(USA TODAY) -- Ask any adult over 50 about memory loss, and you're likely to hear a lot of complaints.

Starting in midlife, the brain's ability to remember names, multi-task or learn something new starts to go downhill.

There's no "smart pill" to ward off that age-related decline, but experts increasingly say lifestyle factors such as physical activity, challenging hobbies and lots of friends or social engagements might help keep the brain more nimble and fit as it ages.

Those protective factors are listed in a report financed by the National Institutes of Health and published in Alzheimer's & Dementia: The Journal of the Alzheimer's Association.

One author of that report, Marilyn Albert of Johns Hopkins University, says seniors who adopt those healthy habits might also be lowering their risk of developing Alzheimer's, a degenerative brain disease that afflicts 4.5 million people in the USA.

"We're learning now there are things that people can do to lower their risk," says Albert, who also is a spokeswoman for the Chicago-based Alzheimer's Association.

Work out

Studies suggest regular physical activity seems to protect against cognitive decline, possibly by spurring the production of natural chemicals that protect brain cells.

There's no consensus on what type of exercise is best, but Fordyce says even modest exercise can help keep the brain alert. "Walking just three times a week for 20 minutes can diminish your chances of getting dementia, including Alzheimer's," she says.

For seniors who are in good shape, Albert recommends dancing, swimming or even playing a round of golf -- provided, naturally, that you walk the course.

Stay curious

Research on animals and humans suggests that mentally challenging activities such as playing bridge, learning a new language or even blogging might help build new connections in the brain, says Molly Wagster at the National Institute on Aging, part of the National Institutes of Health.

To get the most benefit, Fordyce suggests seniors pick an activity that provides them with a challenge, such as learning a language or taking classes at the local university or senior center. Even learning to use a computer or a cellphone can be a challenge for someone who has never used the latest technology, she says.

Stay connected

Seniors who join a book club or some other social activity must use their brains to interact with other people, says Christine Himes Fordyce of the Group Health Cooperative in Seattle. Researchers don't know whether regular social contact builds a better brain or simply reduces stress, which can harm the brain.

What the research can't offer is a one-size-fits-all prescription. Albert says seniors should pick activities they enjoy.

"Figure out what works for you, and then do it," she says.

Copyright 2006 USA TODAY, a division of Gannett Co. Inc.