Older people who began eating right, exercising, playing "brain games" and socializing more often performed better on memory and problem-solving tests than people who maintained their habits, the researchers said.
Earlier studies have observed that each of these lifestyle changes might help fight dementia. But this is the first randomized clinical trial to put those findings to the test, said Keith Fargo, director of scientific programs and outreach for the Alzheimer's Association.
"This is the first study to definitively show that changing your lifestyle will reduce your risk for cognitive decline," Fargo said.
The study involved 1,260 people aged 60 to 77 at risk for dementia and Alzheimer's disease.
Half of the participants received nutritional guidance, physical exercise, brain training, social activities and management of heart health risk factors. The other half just received regular health advice.
After two years, the group that underwent lifestyle changes performed significantly better on memory tests, problem-solving exercises and quick-thinking quizzes.
Only about 11 percent of participants dropped out after two years, which researchers took as a sign that the lifestyle changes weren't too onerous.
The study findings were presented this week at the Alzheimer's Association International Conference in Copenhagen, Denmark.
The findings are in keeping with long-term studies in the United States that found that exercise, diet and social activity can help stave off dementia, said Dr. Gisele Wolf-Klein, director of geriatric education at the North Shore-LIJ Health System in New Hyde Park, N.Y.
"It does not come as a surprise to those of us who have been working for a very long time with patients who are at risk of developing dementia," Wolf-Klein said.
It seems that by giving your brain regular mental and social workouts, you can keep it limber. And diet and exercise can help manage health problems that could affect the brain, she said.
"There's a general feeling that if you eat the right thing and exercise, that's better for your overall health," Wolf-Klein said.
At the same time, healthy living isn't a guarantee against dementia.
"I don't think you can say the risk goes away altogether. As people age, they will have some decline in their cognitive abilities. That's just a part of aging," Fargo said. "But try to maintain healthy activities, a healthy lifestyle, in middle age and later, and that's going to help reduce your risk for cognitive decline."
The study authors plan an extended seven-year follow-up that will track the incidence of dementia and Alzheimer's, and will include brain imaging scans.
For more on dementia, visit the U.S. National Institutes of Health.