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Functional disability has been associated with greater rates of injury, dependency, chronic disease, cognitive decline, and mortality (1,2). Mobility-related disability is directly related to markers of adiposity such as BMI and percentage of fat mass (3,4,5,6,7,8,9,10). The role of other parameters of body composition on disability remains to be defined. Muscle mass is of particular interest because it can be lost with ageing (sarcopenia) (11,12). Nevertheless, a surrogate for muscle mass, fat-free mass, is a marker of disability in some (9,13,14,15,16) but not all studies (6,17,18).

Several explanations have been considered to account for the inconsistent relationship between fat-free mass and disability. First, the elderly can lose muscle mass excessively with weight loss (12). The loss of muscle mass with weight loss may not be fully recovered with weight gain in old age, particularly in men and during periods of illness. Second, the relation of muscle mass to muscle strength is quite linear, but the relation of muscle strength to physical function is not (11). Third, selection bias of participants may weaken the association between muscle mass and disability. Besides, subjects who return to the follow-up examination visit have better baseline lower-extremity performance than those who do not return to follow-up (19). Fourth, most studies use different instruments to measure of physical function. Finally, certain techniques may constitute a potential source of bias in the measurement of body composition (e.g., bioelectrical impedance) (20).

Jankowski et al. examine 109 subjects aged ≥60 years and show that adiposity, but not appendicular skeletal muscle mass, is associated with physical function (21). Direct and indirect measures of physical function are analyzed as well as BMI and two direct measures of body composition (fat index and appendicular skeletal muscle mass by dual-energy X-ray absorptiometry). The authors point out that BMI is as good a correlate of physical function as fat index.

The reported relationship between adiposity and physical function is consistent with previous studies (3,4,5,6,7,8,9,10), but a number of research questions remain unanswered. Few studies have analyzed the ability of markers of central adiposity to predict future disability in older adults (7,22). More significant, disability has been associated with weigh loss in a number of studies (7,8). These experts even recommend maintaining weight to prevent excessive loss of muscle mass in old age (7,9,12). Future studies need to address the limitations of observational studies attempting to assess health effects of weight loss, such as the inability to control adequately for comorbidities and to separate intentional from unintentional weight loss.

Jankowski et al. also report that appendicular skeletal muscle mass is not related to physical function. This result, however, has to be interpreted with caution. First, the number of participants is relatively small. Second, the exclusion of subjects with illnesses may have weakened the relation of muscle mass to physical function. Third, muscle mass may not be as relevant a correlate of mobility-related disability as fat mass in absolute terms, but may still be important relative to amount of fat mass (16). Fourth, progressive loss of muscle is associated with adipose tissue infiltration of muscle mass (23). Fat infiltration into the muscle correlates with low extremity performance (24) and predicts future mobility limitation (25). Fifth, resistance training improves muscle strength and functional performance without significant or small increases in muscle mass (26). In this regard, muscle strength may be a better predictor of lower-extremity function and mortality than skeletal muscle mass (19,27).

Obesity and body composition are relevant to the fact that life expectancy is threatened by disabilities. In women aged ≥75 years, only 6 years of the 13.2 expected years of life will be disability free; in male counterparts, only 3.1 of 9.6 years (28). Functional disability, however, may not be an inevitable consequence of aging (29). One-third of individuals aged ≥75 years with recent loss of autonomy (<1 year of duration) are able to recover their autonomy during the following year (30). Among adults aged ≥65 years, 31% have some type of mobility limitation (31) and the cost of medical care for a disabled older person averages three times that for a nondisabled senior (32). Clearly, the obesity epidemic is of great concern for the well-being and care of older adults, the fastest growing segment of the US population (in 2000, 12%; projections for 2050, 20%) (33). Recent surveys indicate that obesity (defined as BMI ≥30 kg/m2) is present in 31% of adults aged ≥60 years (34). Nevertheless, sarcopenia is of no less importance in the elderly. Sarcopenia (defined as appendicular skeletal muscle mass two standard deviations below the mean of a young reference group) is observed in ∼25% and 50% of individuals aged ≥65 and ≥80 years, respectively (35,36), and is associated with a three- to fourfold increased risk of disability (35). Consequently, clinical trials are needed in older individuals to assess the effect of interventions aiming to modify adiposity and muscle mass on physical function, disability, and mortality.

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The author declared no conflict of interest.

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