Sarcopenia
Muscle and bone mass, as well as their strength/quality, are both lost during aging starting in the late 20s and accelerating in the 50s [11–13]. In the presence of chronic disease, drug therapy, environmental factors, poor nutrition, and physical inactivity, this loss can be accelerated.
One important advantage of using DXA is its ability to estimate appendicular skeletal muscle (ASM) mass which is of particular interest for the assessment of sarcopenia. ASM corresponds to the amount of LBM in the arms and legs, which is mainly muscle (except for a small amount of connective tissue and skin) [14].
Sarcopenia is a term originally used to describe age-related decreases in muscle mass [15]. The term has evolved to indicate a point where skeletal muscle mass and/or strength has declined past a threshold in which health is affected. The European Consensus [16] has recently defined sarcopenia as a progressive, generalized loss of muscle mass and strength which will increase the risk for physical disability, diminished physical performance, and poor quality of life. Although losses of both muscle mass and strength can occur simultaneously, this relationship is not linear [16–18].
Although there are several working definitions of sarcopenia, one of the most commonly used criteria is a level of skeletal muscle mass (ASM index, Table 1) lower than 2 SD below the expected mean for healthy young adults, as established by DXA. This sex-specific definition corresponds to ≤7.26 kg/m2 for men and ≤5.45 kg/m2 for women [19]. Importantly, individuals below this cut point have a significantly increased risk of adverse functional status, such as higher risk of disability, falls, and fractures [20].
It is important to highlight that establishing the threshold to define sarcopenia has been a challenge for the past several years. A variety of cut points in the literature have been reported, which were developed using different body composition techniques such as DXA, bioelectrical impedance, skinfold thickness measurements, and computerized tomography (CT images) [21]. Although most cut points are based on measurements of muscle mass, muscle quality has also been used as a criterion to identify sarcopenia. Measurements of muscle strength (handgrip, hip, or knee strength) are popular for this purpose because of their cost-effectiveness and availability in clinical settings [22]. In summary, definitions of sarcopenia are somewhat arbitrary and are limited to the availability of large clinical and epidemiological studies (population-representative cut points) using gold-standard body composition assessment tools [23]. The reader is referred to an extensive review on the topic for an in depth discussion [24].