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, mm²), 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. © 2014 American Society for Bone and Mineral Research.