Age-Related Trajectories for Bone, Muscle, and Adipose Tissue in Healthy Adults: Impact of Osteosarcopenic Adiposity

Background/Objectives: Age-related changes in body composition (bone, muscle, and adipose tissue) are often assumed to follow linear, sex-specific patterns. Some evidence suggests that these trajectories are nonlinear, and their timing and dynamics remain poorly characterized. Osteosarcopenic adiposity/obesity (OSA), defined by the coexistence of osteopenia/osteoporosis, sarcopenia, and excess/redistributed adiposity, is recognized as a body composition disorder associated with multiple morbidities, but its impact on age-related body composition trajectories has not been fully explored. We aimed to delineate sex-specific, age-related trajectories of bone, muscle, fat mass, and intramuscular adipose tissue (IMAT), identify inflection points across adulthood, and compare patterns in individuals without and with OSA. Methods: Cross-sectional data from 9717 healthy Caucasian adults (aged 20–90 years) enrolled in a multicenter Italian study were analyzed. Body composition was measured using validated bioelectrical impedance analysis. LOESS regression was employed to identify age-related inflection points. Standard diagnostic criteria defined osteopenia/osteoporosis, sarcopenia, adiposity, and OSA. Results: Men exhibited earlier peaks and midlife stability in bone and muscle mass, followed by later decline. Women showed lower baseline values, multiple early-life inflection points, and sharper midlife downturns, particularly around menopause. Fat mass increased steadily in men but followed a multi-phasic pattern in women. IMAT rose progressively with age in both sexes. Adults with OSA, identified in participants even as young as 20 years, demonstrated destabilized trajectories, earlier downturns in bone and muscle, and more complex body fat and IMAT patterns. Conclusions: Distinct sex-specific patterns and mitigating effect of OSA on body composition trajectories were identified. Early detection of OSA may be crucial for preventing acceleration of musculoskeletal decline and rise in adiposity.