Rethinking obesity metrics: the body roundness index as a screening tool for sarcopenia in young and middle-aged adults

Sarcopenia is a progressive musculoskeletal syndrome characterized by a generalized and accelerated decline in skeletal muscle mass, strength, and physical performance. Primarily associated with aging, it is a key determinant of frailty, mobility impairment, and increased risk of adverse health outcomes. Sarcopenia is often overlooked in young and middle-aged adults. Concurrently, rising obesity rates have exacerbated its societal burden. Although conventional obesity indices such as body mass index (BMI) and waist circumference are associated with sarcopenia, their comparative predictive utility in younger populations remains unclear. To address this gap, the present study examines the association between the Body Roundness Index (BRI)—a shape-based metric incorporating both height and waist circumference—and sarcopenia in young and middle-aged adults (mean age 38 years), with the aim of identifying an obesity indicator that may be useful for early risk stratification in this specific population. This study analyzed data from NHANES 2011–2014, including 6,009 adults aged 18–59 years. In this investigation, we assessed obesity via eight anthropometric indices (all calculated from simple body measurements without requiring laboratory results). Sarcopenia was primarily defined in accordance with the criteria established by the European Working Group on Sarcopenia in Older People (EWGSOP). Machine learning was employed to rank the indices based on their relevance to sarcopenia. BRI demonstrated the highest association and was further analyzed using multivariable logistic regression, restricted cubic splines (RCS), and threshold effect analysis. Subgroup analyses (stratified by age, sex, and metabolic factors) were conducted to evaluate consistency. The study included 6,009 participants (inverse-weighted to represent 133,965,501 individuals; mean age 38.25 ± 12.33 years; male-to-female ratio 13:12), among whom 198 had sarcopenia. All eight obesity-related indices showed significant associations with sarcopenia. Machine learning revealed that BRI had the strongest correlation (Model 3: OR = 1.76, 95% CI: 1.62–1.91, p < 0.001). RCS confirmed a nonlinear relationship, and trend analysis indicated that the highest BRI tertile was associated with a 7.69-fold higher prevalence of sarcopenia compared to the lowest tertile (Model 3: OR = 9.31, 95% CI: 5.28–16.4, p < 0.001; p for trend < 0.001). A threshold effect was identified at BRI = 3.33, beyond which sarcopenia risk increased markedly. Across all subgroups (stratified by gender, age, etc.), a positive association between BRI and the prevalence of sarcopenia was consistently observed, lending further support to the robustness of the results. BRI, a simple anthropometric measure, is independently associated with sarcopenia risk in young and middle-aged adults. These findings suggest that BRI may serve as a potentially useful indicator for early risk stratification in obesity-prone populations, highlighting the importance of myogenic obesity in this age group. Further mechanistic and longitudinal studies are warranted to confirm these associations and explore underlying mechanisms.