Skeletal Development During the First Three Decades in Life

Abstract: AbstractBackground: Areal bone mineral density (aBMD) measured by dual-energy X-ray absorptiometry (DXA) is a surrogate estimate of bone strength. The highest value in life is referred to “peak bone mass” (PBM). In the femoral neck (FN), PBM is reached late in the second decade in life, after which there is an age-related decline in aBMD during adulthood. However, a decrease in aBMD may follow a decline in bone mass and/or an increase in bone size, and since the bending strength of tubular structures is related to the width of the structure, a decrease in aBMD could not automatically be translated to reduced bone strength. Since it is counterintuitive in an evolutionary perspective that FN bone strength should start to decline in such early ages, we hypothesized that peak FN.aBMD and peak bone strength do not correlate.A lifestyle factor with great influence on the skeleton is physical activity (PA), where increased PA in childhood is associated with high PBM. PA in childhood could thus hypothetically counteract age-related bone mass attenuation in adult life, possibly postponing the onset of osteoporosis and reducing the number of fractures. It is debated, however, whether long-term interventions during puberty are effective, as children, especially girls, are known to reduce the level of PA in this period. Previous studies have also shown incongruent results as to whether PA-induced musculoskeletal gains are retained after reduction of PA levels.The aims of the studies were to present normative data in young men of DXA and peripheral quantitative computed tomography (pQCT) estimated BMC, BMD, and bone structure, and in cross-sectional analyses, to evaluate whether there are associations between these traits and age. Another aim was to investigate whether a school-based PA intervention program from before to after puberty is associated with musculoskeletal benefits, and if these possible benefits are attenuated after termination of the intervention.Methods: Bone mass and bone structure data were gathered from the cross-sectional MRPEAK study which included 1083 population-based men aged 18–28 years, scanned in the radius and tibia with pQCT and in FN by DXA. We also gathered data from the POP study, which at baseline included 349 children aged 7-9 years in four schools. Children in one school followed a 9-year intervention-program that included 40 minutes’ daily physical education (PE), while children in the three control schools continued with 60 minutes of weekly PE. We assessed the children at baseline, at the end of the intervention, and mean 4 years after the last measurement during the intervention. Assessments included lifestyle evaluation and Tanner staging from questionnaires, measurements of anthropometrics by standard equipment, and musculoskeletal traits by DXA and a Biodex dynamometer.Results: Estimates from pQCT of bone mass and bone density were higher with higher ages in the cortical diaphyseal regions of the radius and tibia, while we found no such associations in the trabecular bone in the ultra- distal regions. After age 19 there was a negative correlation between age and FN aBMD, and a positive correlation between age and bone size. When children were followed from Tanner stage 1 to 5, the intervention program was associated with musculoskeletal benefits in both sexes. After termination of the program, we found attenuation in muscle benefits but not in bone mass benefits. In spite of this, when evaluating the entire period from baseline to several years after program termination, benefits in both muscle strength and bone mass gain were evident in the intervention group.Conclusions: Caution is advised when assessing bone strength and fracture risk using aBMD from DXA, since a decline in aBMD may, at least in men in young adulthood, in part be due to increased bone size that actually increases bone strength. pQCT measurements also indicate that bone strength may increase between ages 18 and 28. In children followed over puberty, daily school PA during the 9 compulsory school years is associated with beneficial gains in musculoskeletal traits that remain in young adulthood, several years after program termination.