In older adults, higher physical activity level has been associated with a lower mortality risk compared with those who report low physical activity. It is unclear whether this risk reduction can be attributed to free-living activity, because self-reported physical activity is an unreliable measure of energy expenditure. This study directly measured energy expenditure through the double-labeled water technique (two isotopes whose relative excretion measures carbon dioxide production). Using this standard, the authors sought to determine to what extent total free-living activity—including purposeful exercise as well as routine activity—reduces mortality risk in older adults.

The study followed 302 adults who were 70 to 82 years of age and measured total energy expenditure with double-labeled water at baseline and after two weeks. Resting metabolic rate also was measured, and activity energy expenditure was calculated in kilocalories per day. Objective calculation of physical activity was achieved by dividing total energy expenditure by the resting metabolic rate. In addition, the researchers obtained self-reported physical activity information about the previous seven days from a questionnaire. To document date of death, participants were followed for eight years and contacted by phone every six months. They also provided self-reported information about their health status, body fat, height, weight, and body mass index.

There was a cumulative mortality of 18.2 percent. For every 287 kilocalories per day in activity energy expenditure, the hazard ratio (HR) for mortality risk was 0.68 (95% confidence interval [CI], 0.48 to 0.96). The increased physical activity, as defined above, showed that mortality risk was similarly lowered. When expressed in terms of tertiles (see accompanying table), the highest energy expenditure had a lower mortality risk compared with the tertile with the lowest energy expenditure (HR = 0.31; 95% CI, 0.14 to 0.69). A similar proportion of patients in each tertile reported that they engaged in high-intensity exercise. However, there were few self-reported behaviors that distinguished one tertile of patients from the others.

In this study, objective measurements showed that higher free-living energy expenditure is associated with lower mortality rates, and every 287 kilocalories of energy expenditure per day lowers mortality risk by 30 percent. Resting metabolic rate, bodily movements, and usual activities—all of which play a role in total energy expenditure—also decrease mortality risk. Daily activities, such as vacuuming, walking up and down stairs, lawn work, and washing windows, constitute energy expenditure.

The authors conclude that self-reported exercise measurements may be underestimated. Likewise, it is important to establish appropriate physical activity levels that are suitable for older adults because active energy expenditure and physical activity levels tend to decrease with age. The emphasis on maintaining these activities will most likely improve the health of patients who are older. Expanding suggestions for physical activity to include a range of other active routines also may lead to a beneficial increase in energy expenditure in older adults.