Inactivity - page 2

That may be because most studies aren't long or large enough to see the link, he adds. "It's a modest effect, but it does look like it's there, mostly for postmenopausal women."

And it looks like women can protect their breasts at the same time they're maintaining other body parts.

"We're not clear how much physical activity you need to reduce the risk of breast cancer," says Lee. "But it's in the same ballpark as for colon cancer--between 30 and 60 minutes of moderate-intensity physical activity a day."

As for some other cancers, she adds, "there's some suggestive evidence, but it's not as conclusive."

3 The Brain

It's bad enough that inactivity can turn your muscles to Jell-O. Can it do the same to your brain?

"The evidence is fairly solid that people who are more physically active are at lower risk for cognitive decline and dementia," says Constantine Lyketsos, director of the division of geriatric psychiatry and neuropsychiatry at Johns Hopkins University School of Medicine in Baltimore.

For example, among more than 3,000 older men and women in the Cardiovascular Health Cognition Study, those who reported engaging in at least four activities (like walking, household chores, gardening, and jogging) during the previous two weeks were half as likely to be diagnosed with dementia five years later than those who took part in no more than one activity. (5)

However, exercise had no link with either Alzheimer's or other dementias in people in the study who had the [epsilon]4 version of the apolipoprotein (ApoE) gene. In contrast, a Finnish study found that exercise protected ApoE [epsilon]4 carriers more than people without the gene. (6) (About 20 percent of people have at least one copy of the [epsilon]4 version of the gene.)

"People with the gene seem to get Alzheimer's about 8 to 10 years earlier than non-carriers," explains Lyketsos, who coauthored the Cardiovascular Health Cognition Study. That usually means it starts in their 70s rather than their 80s.

With or without ApoE [epsilon]4, he acknowledges, the evidence on exercise isn't conclusive. "It's always possible that people are reducing their physical activity because they're in the early stages of dementia."

But clues from animal studies are compelling.

"There's a fairly strong neurobiological basis to suggest that the more physically active you are, the less likely you are to develop dementia," says Lyketsos.

For example, the brains of physically active mice have more nerves, more connections between nerves, fewer clogged arteries, more oxygen flow, and better ability to utilize glucose, he explains.

"All are probably factors in helping prevent cognitive decline and dementia."

Recent studies used mice that are prone to acquire the amyloid plaques that are found in the brains of Alzheimer's patients.

"At least one study suggests that if you take these mice out of their traditional cages, where there's little to do, and put them into stimulating cages with more colors, objects, brighter areas, and little mouse treadmills, you find fewer amyloid deposits in the brain," says Lyketsos.