Although over the lifetime, obesity has been associated with a number of poor health outcomes and greater overall mortality risk, some previous studies have shown that for older adults obesity was associated with a lower overall risk of death. While body mass index (BMI) is a commonly used measure of health in physical checkups and in studies of health and aging, new research suggests that a muscle mass index could be a more useful measurement for predicting health outcomes for older adults. This may be because old age is a time of a changing balance between body fat and muscle mass—which makes the BMI a less useful measurement. In light of other studies showing that muscle strength is associated with decreased risk of death, the researchers examined the association of a muscle mass index with risk of death. In this study of 3,659 older adults, the researchers found that a greater muscle mass index score was significantly associated with a decreased risk of mortality.
Both the body mass index and muscle mass index take height into account, but while BMI calculates a relationship between height and overall weight, the muscle mass index substitutes an electronic measurement of muscle mass from a body composition analyzer. A number of such body composition analysis devices are commercially available.
The men in this study were 55 and better when the study began, and the women were 65 years and better. Women were selected at older ages due to their longer life expectancy and later onset of cardiovascular disease. All participants were followed annually. Individuals who were underweight or who died within the first two years of the study were excluded in order to remove frail older adults and avoid potentially skewing the results. By the conclusion of the study, 2,012 individuals had died, with an average age of 82.7.
When the top 25 percent of participants with the highest muscle mass index scores were compared with the 25 percent with the lowest muscle mass index scores, the researchers found a 30 percent reduction in mortality risk. There was little difference in the mortality rates of the top 50 percent of individuals when compared to the top 25 percent. Similarly there was little difference between the bottom 50 percent and the bottom 25 percent. This suggests that the observed impact of muscle mass was not just being caused by individuals at the extremes of the scale.
To get additional confirmation that these findings reflected muscle mass, the researchers also created a non-muscle mass index to examine whether fat mass could be contributing to mortality outcomes. There were no statistically significant outcomes related to the non-muscle mass index or BMI, suggesting that muscle rather than fat or BMI is responsible for the reduction in mortality risk and rates.
Lastly, the researchers looked at whether the relationship of the muscle mass index with mortality could be explained by traditional cardiovascular risk factors or factors related to diabetes or pre-diabetes. When they took these other factors into account, they found that muscle mass index was independently related to reduced mortality.
The researchers note that despite the increasing recognition that BMI is not a good predictor of mortality among older adults, it remains a standard clinical practice to counsel patients about their BMI. They also note that measuring muscle mass can be easily measured by a device that looks like and takes up as much space as a scale. At a cost of a few hundred dollars, such devices are not prohibitively expensive for use in health screenings.
It is important that older adults are focused on the aspects of their health that have been shown to be independent predictors of mortality; that is why these factors need to be addressed as much as possible through appropriate health interventions. This research suggests that a muscle mass index should be added to health assessments for older adults, and that the types of exercise interventions that best improve muscle mass for older adults be established and made widely known.