Biological Benchmarks for Adult Bone Mass Proportions in Young Females: A Prospective Longitudinal Analysis

In growing humans, densitometric scans of whole-body bone mass "less head" are recommended to circumvent the excessive contribution of youths' proportionally larger heads but potentially inflate inter-scan variation and least significant change due to measurement error. We aimed to determine biological benchmarks for achievement of adult head-body proportions in a sample of US females. Annual whole-body dual energy x-ray absorptiometry (DXA) scans tracked growth, maturation, and bone mass accrual in a prospective longitudinal cohort of girls for up to 19 years (baseline age 7-15 years). We used cubic smoothing spline mixed effects models to generate chronological and gynecological age-based curves for head versus whole-body bone mass proportions (ratios). Females with ≥ 3 annual scans were included (n = 148, age 7-30 years). Models yielded trajectories extending beyond observed age at peak bone mass for our sample. From age 18 years, "adult" mean of means for head vs. whole-body bone mass proportions was 0.204 (n = 66: 95% confidence interval = 0.198-0.210). Individual proportions stabilized to "adult" mean levels circum-menarche (n = 124: mean = 0.198; 95% confidence interval = 0.194-0.202). The minimum age for 95% confidence intervals overlapping with adult values was 12 years, circum-peak height velocity (n = 120: mean = 0.211; 95% confidence interval = 0.207-0.216). In US girls with diverse activity exposures, head vs. whole-body bone mass proportions are "adult" from menarche onward; an "adult" age threshold of 12 years, or age at peak height velocity, may be used in the absence of extreme maturational delay to evaluate whole-body bone mass including the head.