Observational studies have shown that older adults who report low physical activity levels are at a higher risk of death compared with those who report moderate or high levels of activity. These findings have been based on questionnaires asking about physical activity levels, which may not be recalled accurately and are unable to account for many types of daily activity, according to background information in the article. Self-reported physical activity does not provide accurate estimates of absolute amounts of activity (kilocalories per day) and thus is less precise in determining whether higher levels of total activity-induced energy expenditure offer survival advantages.
Todd M. Manini, Ph.D., of the National Institute on Aging, Bethesda, Md., and colleagues conducted a study to determine the association of free-living activity energy expenditure with death from all causes in a group of 302 high-functioning, community-dwelling older adults (aged 70-82 years). The researchers measured energy expenditure over a two week period using a technique that includes determining the rate at which certain isotopes of hydrogen and oxygen, given as "doubly labeled" water, are eliminated from the body as carbon dioxide, a direct measure of total energy expenditure. The resting metabolic rate was also measured. Participants were followed up over an average of 6.15 years (1998-2006). Fifty-five participants (18.2 percent) died during follow-up.
The researchers found that, after adjusting for various factors, higher levels of activity energy expenditure and physical activity were associated with a lower risk of death. Compared with the third of individuals with the lowest activity energy expenditure, those in the highest third had a 69 percent lower risk of death. The absolute risk of death was 12.1 percent in the highest tertile of activity energy expenditure, 17.6 percent in the middle, and 24.7 percent in the lowest tertile. According to self-reports, individuals expending higher levels of free-living activity energy were more likely to work for pay and climb stairs but self-reported high-intensity exercise, walking for exercise, walking other than exercise, volunteering, and caregiving did not differ significantly across the activity energy expenditure tertiles. The authors suggest that this lack of relationship is likely due to the inaccuracies of self-reported activity levels.
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