Association of waist circumference with muscle and fat mass in adults with a normal body mass index

BACKGROUND/OBJECTIVES: We aimed to investigate the association of waist circumference (WC) with body composition among individuals with a normal body mass index (BMI) to distinguish muscle and fat mass, as both affect health differently. SUBJECTS/METHODS: We analyzed dual-energy X-ray absorptiometry data (derived from the Korean National Health and Nutrition Survey, which includes information on fat and lean mass) of 7,493 adults with a normal BMI. Subjects were categorized into four groups of increasing WC. The fourth group was defined as being centrally obese. Each number of subjects are as follows: 1,870, 695, 231, and 39 among men and 3,054, 1,100, 406, and 98 among women. We conducted a sex-stratified linear regression analysis of body composition according to WC group after adjustments for covariates. RESULTS: We observed a positive association of body fat with increasing WC in both men and women (all P for trend: < 0.001). The adjusted mean values for percent body fat with 95% confidence intervals (CIs) according to the four WC groups in ascending order were 17.8 (17.5–18.3), 21.0 (20.6–21.5), 22.1 (21.5–22.8), and 25.1 (24.2–26.1) in men and 29.7 (29.4–30.0), 32.0 (31.6–32.3), 32.9 (32.4–33.4), and 34.7 (33.2–36.1) in women. However, there was an inverted J-shaped association between muscle mass and WC. The fourth group had a higher percent body fat and lower muscle mass than other groups. The adjusted mean values for appendicular skeletal muscle mass index (kg/m(2)) with 95% CIs according to the four WC groups in ascending order were 7.55 (7.51–7.59), 7.62 (7.56–7.68), 7.65 (7.56–7.74), and 7.22 (7.04–7.41) in men and 5.83 (5.80–5.85), 5.96 (5.92–6.00), 6.03 (5.96–6.10), and 5.88 (5.73–6.03). CONCLUSIONS: There was a positive association between body fat and WC among individuals with normal BMI; conversely there was an inverted J-shaped association between lean body mass and WC. Our findings support the WC measurement should be included in obesity evaluations for adults with a normal BMI.