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This longitudinal study examined associations of bone mass with physical activity and vitamin D level over more than 6 years through puberty. A total of 663 participants (320 boys) with mean age 9.6 years at baseline (10-17 years at follow-up), underwent dual energy X-ray absorptiometry, anthropometry and blood samples for vitamin D at least twice during the study period (with three possible time-points). Physical activity was assessed using accelerometers at follow-up. A positive association was found between percent time spent at vigorous physical activity and total-body less head bone mineral content (β = 5.8, p = 0.002). The magnitude of this association increased with maturational development; thus physical activity may have a greater influence on bone mass in the more mature participants. The vitamin D levels were also positively associated with bone mass. A high degree of tracking was observed with changes in anthropometric Z scores predictive of deviation from tracking. No environmental factor predicted deviation from tracking.

Overweight, physical inactivity and sedentary behaviour have become increasing problems during the past decade. Increased sedentary behaviour may change the body composition (BC) by increasing the fat mass relative to the lean mass (LM). These changes may influence bone health to describe how anthropometry and BC predict the development of the bone accruement. The longitudinal study is a part of The CHAMPS study-DK. Children were DXA scanned at baseline and at 2-year follow-up. BC (LM, BF %) and BMC, BMD and BA were measured. The relationship between bone traits, anthropometry and BC was analysed by multilevel regression analyses. Of the invited children, 742/800 (93%) accepted to participate. Of these, 682/742 (92%) participated at follow-up. Mean (range) of age at baseline was 9.5 years (7.7-12.1). Height, BMI, LM and BF % predicted bone mineral accrual and bone size positively and independently. Height and BMI are both positive predictors of bone accruement. LM is a more precise predictor of bone traits than BF % in both genders. The effects of height and BMI and LM on bone accruement are nearly identical in the two genders, while changes in BF % have different but positive effects on bone accretion in both boys and girls.