Selective Mid-Wall Cardiac Dysfunction in Obesity: The Role of Muscle-to-Fat Balance

<b>Objective:</b> This study aims to analyze relationships between body composition, biochemical parameters, and cardiac function in young adults to identify mechanisms of cardiac dysfunction in obesity. <b>Methods:</b> This is a cross-sectional study of 60 young adults (mean age 20.4 years) divided into healthy (<i>n</i> = 29) and overweight/obese (<i>n</i> = 31) groups. Body composition was assessed using bioelectrical impedance analysis. We calculated the SMM-to-Fat ratio (skeletal muscle mass %/body fat %) as a continuous composite metric. Cardiac function was evaluated using 3D speckle-tracking echocardiography, with a 3D global circumferential strain pre-specified as the primary endpoint. <b>Results:</b> The obese group showed unfavorable body composition with lower SMM% (38.0 ± 10.8 vs. 47.1 ± 5.6%), higher body fat% (28.3 ± 12.6 vs. 16.0 ± 8.3%), and lower SMM-to-Fat ratio (2.1 ± 2.3 vs. 4.8 ± 5.1; all <i>p</i> < 0.001). C-peptide was 75% higher (<i>p</i> < 0.001), indicating compensatory hyperinsulinemia. The primary endpoint showed impairment in the obese group (-19.8 ± 4.7 vs. -22.2 ± 2.9%; <i>p</i> = 0.023, <i>d</i> = 0.61), while longitudinal strain was preserved, indicating selective mid-wall dysfunction. The SMM-to-Fat ratio demonstrated a stronger association with circumferential strain (<i>r</i> = -0.467, <i>p</i> = 0.008) than SMM% alone (<i>r</i> = -0.414, <i>p</i> = 0.021) and remained an independent predictor in multivariable analysis (<i>β</i> = -0.88, <i>p</i> = 0.019), whereas SMM% did not achieve significance (<i>p</i> = 0.159). Comprehensive analysis revealed correlation reversal across all body composition parameters between groups, with minerals% and total body water% emerging as additional independent predictors. <b>Conclusions:</b> Young obese adults exhibit selective mid-wall cardiac dysfunction. The SMM-to-Fat ratio, representing muscle-adiposity balance, is superior to SMM% alone for predicting cardiac dysfunction. Our findings suggest that the relative balance, rather than absolute muscle mass, determines cardiac health in obesity, with implications for body composition assessment and intervention strategies.