Physical Activity and Bone Accretion: Isotemporal Modeling and Genetic Interactions.

PURPOSE: This study aimed to determine if replacing time spent in high- and low-impact physical activity (PA) predicts changes in pediatric bone mineral density (BMD) and content (BMC). METHODS: We analyzed data from the longitudinal Bone Mineral Density in Childhood Study (N = 2337 with up to seven visits). The participants were age 5-19 yr at baseline, 51.2% were female, and 80.6% were nonblack. Spine, total hip, and femoral neck areal BMD and total body less head (TBLH) BMC Z-scores were calculated. Hours per day spent in high- and low-impact PA were self-reported. Standard covariate-adjusted (partition model) and time allocation-sensitive isotemporal substitution modeling frameworks were applied to linear mixed models. Statistical interactions with sex, self-reported ancestry, age, and bone fragility genetic scores (percentage of areal BMD-lowering alleles carried) were tested. RESULTS: In standard models, high-impact PA was positively associated with bone Z-score at all four skeletal sites (e.g., TBLH-BMC Z-score: beta = 0.05, P = 2.0 × 10), whereas low-impact PA was not associated with any of the bone Z-scores. In isotemporal substitution models, replacing 1 h·d of low- for high-impact PA was associated with higher bone Z-scores (e.g., TBLH-BMC Z-score: beta = 0.06, P = 2.9 × 10). Conversely, replacing 1 h·d of high- for low-impact PA was associated with lower bone Z-scores (e.g., TBLH-BMC Z-score: beta = -0.06, P = 2.9 × 10). The substitution associations were similar for each sex and ancestry group, and for those with higher and lower genetic scores for bone fragility (P-interactions > 0.05), but increased in strength among the older adolescents (P-age interactions < 0.05). CONCLUSIONS: Time-sensitive models suggest that replacing low-impact PA for high-impact PA would be beneficial for the growing skeleton in the majority of children.

Longitudinal tracking of dual-energy X-ray absorptiometry bone measures over 6 years in children and adolescents: persistence of low bone mass to maturity.

OBJECTIVES: Early assessment of bone mass may be useful for predicting future osteoporosis risk if bone measures "track" during growth. This prospective longitudinal multicenter study examined tracking of bone measures in children and adolescents over 6 years to sexual and skeletal maturity. STUDY DESIGN: A total of 240 healthy male and 293 healthy female patients, ages 6-17 years, underwent yearly evaluations of height, weight, body mass index, skeletal age, Tanner stage, and dual-energy x-ray absorptiometry (DXA) bone measurements of the whole body, spine, hip, and forearm for 6 years. All subjects were sexually and skeletally mature at final follow-up. Correlation was performed between baseline and 6-year follow-up measures, and change in DXA Z-scores was examined for subjects who had baseline Z < -1.5. RESULTS: DXA Z-scores (r = 0.66-0.87) had similar tracking to anthropometric measures (r = 0.64-0.74). Tracking was stronger for bone mineral density compared with bone mineral content and for girls compared with boys. Tracking was weakest during mid- to late puberty but improved when Z-scores were adjusted for height. Almost all subjects with baseline Z < -1.5 had final Z-scores below average, with the majority remaining less than -1.0. CONCLUSIONS: Bone status during childhood is a strong predictor of bone status in young adulthood, when peak bone mass is achieved. This suggests that bone mass measurements in children and adolescents may be useful for early identification of individuals at risk for osteoporosis later in life.

Racial disparity in fracture risk between white and nonwhite children in the United States.

OBJECTIVES: To examine risk factors for fracture in a racially diverse cohort of healthy children in the US. STUDY DESIGN: A total of 1470 healthy children, aged 6-17 years, underwent yearly evaluations of height, weight, body mass index, skeletal age, sexual maturation, calcium intake, physical activity levels, and dual-energy x-ray absorptiometry (DXA) bone and fat measurements for up to 6 years. Fracture information was obtained at each annual visit, and risk factors for fracture were examined using the time-dependent Cox proportional hazards model. RESULTS: The overall fracture incidence was 0.034 fracture per person-year with 212 children reporting a total of 257 fractures. Being white (hazard ratio [HR] = 2.1), being male (HR = 1.8), and having skeletal age of 10-14 years (HR = 2.2) were the strongest risk factors for fracture (all P ≤ .001). Increased sports participation (HR = 1.4), lower body fat percentage (HR = 0.97), and previous fracture in white girls (HR = 2.1) were also significant risk factors (all P ≤ .04). Overall, fracture risk decreased with higher DXA z scores, except in white boys, who had increased fracture risk with higher DXA z scores (HR = 1.7, P < .001). CONCLUSIONS: Boys and girls of European descent had double the fracture risk of children from other backgrounds, suggesting that the genetic predisposition to fractures seen in elderly adults also manifests in children.