that book you want to recommend to a friend, but they’re unlikely to hurt.

If the prospect of inevitable memory decline seems depressing, know this: Some types of cognition improve with age. For instance, older adults outperform young people in vocabulary skills. “It may be a reflection of wisdom,” Wagster says. “We’re collecting information and experience and putting it together.”

The idea that older people can’t learn new things is a myth, Wagster says. “As we age, we may not learn something new quite as quickly, but we’ll learn it and learn it well.”