CAC implies the presence of coronary artery disease regardless of concurrent risk factors or related symptoms [23]. CAC is usually concomitant with advanced atherosclerosis and thus is an established predictor of future cardiac events [24, 25]. In this cross-sectional and longitudinal study, pre-sarcopenic obesity had the highest risk of CAC presence, incidence, and progression among different body composition subtypes, and the risks were significantly higher compared with those of normal subjects. Pre-sarcopenic obesity significantly increased the risk of CAC presence compared to abdominal obesity alone in addition to the risk of CAC incidence and progression compared with either pre-sarcopenia alone or abdominal obesity alone, regardless of other cardiometabolic risk factors.
According to Asian Working Group for Sarcopenia, low ASM with low muscle strength or low physical performance need to diagnose sarcopenia [26]. Therefore, most of the studies on sarcopenia investigated pre-sarcopenia which measured low skeletal muscle mass only. The pre-sarcopenic obesity group defined by ASM/weight < 1 SD and visceral fat area > 100 cm2 on abdominal CT was more susceptible to insulin resistance and CVD than the obesity or pre-sarcopenia alone group [5]. Individuals with pre-sarcopenic obesity defined by ASM/weight < 1 SD and BMI ≥ 25 kg/m2 had significantly higher 10-year CVD risk (≥ 20%) determined using the Framingham risk model, whereas pre-sarcopenic non-obese and non-presarcopenic obese individuals were not associated with 10-year CVD risk [27]. Pre-sarcopenic obesity defined by ASM/height2 less than 7.0 kg/m2 (in men) or 5.4 kg/m2 (in women) and android to gynoid ratio of higher than the sex-specific median value was significantly and positively associated with incident CVD in subjects with type 2 diabetes, while pre-sarcopenia alone or obesity alone was not [28]. However, pre-sarcopenia with obesity classified according to total adiposity parameter (body fat % or BMI) was not associated with CVD events [28]. A large longitudinal study found that high WC with low muscle strength was modestly associated with CVD risk during 10 years of follow-up, but high WC with low muscle mass was not associated with CVD risk [7]. Low skeletal muscle mass, obesity, and pre-sarcopenic obesity were distinctively associated with a higher risk of CVD events than the reference, but these associations lost statistical significance after adjustment for other covariates [2]. Although there have been conflicting results regarding the association between pre-sarcopenic obesity and CVD, our study to evaluate for the first time that subclinical CVD assessed by CAC presence, incidence, and progression had significant and consistent associations between pre-sarcopenic obesity, suggesting possible connections between incremental risk of pre-sarcopenia and obesity for future CVD. Aside from abdominal obesity, pre-sarcopenic obesity still had a significantly higher risk of CAC presence, incidence, and progression when obesity was defined using BMI or body fat% in this study.
Crucially, pre-sarcopenia defined as SMI less than − 1 SD below the mean value of young individuals represents early stage of the sarcopenia, compared with other definitions using a cut-off value of -2 SD below the mean [29] or using absolute value of men < 7.0 kg/m2 and women < 5.7 kg/m2 [26]. If a cut-off value of -2 SD below the mean was used, only 13.7% of total subjects had pre-sarcopenia, and if aforementioned absolute value was used, only 10.2% had pre-sarcopenia. However, even majority of the subjects had modestly low skeletal muscle mass, pre-sarcopenia per se was still associated with an increased risk of CAC, and additively aggravated the risk of CAC with abdominal obesity. Moreover, in relatively younger subjects (< 60 years) who might preserve enough muscle mass and strength, pre-sarcopenic obesity showed similar risk of CAC incidence and progression, compared with elderly subjects. The additive effect of pre-sarcopenia and abdominal obesity on CAC was still significant in the subjects with and without diabetes.
There is no consensus about which skeletal muscle mass or obesity parameters can best predict CVD, particularly among Asians. In regards to the methodology adopted in this study, ASM/weight was used because it is thought to be a better predictor of insulin resistance and cardiometabolic parameters in the Korean population [5, 30]. Our unpublished data also show that weight-adjusted ASM is more closely associated with CAC presence, incidence, and progression compared with other SMI parameters (ASM/BMI and ASM/height2). In fact, brachial-ankle pulse wave velocity, another marker for reflecting atherosclerotic burden, was also higher in sarcopenic subjects than in normal subjects when sarcopenia was defined by ASM/weight but not by ASM/height2 [30]. Compared with Western populations, Asian people tend to have lower muscle mass and higher body fat mass with central distribution. With aging, Asian people present with greater increases in fat mass and higher prevalence of abdominal obesity, especially in women [31]. BMI, WC, and waist to hip ratio are strongly correlated with measures of abdominal adiposity. In one Korean study, the best surrogate of abdominal adiposity across a wide age range was WC [40], and WC has been an important predictor of CVD independent of traditional risk factors, including BMI [32, 33]. BMI itself represents both fat mass as well as nonfat, or muscle mass, and cannot detect regional body fat such as visceral fat [34]. Therefore, both ASM/weight and WC may be good indicators for defining pre-sarcopenic obesity compared with other parameters.
The mechanisms underlying pre-sarcopenic obesity are multifactorial. Aging is one of the most crucial factors that contribute to a gradual loss of muscle mass and parallel increase in visceral fat [35]. A reduction in muscle mass and physical activity decreases total energy expenditure and glucose utilization and finally leads to elevated insulin resistance and obesity [36]. Increases in fat mass, particularly visceral fat, promote low-grade inflammation that results in the secretion of tumor necrosis factor (TNF), leptin, and growth hormone [37, 38], which leads to muscle catabolism. Elevated TNF inhibits adiponectin, arresting muscle protein synthesis and mitochondrial processes [39]. Leptin upregulates the pro-inflammatory IL-6 and TNF, which results in a reduction of the anabolic actions of insulin like growth factor-1 (IGF-1) [40]. Reduced growth hormone and IGF-1 decrease anabolic effects of leptin [41] and increase the likelihood of incident fragility [42]. In addition to visceral obesity, aging stimulates fat infiltration into muscle, and thus the deposition of intramyocellular lipids causes lipotoxicity impairing muscle fiber contractility and new muscle tissue growth, which contributes to sarcopenia [43]. Furthermore, higher fat mass at baseline is associated with a further decline in leg lean mass in both sexes [44]. Therefore, a vicious cycle may exist with low skeletal muscle mass and obesity having a reciprocal influence on each other.
The present study has some limitations. First, BIA could not precisely differentiate lipid, water, and fibrous tissue within muscle mass [45]. Considering the higher adiposity of Asian people, the muscle mass measured by BIA may be overestimated in obese older people with high intramuscular fat infiltration. Second, sarcopenia is also a loss of muscle function in addition to muscle mass. But we could not assess functional outcomes due to the limitations of general health checkups. Third, postmenopausal state or use of estrogen which affects skeletal muscle mass in women could not be assessed due to the limitation of self-report. Finally, the enrolled subjects were all Korean, so our findings may not be generalizable to other ethnic groups, because the interaction between genes and the environment leads to different muscularity and adiposity in different ethnicities [46]. Besides, results could be biased if participants’ characteristics at a single medical institute were not representative of the general Korean population.