Beyond BMI: A Cross-Sectional Analytical Study of the Association Between Body Composition Parameters and Functional Disability in Overweight and Obese Adults With Chronic Low Back Pain

Background Low back pain (LBP) is a significant cause of global disability and is closely associated with overweight and obesity due to increased mechanical load, chronic inflammation, and altered muscle function. The body mass index (BMI) fails to differentiate between fat and lean compartments, thereby limiting its clinical utility. Evidence on the association between specific body composition parameters, such as body fat mass index (BFMI), fat-free mass index (FFMI), and lean body mass (LBM), and functional disability in the Indian population remains scarce. Materials and methods This cross-sectional study included 99 overweight and obese adults with chronic LBP attending a tertiary care teaching hospital in South India, recruited through convenience sampling. Body composition was assessed using bioelectrical impedance analysis, and functional disability was evaluated with the Oswestry Disability Index (ODI). Pearson correlation and linear regression analyses were performed to assess associations between body composition parameters and disability. Results The mean age of participants was 47.6 ± 9.5 years, with females comprising 94.9% of the sample. Obesity was prevalent in 70.7% of participants, and most reported severe pain with long symptom duration. Severe functional disability was observed in 68.7% of cases. ODI showed strong negative correlations with LBM (r = -0.67<i>,</i> p < 0.001) and FFMI (r = -0.67, p < 0.001), and a strong positive correlation with BFMI (r = 0.74, p < 0.001). A multiple linear regression model including BFMI and FFMI explained 69% of the variance in ODI scores. Conclusions Body composition parameters are strongly associated with functional disability in overweight and obese adults with LBP. Higher adiposity is linked to greater disability, whereas higher lean mass is protective. Assessing body composition beyond BMI may aid clinical decision-making and inform targeted lifestyle and rehabilitative interventions.