BACKGROUND: Bone mineral density (BMD) is influenced by many factors. Despite the reported association between body components and BMD, most of these studies investigated the relationship between absolute muscle mass or fat mass and BMD in postmenopausal women or elderly subjects. The aim of this study is to investigate the association between muscle mass deficits (MMD) estimated from bioelectrical impedance analysis (BIA) and lumbar spinal BMD in Korean adults 20 to 49 years of age. METHODS: This cross-sectional study included 1,765 men and women who visited a health promotion center for a routine checkup. The lumbar spinal BMD was measured by dual energy X-ray absorptiometry. Body composition analysis was performed using BIA. RESULTS: The mean age of the subjects was 40.2±6.3 years. Ten thousand subjects (56.7%) were males and 126 subjects (7.1%) belonged to the low BMD (Z-score ≤-2.0). MMD had the strongest influence on BMD after adjusting for all covariates. The adjusted odds ratio of Group 3 (MDD >2.6 kg) for low BMD was 2.74 (95% CI, 1.46-5.15) after adjusting for age, gender, body mass index, height, and smoking. CONCLUSIONS: MMD estimated by BIA showed a significant association with BMD and could be regarded as an independent risk factor for low BMD in adults 20 to 49 years of age. These findings support that interventions such as physical activity or lifestyle changes may simultaneously modify both muscle and bone health in this age group.