The aging of the population along with the rise in the prevalence of obesity are two major public health issues with serious economic consequences. Indeed, both these factors are closely linked to health and productivity metrics such as physical disability. Traditionally assessed by the body mass index (BMI, calculated as weight in kilograms divided by height in meter square), obesity has been linked to physical disability. However, whether a more precise assessment of body composition would better identify people who are at greater risk of physical disability was uncertain until a report published in the AGE journal by the investigators of the Framingham Heart Study in Boston shed light on this issue.

In this study, BMI and muscle fat accumulation were measured in 1152 men and women (mean age of 66 years) from the general population who covered a wide range of adiposity. Muscle fat accumulation was measured in the paraspinous muscle at the mid-abdominal level by computed tomography (CT). Study participants also underwent a series of measurements of mobility disability and physical performance such as the ability to walk a half-mile or climb a flight of stairs, walking speed and dynamometer-measured grip strength. The results of this study showed that compared to those with less muscle fat, those with more muscle fat were three times more likely to have a walking speed of less than 1 meter/second, a finding that was remarkably consistent among men and women. Interestingly, the association between muscle fat and walking speed was independent of BMI. The authors also reported that the association between muscle fat and grip strength was strong (in men) and independent of BMI.

Although this is one of the largest studies conducted on this topic and that the investigators thoroughly assessed body composition, this study has some limitations. For instance, this was an observational study and, consequently, the researchers cannot conclude that there is a causal association between muscle fat and mobility disability or physical performance. Additionally, although muscle fat measured by CT correlates reasonably well with muscle density, it was only measured in one site. Nevertheless, this study has important implications. For instance, because interventions targeting physical activity levels have been shown to decrease muscle fat infiltration and increase muscle strength, this study suggests that physical activity does not have to induce weight loss to yield substantial health benefits, even in people who are overweight or obese. On the other hand, this study reinforces the notion that people who have a normal weight (at least according to their BMI) could nevertheless be at risk of developing impaired mobility or a decline in physical performance if they are sedentary with muscle fat infiltration. Taken together, the latest studies on physical activity and body composition suggest that in our aging population, increasing physical activity levels could improve population health and prevent numerous cases of physical disability.