This is the first study that investigated the relationship between appendicular muscle mass indices and physical function parameters in Turkey. We observed statistical correlations between ASM/BMI and all measures of physical function. Furthermore, we established independent associations between ASM/BMI and dynapenia.
Total body muscle mass or appendicular muscle mass is strongly related to body size(14). Muscle mass also varies according to different ethnic groups. For example, healthy black individuals have a higher fat-free mass than white healthy individuals(15), while the percentage of fat mass is higher in the Asian race(16). Therefore, cut-off values must be population-specific.
The EWGSOP-2 recommended ASM-derived muscle mass indices (adjusted for height², weight, or BMI) for the accurate detection of low muscle mass in older adults, but cut-off values are only given for height-adjusted appendicular muscle mass(2). Consequently, it remains unclear which muscle mass index is superior. Previous research has explored the association between muscle mass indices and physical function in older adults.
In the study conducted by Janssen et al., using the height-adjusted SMM, a significant relationship was found between severe sarcopenia and compared to moderate sarcopenia(17). Another study in older adults found that the height-adjusted SMM index was better related to muscle function than the weight-adjusted SMM index(18). However, the important striking detail of the study was that the prevalence of sarcopenia was higher in men than in women using the height-adjusted SMM index, which was attributed to the shortening of women's height due to osteoporosis in the postmenopausal period. In contrast to those, Kittiskulnam et al. reported in a study with hemodialysis patients that a low weight-adjusted SMM was better associated with walking speed than a low height-adjusted SMM(19). Moreover, in the same study, Kittiskulnam et al. found that a low SMM/BMI had a better correlation with hand grip strength and walking speed than other muscle mass indices(19). Considering that all these studies were conducted among different populations and patient groups, it is unsurprising that different results were obtained.
In the study, we demonstrated a significant correlation between BMI-adjusted skeletal muscle mass indices (including ASM/BMI and SMM/BMI), and SMM/kg with all physical function parameters. Moreover, BMI-adjusted muscle mass indices were identified as independently related to dynapenia. These results align with those conducted by Bahat et al., which found that BMI-adjusted SMM was better associated with functional parameters than height and weight-adjusted SMM(8). Unlike the research by Bahat et al., our study stands out as the first to utilize appendicular muscle mass as a metric for muscle mass. Unlike total muscle mass, appendicular muscle mass allows direct measurement without the need for a correction factor. This distinction holds significance as appendicular muscle mass offers a more holistic evaluation of muscle mass in older adults.
Identifying sarcopenia in obese older adults continues to pose a challenge, primarily due to the absence of a universally accepted gold standard for both muscle mass and muscle quality. In a study, they found that BMI-adjusted ASM exhibited the most association with visceral obesity and metabolic syndrome(20). Another recent study showed a significant correlation between BMI-adjusted ASM and visceral fat area (VFA)(21). In a recent study involving pre-frail individuals with obesity, a significant association was observed between ASM/BMI and both the short physical performance battery (SPPB) and handgrip(22). Graf et al. showed that fat-free mass or height-adjusted low muscle mass index was lower in people with obesity (BMI > 30 kg/m2)(23). As a result, the definition of sarcopenia based on height-adjusted low muscle mass may be misdiagnosed in those with obesity. Considering these different and contradictory results, the Foundation's National Institutes of Health group (FNIH) recommended in 2014 that ASM should be adjusted according to BMI(7). According to the FNIH definition, people with high BMI and fat mass are more likely to be diagnosed with sarcopenia.
As of the 2022 data from the Turkish Statistical Institute, 41.6% of individuals aged 65 years and older have a BMI ≥ 25 kg/m² (overweight) in Türkiye(24). Considering the increasing obesity problem in our country today, correction with body weight or BMI should be taken into account when diagnosing sarcopenia in geriatric patients. Our study showed that BMI-adjusted skeletal muscle mass indices (SMM/BMI and ASM/BMI) were independently associated with low hand grip strength in overweight patients. Hence, our study supports the preference of this index in sarcopenia assessments in the Turkish population.
The present study exhibits both strengths and limitations. Recruiting participants from a geriatric outpatient clinic, rather than utilizing a population-based sample of community-dwelling older adults, could be perceived as a limitation in our study. In addition, as this was a cross-sectional study, we were not able to determine the relationship between longitudinal changes in physical function and body composition. Nevertheless, we meticulously excluded patients with confounding factors affecting muscle functions and adjusted the analyses for other potential variables that might influence the correlation between skeletal muscle mass indices and physical performance. The strength of this study lies in being the first to investigate the relationship between different appendicular muscle mass indices and physical performance parameters. However, further studies are needed to determine the value of different appendicular muscle mass indices in predicting functional adverse outcomes.