Osteoporosis is a major public health problem, and a low bone mass is an important risk factor for osteoporosis.1 Dual-energy X-ray absorptiometry (DXA) is a clinical method for assessing bone mineral content (BMC) and bone mineral density (BMD) to diagnose osteoporosis and bone health.2 BMD indicates how dense the bones are, while BMC provides insights into the total bone mass available, which can be crucial in understanding overall bone health and assessing treatment responses. Both BMD and BMC assessment substantially improves the robustness of findings related to fracture risks and bone health. DXA is a special imaging modality used for assessing bone mineral density, but it can also be utilized to measure body composition.3 According to the fundamental principles of DXA measurement, the technique employs two distinct monochromatic X-ray energies—high and low—to simultaneously calculate the mass of two unknown materials. For simplicity, DXA was designed to measure bone mass and body composition using a two-component system. This system assesses bone mineral mass and soft tissue when bone is present, and distinguishes between fat mass (FM) and lean mass (LM) when bone is absent in the image pixels. Thus, both FM and LM play a role in determining bone mineral density and body composition across all body regions.4 Therefore, extracting body composition distribution patterns from whole-body and regional DXA scans could provide valuable insights into their relationships with metabolic syndrome, osteoporosis, cardiovascular disease, insulin resistance, and overall mortality.

Body weight is a strong predictor of BMD,5,6 but it remains unclear whether FM or LM has a greater association with BMD. The relationship between body composition and bone mass has been reported to differ between men and women.7 Research indicates that lean body mass is a critical determinant of BMD in premenopausal women, while fat mass has a greater influence on BMD in postmenopausal women.8 In addition, the relationships between body composition and bone mineral density in different ethnicities are inconsistent. Studies always focused on the relationship between total lean and fat body composition with BMD. But, the specific distribution pattern of body composition may be more important for BMD an individual's health. Specifically, excess fat in central body areas is more strongly linked to health than total body fat.9,10 The relationship between central fat mass and BMD is complex and multifaceted. While mechanical loading may have a protective effect on bone health11, metabolic and hormonal dysregulation associated with central fat mass can lead to bone loss.12 Understanding these dual pathways is critical for developing targeted interventions to improve bone health in individuals with central adiposity. Central lean mass is strongly associated with higher BMD due to mechanical loading. Studies have shown that greater lean mass is positively correlated with BMD, particularly in weight-bearing bones such as the hip and spine.13,14 Mechanical loading increases bone density by activating signaling pathways such as Wnt/β-catenin, which promote osteogenesis.15

DXA is used to determine whole body composition as well as specific regions such as the arms, legs, and trunk region. The trunk region refers to the central part of the body including the chest, midriff, and pelvis. As previously noted, body composition is closely linked to bone mineral density and bone mineral content. However, there is limited data available, both in Iranian and other populations, regarding whether the relative contributions of trunk region body composition (including the chest, midriff, and pelvis) to BMD and BMC differ between men, premenopausal women, and postmenopausal women. The objective of this study was to examine the relationship between trunk components with BMC and BMD at the lumbar spine (L2-L4) with respect to gender and menopausal status.