With the acceleration of global population aging, frailty has become particularly prominent in the elderly population (1). Frailty is a combined state accompanied by the decline of multiple physiological systems that significantly increases the risk of falls, disability, hospitalization, and even death (2). Obesity, particularly abdominal obesity, is widely recognized as one of the main risk factors for frailty. However, the traditional BMI, although widely used in the assessment of obesity (3), is deficient in distinguishing visceral fat from subcutaneous fat, making it difficult to accurately reflect the accumulation of abdominal fat (4), and therefore has limitations in measuring the impact of obesity on frailty. Waist circumference (WC) is a good indicator of abdominal fat accumulation, but it does not provide insight into fat distribution (5). The waist-to-hip ratio provides information on the distribution of abdominal fat, but it does not accurately assess fat metabolism or muscle mass (6). Especially in the elderly population, the lack of age- and gender-specific criteria limits its validity in the prediction of frailty.

The BRI, which combines measurements of height and WC to more accurately capture the features of body fat distribution, particularly the buildup of abdominal fat, was created in order to better evaluate the health risks associated with obesity (7). In contrast to BMI, the BRI has been shown to be more effective in characterizing metabolic syndrome (8), non-alcoholic fatty liver disease (NAFLD) (9), and osteoarthritis (10), among many other health problems, showed higher sensitivity in risk prediction. Additionally, the study found a U-shaped correlation between mortality prevalence and BRI, indicating that both high and low BRI values may elevate mortality rates (11). This finding further underscores the potential of BRI as a valuable tool for health monitoring and prevalence assessment. These studies provide insights into the association of BRI with health outcomes, but BRI is not yet uniformly defined and its clinical application is still under investigation.

While the link between abdominal obesity and frailty has been established in several studies, research examining the relationship between BRI and frailty is still scarce. To address this gap, this study used the National Health and Nutrition Examination Survey (NHANES) data from 2007 to 2018 to investigate the applicability of BRI in older U.S. adults and evaluate its potential role in predicting frailty prevalence. This study aims to enhance our understanding of BRI’s role in frailty screening and provide new support for early intervention in the elderly population.

Using NHANES data from 2007 to 2018, this study examined the relationship between BRI and the prevalence of frailty in U.S. persons 60 years of age and older. The findings showed that BRI and a higher prevalence of frailty were significantly and independently correlated. A 34% increase in the prevalence of frailty was associated with every unit rise in BRI in the fully adjusted model. GAM analysis further highlighted a nonlinear relationship, showing a 37% increase in frailty prevalence for each unit rise in BRI when BRI exceeded 4.42. The results of the ROC curve analysis indicated that BRI outperformed BMI and WC in the prediction of frailty prevalence. The AUC values of BRI were significantly higher than those of BMI and WC (p < 0.05), further demonstrating the superiority of BRI in distinguishing high-risk populations. In addition, to enhance the robustness of the results, sensitivity analysis, and subgroup analysis were performed in this study. After adjusting for BMI and WC, the association between BRI and frailty prevalence remained significant, indicating strong stability of the study results. In subgroup analyses, significant associations between BRI and frailty were observed in groups of different age, gender, race, smoking, and drinking statuses. This suggests that the BRI has good applicability and predictive power for a wide range of older populations.

Research on frailty has extensively explored the effects of obesity, BMI, and metabolic health states (15). The study by He et al. (16) states that metabolic status plays a key role in the progression of frailty, and metabolically unhealthy obesity (MUOO) significantly accelerated the progression of frailty, while metabolically healthy obesity (MHOO) had a lesser effect on frailty (16). Several studies have revealed that BRI, an important indicator of body fat distribution, is strongly associated with a variety of health problems. For instance, research by Zhang et al. (17) identified a significant association between BRI and the prevalence of non-alcoholic fatty liver disease (NAFLD), highlighting a non-linear relationship. The study by Li et al. (8) noted that BRI is a significant predictor of metabolic syndrome, and its nonlinear characterization suggests that individuals with high BRI have a significantly increased prevalence of metabolic disorders. BRI has been linked to a higher prevalence of depression among U.S. adults (18). In addition, the BRI has shown superiority in predicting the prevalence of osteoporosis and osteoarthritis (10, 19). It should be noted that the systematic review and meta-analysis by Yuan et al. (3) showed that abdominal obesity was significantly associated with the risk of frailty (RR = 1.57). The study also found that both excessively high and low BMI levels were associated with higher frailty rates, indicating a U-shaped relationship between BMI and frailty. Meanwhile, Zhang et al. (11) demonstrated a U-shaped relationship between BRI and adult mortality in the United States, suggesting that either too low or too high a BRI value may be associated with increased mortality. Although our study focused only on the positive correlation between BRI and frailty, this result should not be interpreted to mean that only higher BRI is associated with frailty. Possible reasons for this include the fact that this study included only the elderly population in the United States, of which only 79 had a BMI below the normal range, a relatively small sample size, which limited our ability to effectively assess the potential association between low BRI and frailty. Low BRI may be associated with low body fat or insufficient fat reserves, which is also a risk factor for frailty (20). Therefore, future studies should be conducted in larger sample sizes of older adults and in different populations. While prior research has examined the connection between BMI, abdominal obesity, and frailty, studies specifically investigating the link between BRI and frailty remain limited. The present study reveals for the first time the potential of BRI, an emerging metric, in the prediction of frailty prevalence, especially in revealing the nonlinear characteristics and threshold effects of BRI. This study aligns with the findings of Yuan et al. (3) and He et al. (16) in supporting the positive correlation between obesity and frailty prevalence. However, the present study used BRI instead of BMI or WC as a measure of obesity, which better reflects the comprehensive nature of body fat distribution.

