Associations between BMI trajectories and young adult bone: a 10-year follow-up from the Norwegian Fit Futures cohort

BMI is a well-established determinant of bone health; however, most evidence is derived from cross-sectional or short-term studies, providing limited insight into how long-term BMI development influences skeletal maturation. We aimed to investigate the associations between long-term BMI trajectories and skeletal development from adolescence into young adulthood. Data were derived from 1,114 participants in the population-based Fit Futures study with repeated measurements (2010–2011 [FF1], 2012–2013 [FF2], and 2021–2022 [FF3]). Group-based trajectory modelling identified three BMI trajectories: low-stable, moderate-increasing, and high-stable. Sex-stratified linear mixed-effects models estimated adjusted mean DXA-derived areal BMD (aBMD) and BMC at the total body, total hip, and femoral neck. Participants in the moderate-increasing and high-stable trajectories consistently exhibited higher aBMD and BMC than those in the low-stable trajectory across all skeletal sites throughout follow-up. For total body aBMD, compared with the low-stable trajectory, the moderate-increasing trajectory was associated with β = 0.06 g/cm2 (95% CI: 0.05–0.08) in females and β = 0.08 g/cm2 (95% CI: 0.06–0.10) in males, whereas the corresponding estimates for the high-stable trajectory were β = 0.10 g/cm2 (95% CI: 0.07–0.17) and β = 0.10 g/cm2 (95% CI: 0.06–0.15), respectively (all p < 0.001). Despite continued increases in BMI, the moderate-increasing trajectory was not associated with proportionally greater skeletal accrual than the high-stable trajectory, and statistically significant trajectory-by-time interactions were limited to total body outcomes. BMI trajectories established during adolescence were associated with persistent differences in attained skeletal levels rather than differential skeletal accrual into young adulthood. These findings underscore the importance of adolescence as a critical period for the attainment of peak bone mass.