Summary Bone mass achievement predicts later fracture risk. This population-based study describes bone mineral density levels (BMD) and associated factors in Norwegian adolescents. Compared with international reference ranges, BMD levels appear higher and physical activity levels are positively associated with BMD.Purpose Norway has one of the highest reported incidences of osteoporotic fractures. Maximization of peak bone mass may prevent later fractures. This population-based study compared BMD levels of Norwegian adolescents with international reference ranges and explored associated factors.Methods All first year upper secondary school students, aged 15-19 years in the Tromsø region were invited to the Fit Futures study in 2010-2011. Over 90% of the invited participants attended, 508 girls and 530 boys. BMD was measured at total hip, femoral neck and total body by dual x-ray absorptiometry.Lifestyle variables were collected by self-administered questionnaires and interviews. All analyses were performed sex stratified, using linear regression models.Results In girls mean BMD (SD) was 1.060 (0.124), 1.066 (0.123) and 1.142 (0.077) g/cm² at the total hip, femoral neck and total body respectively. In boys corresponding values were 1.116 (0.147), 1.103 (0.150) and 1.182 (0.097), with significant higher values than the Lunar pediatric reference at 16 years of age In girls, height and self-reported intensive physical activity of more than four hours a week and early sexual maturation were positively associated with BMD at both femoral sites (p<0.047). Among boys age, height, body mass index, physical activity and alcohol intake were positively (p<0.038), whereas early stages of sexual maturation and smoking was negatively (p<0.047) related to BMD.Conclusions Despite the heavy fracture burden, Norwegian adolescents´ BMD levels are higher than agematched Caucasians. Physical activity is associated with 1 SD increased BMD levels in those involved in competition or hard training.

ObjectivesLow levels of physical activity may have considerable negative effects on bone health in adolescence, and increasing screen time in place of sporting activity during growth is worrying. This study explored the associations between self-reported screen time at weekends and bone mineral density (BMD).DesignIn 2010/2011, 1038 (93%) of the region’s first-year upper-secondary school students (15–18 years) attended the Tromsø Study, Fit Futures 1 (FF1). A follow-up survey (FF2) took place in 2012/2013. BMD at total hip, femoral neck and total body was measured as g/cm² by dual X-ray absorptiometry (GE Lunar prodigy). Lifestyle variables were self-reported, including questions on hours per day spent in front of television/computer during weekends and hours spent on leisure time physical activities. Complete data sets for 388/312 girls and 359/231 boys at FF1/FF2, respectively, were used in analyses. Sex stratified multiple regression analyses were performed.ResultsMany adolescents balanced 2–4 h screen time with moderate or high physical activity levels. Screen time was positively related to body mass index (BMI) in boys (p=0.002), who spent more time in front of the computer than girls did (p<0.001). In boys, screen time was adversely associated with BMDFF1 at all sites, and these associations remained robust to adjustments for age, puberty, height, BMI, physical activity, vitamin D levels, smoking, alcohol, calcium and carbonated drink consumption (p<0.05). Screen time was also negatively associated with total hip BMDFF2 (p=0.031). In contrast, girls who spent 4–6 h in front of the computer had higher BMD than the reference (<2 h).ConclusionsIn Norwegian boys, time spent on screen-based sedentary activity was negatively associated with BMD levels; this relationship persisted 2 years later. Such negative associations were not present among girls. Whether this surprising result is explained by biological differences remains unclear.

The overall incidence of fractures in childhood in Northern Norway corresponds with other reports from Scandinavia, although the proportion of fractures in girls is higher than in other studies. Both sexes seem especially vulnerable at stages related to sexual maturation. Whether this reflects bone vulnerability or other changes related to puberty requires further investigation.