LAUSR

Biochemical markers of bone turnover are higher in young adult men than in middle‐aged men or young adult women. Nonetheless, little is known about the determinants and clinical significance hereof. The present study examined determinants of serum bone turnover markers in men around peak bone mass age, and explored whether bone turnover at this age predicts subsequent changes in bone mass. We used cross‐sectional and longitudinal data from 973 and 428 healthy men, respectively, aged 25 to 45 years at baseline, including baseline procollagen type I amino‐terminal propeptide (P1NP), osteocalcin, and C‐terminal telopeptide of type I collagen (CTX) from fasting serum samples, baseline questionnaire‐assessed physical activity levels, and baseline and follow‐up dual‐energy X‐ray absorptiometry–derived areal bone mineral density (aBMD) and body composition. Mean follow‐up time was 12.4 ± 0.4 years. At baseline, all bone turnover markers were inversely associated with total body fat mass (β ≤ –0.20, p < 0.001), and positively with physical activity during sports activities (β ≥ 0.09, p ≤ 0.003), and, albeit not independently from fat mass, total body lean mass (β ≥ 0.20, p ≤ 0.003). Mean annual aBMD changes in the longitudinal cohort were –0.19% ± 0.24% at the total body, –0.14% ± 0.42% at the spine, –0.49% ± 0.47% at the femoral neck, and –0.25% ± 0.37% at the total hip (all p < 0.001). Higher bone turnover markers at baseline were associated with larger decreases in aBMD at all measurement sites (β ≤ –0.08, p ≤ 0.081 for P1NP; β ≤ –0.16, p ≤ 0.002 for osteocalcin; and β ≤ –0.21, p < 0.001 for CTX). In conclusion, our findings show that sports activities and body composition, primarily fat mass, are the main identified determinants of bone turnover in men around peak bone mass age. Further, bone turnover at this age is an important determinant of subsequent changes in bone mass, with higher levels of bone turnover markers being associated with greater decreases in aBMD. © 2017 American Society for Bone and Mineral Research.