Physical activity and its effect on bone in the short- and long- term perspective

Abstract: Osteoporosis is a growing problem worldwide where Sweden has one of the highest fragility fracture incidences in the world. The growing years are thought to be an opportune time to build strong bones and the enhancement of peak bone mass (PBM) has been suggested as a prevention strategy for osteoporosis. Most exercise intervention studies in children, evaluating the accrual of areal bone mineral density (aBMD), include volunteers and use specifically designed osteogenic exercise programs. The aim of the prospective controlled paediatric osteoporosis prevention (POP) study is to evaluate a daily general curriculum-based exercise intervention program of 40min/school day in a population-based cohort of children, aged 7-9 years at study start. All children in grades 1 and 2 in the intervention school were invited and 93% agreed to participate. Age-matched children from 3 nearby schools assigned to the ordinary Swedish school curriculum of 60 minutes per week served as controls. Bone mineral content (BMC; g) and aBMD (g/cm2) were measured with DXA at the total body, lumbar spine and the hip. Bone size and volumetric bone mineral density (vBMD; g/cm3) of the femoral neck and the third lumbar vertebrae (L3) were calculated from the dual energy X-ray absorbtiometry (DXA) scans. A questionnaire previously used in several studies but modified for children evaluated lifestyle factors. All participants remained in Tanner stage I during the study period.

The data presented in this thesis are the results from the first 2 years in 99 girls and 137 boys. There were no differences between the intervention group and the controls at baseline in anthropometrics, bone parameters or lifestyle factors such as dietary habits, chronic diseases, ongoing medication, fractures, smoking and alcohol intake. The only difference found were that the girls in the control group exercised more during leisure time (0.7 ± 0.7 vs. 1.3 ± 1.6 h/week, p=0.02). After the intervention was initiated, the intervention group spent more time on physical activity both in school and in total compared with the controls. The mean annual gain in bone mineral accrual and bone width in the lumbar spine was greater in the intervention group than in the controls. In addition, in girls there was also a difference between the groups in the annual gain in total body bone mineral accrual and femoral neck width. When all individuals gender specific were included in one cohort, the total duration of exercise including both school-based and spare-time organized physical activity correlated with L3 BMC, L3 BMD and L3 width. No such correlations were found for femoral neck parameters.

In summary, the data from the POP-study show that daily physical activity within the school curriculum seems to be associated with benefits in the accrual of BMC, aBMD and gain in bone width, all traits important for bone strength. The benefits are present already after 1 year of intervention and remains after a two year study period. This thesis supports the view that general moderately intense physical activity could be recommended as a strategy to increase BMC, aBMD and bone size in prepubertal children.

If exercise during growth is to be recommended as a prevention strategy for osteoporosis, benefits in aBMD must be maintained in old age when fragility fractures occur. Our study in 22 active soccer players, 128 former soccer players and 138 age-matched controls, evaluates the relationship between exercise during growth and bone mineral density and fractures in old age. Bone mass was measured by DXA and the frequency of fractures was obtained in 284 former soccer players and in 568 controls identified from the computerized city files of Malmö. Relative to controls, the aBMD in the leg was almost 12% higher in the active soccer players. During the more than 35 years following retirement the diminution in leg aBMD, estimated from the regression line, was 0.33% per year in the former soccer players compared with 0.21% in the controls. A greater proportion of former soccer players than of controls had fractures when they were active and below 35 years of age, but the proportion of fragility fractures in old age was no different between the groups. This study provides evidence that exercise during growth results in biologically important benefits in peak aBMD at weight-bearing sites. The data suggests that cessation of exercise may result in loss of the benefit in aBMD, leaving only modest residual benefits in middle age and perhaps none in old age, when fracture occurs. This study does not support the notion that vigorous exercise during growth and young adulthood reduces the risk of fracture in old age. However, we are aware of the power problem when evaluating the fracture data, and that later studies from our group, including a larger sample size, have opposed this view. Therefore, our view today is that vigorous exercise during growth and young adulthood is associated with reduced fracture risk in old age.