Downturn in Childhood Bone Mass: A Cross-Sectional Study Over Four Decades

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.