That half-used bottle of ginkgo biloba in the medicine chest? You can probably toss it. The stack of crossword and Sudoku puzzles by your chair? Fine if you enjoy them, but don't force yourself to slog through them like your 10th-grade homework. The treadmill in the basement? Keep on treading -- not only is physical exercise good for your heart and waistline, but it just might help keep your brain in shape, too.

That's the latest advice for keeping your mind sharp as you get older, based on what scientists know so far; unfortunately, that's not much.

Researchers have found little proof that any factors within our control can help ward off Alzheimer's disease, diminish dementia or preserve our ability to form memories, make decisions and learn new information.

"If you drill down into the hard-nosed, scientific studies, if you say, 'Show me that you can prevent Alzheimer's by doing A, B or C,' you just can't do it," said Dr. Ronald C. Petersen, director of the Alzheimer's Disease Research Center at the Mayo Clinic in Rochester. "The data are not that convincing for any single intervention."

This might come as a surprise to anyone who watches TV, reads magazines or surfs the Internet. There's plenty of advice: Eat more berries (or fish, or curry). Take Vitamin E. Get more sleep. Meditate. Drink one glass of wine a day. Have an active social life. Think positive thoughts.

Often these prescriptions for brain health are presented as if backed by incontrovertible proof. But real evidence is lacking, according to an independent panel of doctors and experts who met last year at a conference organized by the National Institutes of Health. After examining research and hearing from investigators in the field, the panel wrote that, for now, "firm conclusions cannot be drawn about the association of any modifiable risk factor with cognitive decline or Alzheimer's disease."

Those helpful tips in the media are generally based on studies showing correlation between brain health and other factors. But those studies -- most of them small, methodologically flawed, inconsistent or limited by practical constraints -- don't actually prove that those factors are what caused the healthy minds.

For example, researchers have observed that people who are socially active are more likely to be mentally sharp. Is this because social activity stimulates the brain and forestalls cognitive impairment? Or is it that people who are mentally agile enjoy getting together with friends and family, while their peers who have trouble following a conversation are inclined to withdraw? A team from Duke University combed through 25 years of research, examined nearly 300 studies, then reported that hardly any of them indicated which direction the causal arrow points.

The Duke team did find strong research regarding ginkgo biloba's effect on Alzheimer's -- unfortunately, it doesn't work. It also found some evidence that Vitamin E supplements do no good.

Train your brain

On the bright side, there is some encouraging news about the potential benefits of training your brain. The NIH panel cited a large, well-designed study in which people who underwent five or six weeks of cognitive training (focusing on memory, reasoning and speed) showed modest improvements in cognitive functioning. Five years later, they still showed a bit less age-related decline than those who hadn't taken the training.

It's not clear whether similar benefits can be gained from more ordinary pursuits such as crosswords, Sudoku or reading. Petersen's advice is to challenge yourself with activities you enjoy, but "if it's, 'Gee, I've got to do my five New York Times crossword puzzles today or I'll fall behind,' then don't bother," he said.

Other studies have suggested that physical activity may help keep the mind fit. "Not necessarily marathon running," Petersen said. "Brisk walking on a regular basis would work."

For example, in an experiment using mice genetically predisposed to develop Alzheimer-like plaques in the brain, the mice developed fewer symptoms the more time they spent on an exercise wheel. Though it's not clear that translates to humans, "at least it showed that physical exercise can affect the biological process in the brain that is relevant to Alzheimer's disease," Petersen said.

Petersen, the NIH panel and the Duke team all emphasized the need for more and better research. But good studies, often involving years of observation, are expensive, and federal funding for research is dwindling, Petersen said. Meanwhile, Alzheimer's cases are poised to explode. An estimated 5.1 million Americans currently have the disease, a number projected to double or even triple by 2050.

"With baby boomers just coming into age 65 this year, the problem is only going to mushroom and get much worse, very soon," he said. "And the research support is going in the other direction."

Katy Read • 612-673-4583