Weight-adjusted waist index reflects fat and muscle mass in the opposite direction in older adults

Background Age-related changes in body composition include decreased muscle mass and preserved or increased fat mass. There is no anthropometric index to assess both muscle and fat mass. Methods Using a cross-sectional sample of 602 participants aged ≥65 years from the Ansan Geriatric study, we evaluated the association of weight-adjusted waist index (WWI) with muscle and fat mass and compared these with body mass index (BMI) and waist circumference (WC). WWI was calculated as WC (cm) divided by the square root of body weight (kg). Body composition was measured using bioelectrical impedance analysis, dual-energy X-ray absorptiometry and abdominal computed tomography. Results WWI positively correlated with total abdominal fat area (TFA) (r = 0.421, P < 0.001), visceral fat area (VFA) (r = 0.264, P < 0.001), and percentage of total tissue fat (r = 0.465, P < 0.001), but negatively correlated with appendicular skeletal muscle mass (ASM) (r = −0.511, P < 0.001) and ASM/height² (r = −0.324, P < 0.001). Mean ASM was highest in the first quartile of WWI (17.85 kg/m²) and showed a decreasing trend, with the lowest value in the fourth WWI quartile (13.21 kg/m², P for trend <0.001). In contrast, mean TFA was lowest in the first quartile and highest in the fourth WWI quartile (P for trend <0.001). The probability of combined low muscle mass and high fat mass was >3× higher in the fourth WWI quartile than in the lowest quartile (odds ratio 3.22, 95% confidence interval 1.32–7.83). Conclusions WWI is an anthropometric index positively associated with fat mass and negatively associated with muscle mass in older adults.