This study is the first to use NHANES data to investigate the association between WWI and combined grip strength, examining data from 4,179 eligible subjects. A significant inverse relationship between WWI and combined grip strength was observed, with a saturation effect for grip strength in men and WWI at an inflection point of 13.40 (cm/√kg). The subgroup analysis revealed notable differences between gender groups.
WWI, a novel index for measuring obesity, is calculated as waist circumference divided by the square root of body weight (cm/√kg) and was initially developed by Park et al[8]. As a reflection of central obesity, WWI has been linked to various health conditions, including heart failure[21], abdominal aorta calcification[19], hyperuricemia[22], and others. Our results demonstrate that a higher WWI is associated with lower grip strength, indicating that weakened muscle strength can adversely affect sarcopenia. Thus, WWI may help diagnose and manage sarcopenia by responding to muscle strength and rating the degree of muscle loss.
A significant correlation between higher WWI and lower combined grip strength was found in this study. Obesity has also been shown to be negatively correlated with grip strength in a meta-analysis by Hsu et al[23]. which used BMI to assess obesity. As both BMI and WWI are indicators of obesity, their study provides support for our findings. The inclusion of arthritis as a covariate and subgroup analysis in this project was based on a cohort study of 300 patients with arthritis and grip strength, where the severity of thumb arthritis was significantly negatively associated with grip strength (β = -0.79 ,95% CI -1.10- -0.49,p༜0.05)[24]. Our study suggests that patients with arthritis are more likely to have lower combined grip strength and that arthritis status alters the association between WWI and grip strength. Furthermore, our study reveals that combined grip strength in men is no longer significantly correlated with higher WWI when WWI is greater than 13.40 (cm/√kg). The correlation between grip strength and WWI in women remained consistently negative. Factors such as hormone levels[25], dietary patterns [26], fat distribution[27], and others can account for gender differences. A prospective study with a larger sample size is required to confirm the relationship between combined grip strength in men and women and WWI.
To our knowledge, few clinical studies have examined the association between WWI and muscle strength in US adults. In a cross-sectional study in Korea, researchers found that among 602 older adults aged over 65, WWI was negatively associated with muscle mass and positively associated with adiposity[28]. Additionally, an NHANES cross-sectional study revealed a negative association between WWI and abdominal muscle mass in patients with atherosclerosis[29]. Kim et al[30]. found that different body measurements had the strongest correlation with WWI in the evaluation of sarcopenic obesity in a descriptive clinical study. These studies primarily focused on WWI and muscle mass. Previous research has suggested that the negative effect of obesity on muscle strength is present in both adolescents and the elderly and that grip strength is negatively associated with the risk of obesity prevalence in men[31, 32]. A cross-sectional study involving 423 Spanish children aged 11 examined the relationship between physical obesity and muscle movement, discovering that obese children performed worse on tests such as jumping height and running speed[33]. All three studies assessed obesity using BMI, WC, and other indicators, and none used WWI as an assessment indicator.
Based on previous research, there are several plausible explanations for the mechanisms underlying the negative association between WWI and muscle strength. Satellite cells (SC) play a crucial role in promoting skeletal muscle growth and development. However, their functions are adversely affected by fatty diets and obesity. A cellular study demonstrated that excessive glucose and fat intake impairs SC cell function, consequently impacting skeletal muscle mass and strength[34]. Additionally, an animal experiment showed that muscle hypertrophy was linked to reduced fat and blood glucose levels when mice were fed a high-fat, high-sugar diet, followed by rapid type II glycolysis of muscle fibers [35]. Obesity also contributes to increased levels of non-esterified fatty acid (NEFA), which in turn upregulates various inflammatory factors such as tumor necrosis factor-alpha (TNF-α), interleukin-1 (IL-1), and interleukin-6 (IL-6) through adipokine regulation or Toll-like receptor activation, resulting in chronic inflammation[36, 37]. These cytokines activate the ubiquitin-proteasome system (UPS), thereby increasing the likelihood of muscle wasting [38]. These findings illustrate the negative correlation between obesity and grip strength and indirectly reflect the mechanism of association between WWI, as an obesity indicator, and grip strength.
One of the strengths of this study is its foundation on the extensive NHANES database, which is more representative of the study population. Furthermore, we were the first research team to investigate the relationship between WWI and combined grip strength, identifying a negative association within the WWI range and combined grip strength. Stratified subgroup analysis was also conducted for factors such as gender. However, the study has some limitations. Firstly, the project did not analyze and study all age groups, focusing instead on the association between grip strength and WWI in Americans over 50. Secondly, it is essential to acknowledge that the cross-sectional nature of this study limits our ability to establish causality. A considerable number of prospective trials must be conducted to confirm our findings and demonstrate causality.