This study investigated the associations between obesity according to the BMI criteria, central obesity by WC, and sarcopenia according to sex. In our study, a high BMI was associated with a low prevalence of low ASMI and sarcopenia, defined based on the AWGS criteria. Central obesity was associated with a low prevalence of sarcopenia in women. However, obesity was not found to be positively associated with muscle strength or physical performance. These results are consistent with those of previous studies showing that low BMI is positively associated with sarcopenia and that obesity has a positive effect on sarcopenia [16,17,18].
In both sexes, obesity, defined as high BMI, was associated with a low prevalence of low muscle mass and sarcopenia. The association between BMI and sarcopenia has been investigated in a few studies. According to previous studies, low BMI was positively related to sarcopenia, and obesity determined by BMI had a protective effect against sarcopenia. Senior et al. studied the risk factors of sarcopenia among adults living in nursing homes and found that low BMI was a predictive factor for sarcopenia [16]. Sato et al. conducted a cross-sectional study of older adults to investigate the risk factors for sarcopenia and showed that overweight defined by a BMI of ≥ 25 kg/m2 had a protective effect against sarcopenia (OR = 0.95, 95% CI = 0.00–0.06) [17]. In addition, Yu et al. examined the incidence of sarcopenia and its associated factors over 4 years and revealed that low BMI was associated with the development of sarcopenia during the 4-year follow-up (OR = 0.66, 95% CI = 0.62–0.70) [18].
The relationship between high BMI and ASMI can be explained by the following hypothesis. First, since BMI is calculated by dividing the body mass by the square of the height, individuals with high BMI have high body mass. High body mass in obese individuals may have a chronic overload stimulus on antigravity muscles helping maintain an upright, balanced position (e.g., quadriceps, calf), which increases the muscle size and strength [19, 20]. A few previous studies have revealed that a high BMI is significantly associated with stronger antigravity muscles and increased lower-limb skeletal muscle volume as measured by CT. However, handgrip strength was not significantly related to obesity [19, 20]. Second, since BMI cannot be used to distinguish lean mass from body fat mass, individuals with a high BMI can have a high lean mass. A relatively poor correlation between the percentage of body fat mass and BMI has been found in a few studies, and a previous study revealed that BMI correlated better with lean body mass than fat mass [21].
By contrast, another study showed an interaction between adipocytes and myocytes [22]. Increased leptin produced by adipocytes contributes to ectopic fat deposition in the muscles, leading to loss of muscle quality [23, 24]. The adipose tissue of an obese person has high levels of tumor necrosis factor-alpha, which promotes the production and secretion of several pro-inflammatory cytokines. Based on these pathways, fat gain and muscle loss seem to be linked to each other, while the adipocytes seem to have an interaction with myocytes [25]. Surprisingly, in this study, central obesity was associated with a low prevalence of sarcopenia only in women. In a previous study, Chen et al. reported that central obesity is associated with a lower risk of muscle mass loss in menopausal women [9]. Abdominal fat is thought to store high concentrations of sex hormones and has positive effects on skeletal muscle mass [26, 27]. Healthy adipocytes secrete adiponectin, an anti-inflammatory and insulin sensitizer, which is positively related to muscle mass [28]. This supports the hypothesis that our results are only applicable to women.
Other sarcopenia parameters, including HGS (representing muscle strength), SPPB (representing physical performance), and prevalence of severe sarcopenia, were not significantly associated with high BMI.
Additionally, in the obese group, defined by both BMI and WC, the results were not significantly different from those of obesity defined by high BMI, except for physical performance. The prevalence of low SPPB in the obese group was significantly higher than that in the non-obese group only in women. This finding indicates that obese women may have a higher probability of exhibiting lower physical performance, which is thought to be due to differences in body composition between the sexes. Since women have higher body fat and lower muscle mass, balance stability in this group may be more strongly affected by alterations in body fat composition than that in their men counterparts [29, 30]. In a previous study, Waters et al. found that poor balance was strongly associated with fat mass in women, whereas poor balance was associated with muscle mass in men [29].
In summary, high BMI was associated with a protective impact on low ASMI and sarcopenia defined based on the AWGS criteria, which may be due to the high ASMI in the obese group. Central obesity also had a protective impact against sarcopenia in women alone, which may be due to the effects of sex hormones and adiponectin. However, obesity was not positively associated with muscle strength or physical performance. Obesity and central obesity appear to have a protective effect against sarcopenia, as defined based on the AWGS criteria. However, since obesity does not have a positive impact on the functional parameters of sarcopenia, further studies are needed to investigate the association of clinical outcomes between obesity and sarcopenia.
This study has several limitations. First, this was a cross-sectional study. However, the results of this large cohort study with 1,827 participants are meaningful, as it is the first study to examine the association between obesity and sarcopenia and the component parameters of sarcopenia. Second, the association between body fat indices measured using dual-energy DXA and sarcopenia was not included in this study. Further prospective studies are required to confirm these findings. Third, in this study, the number of patients with severe obesity was relatively small (3.6% of the total participants); therefore, our results might not be properly reflected in the case of severe obesity. Lastly, this study might not apply to other races because it was conducted on a single race, the East Asian Korean population. Because body composition also differs between races, studies in other races are also warranted [31].