Cause and effect are hard to prove, but research suggests that taking care of your heart may help protect your brain.

In December 2006, findings in the Journal of the American Medical Association cast some doubt on whether the mental workouts from crossword puzzles, sudoku, and the like have long-term benefits for our brains. They may be fun, but a large, five-year study concluded that the sort of stimulation they provide probably does little to improve people’s ability to manage the activities of daily life.

But there’s better news on another front: Taking care of your heart — and the circulatory system in general — may help avert dementia in later life.

Guilt by association

Dementia refers to mental difficulties that are severe enough to interfere with day-to-day living. It often includes poor memory and impaired “executive function” — the ability to start and stop actions and plan ahead.

Alzheimer’s disease is believed to be the main cause of dementia, accounting for roughly 75% of all cases. There’s some debate about the changes in the brain that cause Alzheimer’s, but the dominant theory is that clumps, or plaques, of a protein called beta-amyloid damage and kill brain cells.

In the September 2006 Journal of Internal Medicine, Dr. Meir Stampfer, an epidemiologist at the Harvard School of Public Health, assembled evidence from dozens of epidemiologic studies of cardiovascular disease (heart attack, stroke, and so on) and dementia. Time and time again, researchers have found the conditions to be closely related.

The association with Alzheimer’s extends to some well-known cardiovascular risk factors. For example, diabetes, which greatly increases the risk for having a heart attack, also increases the risk of developing dementia, although by how much varies greatly from study to study. High blood pressure in middle age correlates with dementia in old age.

Research has also shown that high total and LDL cholesterol — again, especially at midlife — are associated with lower mental performance in later life. In some early studies, the LDL-lowering statin drugs (Lipitor, Zocor, other brands) appeared to prevent Alzheimer’s disease. But results from more recent studies have been far less promising.

Cause and effect?

Showing an association is one thing. Proving a cause-and-effect relationship is another.

Experiments have provided some tantalizing clues that there is one. Scientists have reported that cholesterol and beta-amyloid may bind to one another to form a substance that seems to interfere with memory formation. Researchers at the University of Leeds in Britain have found that when brain cells are denied sufficient oxygen (as a result of stroke, for example), they unleash a stream of neurotransmitters that increases levels of beta-amyloid.