Obesity increases the prevalence of frailty through multiple pathophysiologic processes. First, abdominal fat is a metabolically active tissue that secretes a variety of pro-inflammatory factors (e.g., tumor necrosis factor alpha, interleukin-6), leading to systemic chronic low-grade inflammation (21). This inflammatory state is thought to be one of the main drivers of frailty, as inflammation not only accelerates the loss of muscle mass and strength but may also accelerate multisystem decline through oxidative stress (22). Individuals with a high BRI are typically accompanied by higher abdominal fat stores and are thus more susceptible to the negative effects of inflammation. It may also give rise to sarcopenic obesity, a coexisting state of excess adipose tissue, and deficient muscle mass, which is often closely associated with elevated BRI (23). In this state, muscle is replaced by adipose tissue, leading to decreased muscle function and strength, as well as reduced metabolic efficiency. This dual effect of muscle deficiency and obesity makes the probability of weakness significantly higher. Second, the accumulation of abdominal fat is directly related to the development of insulin resistance, the latter of which can further lead to metabolic disorders (e.g., diabetes, hyperlipidemia, etc.) (24). Metabolic disorders may accelerate the onset of frailty through a number of pathways, e.g., affecting muscle glucose metabolism capacity (25) decreasing the rate of protein synthesis (26), and impaired muscle repair (27). Additionally, there is a substantial correlation between on the prevalence of cardiovascular disease, a significant risk factor for frailty, and abdominal obesity, and a higher BRI may further accelerate the progression of frailty through damage to the cardiovascular system. Specific mechanisms include increased atherosclerosis, decreased cardiac output, and deterioration of the heart’s pumping function (28). All of these factors impair physical performance and mobility in older adults. At the same time, abdominal obesity is associated with several hormonal imbalances, including leptin (29), insulin-like growth factor (IGF-1) (30), and abnormalities in sex hormone binding globulin (SHBG) (31). These hormones are closely related not only to energy metabolism and fat distribution but also have important effects on muscle mass and bone strength. Individuals with high BRI tend to exhibit higher leptin resistance and low levels of IGF-1 (32) which may further exacerbate the frail state. Finally, excessive obesity is usually accompanied by higher levels of psychological problems such as depression and anxiety (33), and these factors may further accelerate the onset of frailty through indirect effects such as reduced physical activity levels, altered dietary habits, and exacerbation of chronic inflammation. In addition, the present study observed a threshold effect between BRI and frailty, which may reflect the body’s ability to adapt to fat accumulation within a certain range. When BRI is low, the body may maintain homeostasis through metabolic compensation mechanisms, but when BRI exceeds a certain level, the metabolic burden increases rapidly, leading to increased inflammatory responses and metabolic disorders, thus significantly increasing the prevalence of frailty.

This study’s use of NHANES data, which offers a sizable, nationally representative sample and improves the findings’ generalisability and dependability, is one of its main strengths. The study adjusted for a variety of potential confounders, including age, gender, race, and lifestyle factors, enhancing the reliability of the findings. Subgroup analyses confirmed a significant association between BRI and frailty across various populations, reinforcing the robustness of the findings. However, the study also had certain limitations. A causal association between BRI and frailty could not be demonstrated since the data came from a cross-sectional survey; more longitudinal research is required to confirm this. Despite adjusting for multiple variables, there may be potential confounders (e.g., genes, etc.) that were not captred. The study population was an older U.S. population, and the applicability of the results to other regions or ethnicities requires further validation.

According to this study, BRI and frailty among US older persons are significantly positively correlated. Compared to standard BMI and WC, the BRI offers a higher predictive potential for frailty. These results highlight BRI’s promise as a screening and management tool for frailty, but larger prospective trials are required for additional validation.