Previous studies of the effects of body fat on bone mineral density (BMD) have shown conflicting results depending on the age, sex, and menopausal status of the subjects. The purpose of the present study was to investigate the effects of body fat on BMD using data from the Fifth Korea National Health and Nutrition Examination Survey, which was conducted in 2010. Our data were drawn from a relevant population within the survey's range of participants, yielding a total of 4,138 subjects aged 20-93 years (1,133 men <50 years, 946 men ≥ 50 years, 1,332 premenopausal women, and 727 postmenopausal women). Percent body fat, fat mass (FM), lean mass (LM), and BMD of the total hip, lumbar spine, and arm were measured in the original survey using dual-energy X-ray absorptiometry. Relationships between FM and BMD were evaluated using multiple linear regression analyses. LM was the strongest determinant of BMD. Although FM also showed a significant positive correlation, even after adjusting for LM, with total hip and lumbar spine BMD in men ≥ 50 years and pre- and postmenopausal women, this positive correlation was not observed in the arm. In an analysis of obese and non-obese subgroups, the positive correlation between FM and BMD disappeared in obese subjects. Furthermore, FM and BMD exhibited a negative correlation in premenopausal obese women. These results suggest that FM contributes to BMD primarily through mechanical loading. FM, then, is not beneficial and may be detrimental to bone health in obese people.