Tracking of environmental determinants of bone structure and strength development in healthy boys: an eight-year follow up study on the positive interaction between physical activity and protein intake from prepuberty to mid-late adolescence.

High protein (> median:Hprot) vs. moderate (< median:MProt) intake was shown to enhance the positive impact of high physical activity (HPA) on proximal femur BMC/aBMD/Area in healthy prepubertal boys. We tested the hypothesis that this synergistic effect would track and influence bone structure and strength until mid-adolescence. BMC/aBMD/Area was measured at femoral neck (FN) and total hip (TotHip) by DXA in 176 boys at 7.4 ± 0.4 and 15.2 ± 0.5 years (± SD). Distal tibia (DistTib) microstructure and strength were also assessed at 15.2 years by high-resolution peripheral computerized tomography (HR-pQCT) and micro-finite element analysis (µFEA). The positive impact of HProt vs. MProt on FN and TotHip BMC/aBMD/Area, recorded at 7.4 years remained unabated at 15.2 years. At this age, at DistTib, HProt-HPA vs. MProt-HPA was associated (p < 0.001) with larger cross-sectional area (CSA, mm(2) ), trabecular number (Tb.N, mm(-1) ) and lower trabecular separation (Tb.Sp, µm). The interaction between physical activity and protein intake was significant for CSA (p = 0.012) and Tb.N (p = 0.043). Under MProt (38.0 ± 6.9 g.d(-1)), a difference in PA from 168 ± 40 to 303 ± 54 kcal.d(-1) was associated with greater stiffness (kN/mm) and failure load (N) of +0.16 and +0.14 Z-score, respectively. In contrast, under HProt (56.2 ± 9.5 g.d(-1) ), a difference in PA of similar magnitude, from 167 ± 33 to 324 ± 80 kcal.d(-1) , was associated with a larger difference in stiffness and failure load of +0.50 and +0.57 Z-score, respectively. In conclusion, the positive influence of relatively HProt on the impact of HPA on proximal femur macrostructure tracks from prepuberty to mid-late puberty. At this stage, the impact of HProt on HPA is also associated with microstructural changes that should confer greater mechanical resistance to weight-bearing bones. These results underscore the importance of protein intake and exercise synergistic interaction in the early prevention of adult osteoporosis.