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ABSTRACT Evidence suggests that single photon absorptiometry (SPA)‐measured forearm bone mineral density (BMD) is lower in contemporary children in Malmö than it was four decades ago, but the fracture incidence in the at‐risk population (all Malmö children) has been stable during the same period. The aim of this study was to evaluate if improvements in skeletal structure over time may explain this observation. In 2017–2018 we measured distal forearm bone mineral content (BMC; mg/cm) and periosteal diameter (mm) in 238 boys and 204 girls aged 7–15 using SPA. Based on the SPA measurements, we calculated forearm BMD (mg/cm2), bone mineral apparent density (BMAD, mg/cm3), section modulus, and strength index (BMAD × section modulus). The results were compared with those derived from measurements of 55 boys and 61 girls of the same ages using the same scanner in 1979–1981. We used log‐linear regression with age, sex, and cohort as predictors to investigate differences in trait trajectories (trait versus age slopes [mean percent difference in beta values (95% confidence interval)]). SPA‐measured forearm BMC was lower at each age in 2017–2018 compared to 1979–1981 (a mean age and sex adjusted relative difference of 9.1%), the forearm BMC trajectory was similar in 2017–2018 to that in 1979–1981 (reference) [0.0%/year (−1.0%, 1.0%)], while the 2017–2018 forearm periosteal diameter trajectory was steeper [1.1%/year (0.3%, 2.0%)]. Since bone size influences both BMD (BMC divided by scanned area) and mechanical characteristics, the forearm BMD trajectory was flatter in 2017–2018 [−1.1%/year (−2.0%, −0.2%)] and the forearm section modulus trajectory steeper [3.9%/year (1.4%, 6.4%)]. Forearm strength index trajectory was similar [1.8%/year (−0.5%, 4.1%)]. The lower SPA‐measured forearm BMD trajectory in contemporary children compared to four decades ago may be offset by changes in forearm bone structure, resulting in similar overall bone strength. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

ABSTRACT Screen time and physical inactivity have increased among children. As physical activity is a determinant of bone mass, there is a concern that children today have lower bone mass than earlier. If this is true, fractures may become more common in the future. In 2017–2018, we used single‐photon absorptiometry (SPA) to measure distal forearm bone mineral density (BMD; mg/cm2) in a normative cohort of 238 boys and 204 girls aged 7 to 15 years. We compared these results to BMD in a normative cohort collected in 1979–1981 (55 boys and 61 girls aged 7 to 15 years) measured by the same scanner. To investigate difference between the two cohorts, we used multiple linear regression with age, sex, and cohort as predictors. Predicted bone density at age 16 years was estimated through the slope values. The bone density‐age slope was flatter in the cohort measured in 2017–2018 than in the cohort measured 1979–1981 (−5.6 mg/cm2/yr [95% confidence interval −9.6 to −1.5]). Predicted bone density was at age 16 years in 2017–2018 in boys was 10% lower (−0.9 SD) and in girls 11% lower (−1.1 SD) than in their counterparts measured in 1979–1981. We found indications that children nowadays develop lower bone mass than four decades ago, giving concern that they may have a higher risk of osteoporosis and fragility fractures as they grow old. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.