In this study, we present reference values for waist circumference (WC), percentage body fat (PBF), fat mass index (FMI) and fat mass to fat-free mass (FM/FFM) ratio, as well as sex-specific cut-off values for PBF and FMI, to define alternative phenotypic representations of sarcopenic obesity (SO). We describe the corresponding SO prevalences using AWGS guidelines for sarcopenia, across the age groups of healthy adults. Our estimated prevalence of SO for adults aged ≥21 years and ≥60 years were 7.6% and 21.6% (WC-based), 5.1% and 16.1% (PBF-based), 2.7% and 9.5% (FMI-based), and 1.5% and 4.8% (FM/FFM-based) respectively.
Comparatively, a recent study on 200 cognitively-intact and functionally-independent community-dwelling adults in Singapore (≥50 years) reported a prevalence of 10.5% and 10.0% based on WC-based and PBF-based definitions of SO respectively (5), slightly lower than the 15.4% and 10.7% in the present study (≥50 years). Notably, the authors used the original AWGS criteria to define sarcopenia (19), with lower cut-offs for muscle strength and gait speed and thus lower detection rates compared to the updated AWGS criteria (2). Another study involving 591 healthy volunteers in Korea found a prevalence of 10.9% (40-59 years) and 18.0% (≥60 years) using the PBF-based definition (4), higher than the 2.1% (40-59 years) and 16.1% (≥60 years) in the present study. However, their criteria did not include the functional components of sarcopenia (2), and their population-derived cut-offs for low muscle mass were much higher at 8.81 and 7.36 kg/m2 compared to AWGS’ 7.0 and 5.4 kg/m2 used in the present study (2,19).
Using one-way analysis of variance (ANOVA) with Bonferroni correction for post-hoc comparisons, the SO phenotype consistently performed poorer than the obese-only group in all measures of muscle/physical function for the WC, PBF and FMI-based definitions of SO. Compared to the sarcopenic-only group, the SO phenotype performed worse in the SPPB and TUG assessments for the PBF-based and FMI-based SO definitions, and only in TUG for FM/FFM-based SO phenotype. Physical function impairment in the absence of disability likely represents the shared core of sarcopenia and physical frailty. Such functional deterioration with deficits in gait speed, balance, and muscle strength, can be objectively assessed through the SPPB (20). Given that the SPPB is considered one of the most reliable and valid assessments for functional performance (5,17,20), our findings suggest PBF and FMI to be the preferred obesity measures for defining SO. On the other hand, multiple linear regression results for SPPB revealed that the total variance explained by the different regression models was highest for the FM/FFM definition, followed by FMI, PBF and WC. In addition, only FMI-based and FM/FFM-based definitions of SO were significantly associated with poorer SPPB scores, suggesting FMI and FM/FFM to be the preferred obesity measures.
On the whole, our results indicate that FMI is the most preferred measure of obesity for defining SO. PBF gives an objective indication of total body fat, but does not discern between visceral and subcutaneous fat (5). While WC provides an estimate of visceral adiposity which is associated with higher morbidity than its subcutaneous counterpart (21), it is not adjusted for height and is thus insensitive to body size (5-7). In corroboration with the literature, fat mass was previously reported to be the most frequently used adiposity index for the classification of SO, and its adjustment to height squared (FMI) has been the preferred method to account for differences in body size across age and between genders (6). In terms of physical performance, FMI is also considered an accurate indicator of total body adiposity that could improve the predictive value of SO in functional deterioration (6,7). In addition, FMI was found to be a better screening tool in the prediction of metabolic syndrome in Chinese men and women (22) and more accurately assessed obesity in Mexican Americans (23) compared to BMI or PBF.
Similar to the findings of previous studies, the FM/FFM definition led to a markedly disproportionate low number of males identified with SO (6,7,24), with only 5.3% of those detected being males, amounting to a population-adjusted prevalence of <0.1% in our study. Women inherently have much higher relative fat mass than men (25), and conversely, men have higher relative fat-free mass (total body water, muscle and bone mass) than women at all ages (25,26). This is primarily due to the hormonal differences between men and women; men have higher testosterone levels which exhibits anabolic effects on muscle and bone (27), while higher estrogen levels in women promote subcutaneous fat deposition especially in the hips, thighs and chest (28). In addition, approximately 75% of skeletal muscle tissue is composed of water (29). Thus, with higher muscle mass, men inadvertently hold more total body water, further contributing to the discrepancy in fat mass and fat-free mass between men and women. To address the underlying gender-bias of the FM/FFM criteria and improve its accuracy in identifying gender-specific obesity and SO prevalence, different cut-off values for men and women (lower cut-off values for men) should be explored.
A recent study on 1235 adults with type 2 diabetes (T2D) in Singapore (≥45 years) reported a SO prevalence of 19.4% using the FM/FFM-based definition, higher than the 12.7% reported in this study, although the criteria for diagnosing SO in that study did not include the AWGS functional components for sarcopenia, which could possibly have inflated the proportions identified with SO (6). Furthermore, previous studies have shown a close link between sarcopenia and obesity through insulin resistance (3). Visceral fat accumulation (which promotes secretion of pro-inflammatory cytokines) is a contributing factor to the loss of skeletal muscle (which is the largest insulin-responsive tissue). Obesity and sarcopenia have a synergistic effect on promoting insulin resistance which could exacerbate T2D (4). In addition, patients with T2D exhibit insulin resistance, systemic inflammation and metabolic complications that could in turn perpetuate excess adiposity accumulation and loss of muscle mass (3), leading to a vicious cycle of worsening insulin resistance, T2D, sarcopenia and obesity (4).
This study has several limitations. It presents cross-sectional data on obesity, muscular health and function of Singaporeans, which precludes inferences on causality. Agreement amongst the obesity definitions was also not investigated, though it has previously been established that different obesity definitions intrinsically measure different constructs and are therefore not interchangeable (5). Finally, the participants were healthy, community-dwelling adults; thus, the findings may not be generalisable to hospitalized, institutionalised or disabled individuals.