Disease Proof

His team's six-year study of 302 people between 70 and 82 years of age found that any sort of energy expenditure through physical activity was associated with a lower risk of death.

That finding, published in the July 12 issue of the Journal of the American Medical Association, is not entirely surprising. Organizations such as the American Heart Association have long said that some physical activity is better than none. What was unusual about this study was the exquisitely detailed measurements used to determine levels of physical activity, Manini said.

In the study, the researchers had volunteers drink water containing two harmless isotopes, oxygen-18 and hydrogen-2. Oxygen-18 is eliminated from the body in water, while hydrogen-2 is eliminated not only in water but also in carbon dioxide, which is produced during energy expenditure. So, by measuring levels of water and carbon dioxide leaving the body, the researchers were able to get accurate readings of daily energy expenditure.

"The technique has been around for use in humans for 20 years," Manini said. "It is kind of expensive for a large-scale study, and also requires special expertise."

Following the participants for six years, the researchers found that death rates went down as daily energy expenditure went up. In fact, seniors in the highest third of daily energy expenditure had a 69 percent lower risk of dying than those in the lowest third.