People joke, especially as they reach middle age, about having a "senior moment" -- often when they've reached in vain for the name of an actor or walked into a room and forgotten why. It's a joke, but also a worry: Am I not as swift as I used to be?

It's true that the brain, like every other part of the human body, can lose some of its power with age. It experiences both a gradual loss of cells and some shrinkage of tissues. But in recent years scientists have identified a number of activities that seem to protect the brain throughout young adulthood, midlife, and old age. These beneficial practices, which touch upon dietary choices, exercise habits, and even leisure pursuits, appear to reduce the risk of age-related memory loss as well as neurological disorders such as Alzheimer's disease.

Mental Preparedness
It's intuitive, really: To stay in shape, a brain needs to work out. Scientists who study everyone from 55-year-olds on up to centenarians have found that people with advanced educations and those who regularly engage in mental exercise -- for example, by reading, taking classes, sharing in stimulating conversations, or speaking more than one language -- build a bank of brain-power that defends against decline. Keeping the mind active appears to generate new brain cells and also strengthen the signals between them, forming a robust mental network.

But all kinds of activities, even everyday diversions, appear to spur the brain. In a study of 469 people older than 75, published in The New England Journal of Medicine in 2003, Dr. Joe Verghese, an associate professor of neurology at Albert Einstein College of Medicine in the Bronx, New York, found that playing musical instruments and board games, reading, and dancing were associated with a reduced risk of dementia. (Dementia is an umbrella term that includes several conditions, including vascular dementia, which is frequently connected with stroke, and Alzheimer's.)

The hobbies in Verghese's study were ones the group happened to favor. But scientists say that knitting, traveling, doing odd jobs, and taking a crack at crossword puzzles, sudoku, and all sorts of brainteasers also can help preserve mental agility.

"It's important to find a set of activities that you find enjoyable and start them at an early age," Verghese says. Of his subjects' pursuits, most were things the people started doing well before they turned 75.

Follow the Heart
Less obvious, perhaps, but no less critical is physical exercise. Several scientists have studied the benefits of walking, for example, on cognitive health. A two-year study of more than 2,000 physically able men ages 71 to 93 in Hawaii, published in The Journal of the American Medical Association in 2004, showed that those who walked at least a mile a day had a reduced risk of dementia. Those who walked two miles or more seemed to be even more protected. And a recent review of several other studies of exercise and brain function concluded that people who get 15 minutes or more of physical activity three times a week are less likely to develop Alzheimer's, even if they are genetically predisposed to the condition.

People should not have the impression that only strenuous activity will have a salutary effect, says Dr. Brent Ridge, an assistant professor at Mount Sinai School of Medicine in New York City and vice president of healthy living at Martha Stewart Living Omnimedia. Some of the best activities, scientists say, are those that vitalize the mind and the body at once, such as gardening and dancing.

Limited blood flow to the brain is thought to set the stage for mental decline in various ways. In this regard, scientists speculate that exercise aids the brain just as it does the heart, by keeping blood vessels healthy and elastic. Another theory is that physical exertion, like the mental variety, sparks the development of neurons.

Food for Thought
Many conditions linked with cardiovascular disease, including high cholesterol, high blood pressure, obesity, and diabetes, also seem to heighten the risk of dementia. Conversely, many nutritional practices that are considered beneficial to the heart might help keep the brain sharp.

Since 1993, scientists at Rush University Medical Center in Chicago have been tracking the dietary habits of a large population of Chicagoans age 65 and older, assessing their brain function every three years. Over time, the participants' test scores have declined a little overall. But subjects with certain eating habits seem more likely to maintain their cognitive health.

A nutritional regimen abundant in foods containing omega-3 fatty acids -- especially the DHA (docosahexaenoic acid) type in fish -- is associated with an 80 percent reduction in the risk of developing Alzheimer's, says Martha Clare Morris, an associate professor at Rush. (Mackerel, salmon, tuna, and sardines are some leading sources.)

Individuals in the study who reported eating the greatest amounts of polyunsaturated fat (found in nuts, seeds, and their oils) and monounsaturated fat (available from avocados, canola oil, and olive oil) had a 70 percent to 80 percent lower risk of dementia than average. High intake of saturated fat, the kind present in meat, butter, cheese, whole milk, and processed and fast foods, on the other hand, seemed to double the risk of dementia.

A diet high in vitamin E-rich foods, such as vegetable oil; seeds and nuts; and dark-green, leafy vegetables, is associated with a 60 percent reduction in Alzheimer's risk, Morris says.

Notice the dietary correlations involve foods, not pills. Scientists have not found strong evidence that supplements protect the brain. William Thies, vice president of medical and scientific relations for the Alzheimer's Association in Chicago, advises against fixating on specific nutrients. Instead, he says, adopt a strategy that's easier to remember: Eat more vegetables and less saturated fat.

It is also important to watch alcohol intake. Many studies have shown that moderate drinking is better for the brain than abstention. But many Americans overestimate what a modest amount is. "I always caution people," Morris says. "We're talking about low levels of alcohol." According to public health officials, this means one drink a day for women and two for men. (One drink equals 12 ounces of beer, 5 ounces of wine, or 1 1/2 ounces of spirits.) High consumption of alcohol has a long-lasting negative effect on mental clarity.

Defining "Sharp"
The kind of keenness that people have a tendency to lose as they get older is not the same thing as basic intelligence. As the brain ages, its processing time merely slows down. Many individuals often notice, for example, that it takes them longer to remember the names of things and people. They also might have a shortened attention span. Or they may find that their executive functioning -- the ability to balance a checkbook, for instance -- is flagging.

But there is a significant difference between small struggles with memory and real dementia. Many experts explain the difference with this example: It's one thing to forget where you put your car keys; it's another thing to forget what car keys are for.

"The brain is still a bit of a mystery to us in how it does all that it does, but it is still clearly biological," Thies says. "To the extent that you manage its biological environment for optimum performance, you're going to get better function." Said in another way: Take care of your body and your brain will take care of you.