To our knowledge, this is the first large longitudinal population-based study investigating physical activity in relation to aBMD and bone structure development in men at the age when peak bone mass is believed to be attained. In the present study, increased physical activity was related to greater gains in aBMD at the lumbar spine and total body. Interestingly, we found that men who increased their amount of physical activity also increased their hip aBMD in contrast to men who remained or reduced their activity level, in whom a reduction in hip aBMD was apparent. Our data indicate that the physical activity accompanied increase in aBMD was due to increased cortical bone size and trabecular vBMD.
Both cortical bone size and BMD are important determinants of bone strength and resistance against fracture.(24
) As the resistance of bone to bending and torsion forces is related exponentially to its diameter, even a small difference in the outer circumference could make a substantial contribution to its strength and resistance to fracture.(24
) In the present study, we found that increased physical activity was related to augmented cortical bone size (cross sectional area) via actions on the outer cortical envelope (periosteal circumference), indicating attained benefits of physical activity in enlarging the cortical shell, even though physical activity was increased after entering adulthood. Thus, our results suggest that increased physical activity during this age has the ability to prevent the age-dependent decline in aBMD at the hip and increase the cortical envelope, having a positive effect on the attainment of peak bone mass, both in terms of improved cortical structure and in vBMD, in men. We found similar but weaker associations between change in physical activity and change in cortical CSA as well as trabecular vBMD at the radius. We speculate that this finding was due to a much higher degree of loading, by sports activity, exerted on the weight-bearing tibia than on the mostly unloaded radius. In this study, we performed an analysis taking mechanical loading in to account using the osteogenic index, based on ground reaction forces of physical activity. In this analysis, the osteogenic index could explain the change, to a somewhat higher degree than amount of physical activity, in several bone variables, including the cortical CSA and trabecular vBMD. When alterations in lean and fat mass were included as covariates in the linear regression analyses the association between amount of physical activity and bone parameters was weakened or lost, indicating that the effect of change in physical activity on change in bone variables is at least partly mediated via changes in body composition.
The differences in bone parameters according to physical activity behavior during this relatively short period were significant. Hence, the net difference between a man who reduced his activity by 2.5 h/week and a man who increased his activity by 2.5 h/week would equal 0.023 g/cm2
for the spine and 0.022 g/cm2
for the hip, changes corresponding to 16% and 14%, respectively, of an SD in aBMD.(19
) Given that every SD decrease in femoral neck aBMD is associated with approximately a doubled increased hip fracture risk, an alteration dependent on change in physical activity behavior in hip aBMD, as seen in the present study, could result in future increased risk of developing low bone mass and to some extent be of clinical significance.(26
Trabecular vBMD is thought to be of importance for the structural integrity and strength of the long bones, and has been found to be lower in both young men and postmenopausal women with prevalent fractures than in their nonfracture counterparts.(28
) Thus, our data indicate that increased physical activity could have a positive impact on the development of this bone trait, which could have a future impact of fractures associated with trabecular bone loss.
Although our results clearly demonstrate that increased physical activity was associated with greater 5-year gains in aBMD and cortical bone size, it must be emphasized that the men who continued on a high level of physical activity had the highest aBMD and the greatest cortical bone size at the follow-up visit, probably as a result of their long duration and early initiation of physical activity. These findings support previous research indicating the importance of maintenance of a sufficient physical activity to avoid bone loss.(30
) In addition, since falling is an even more important risk factor of hip fractures than aBMD,(31
) and exercise to maintain physical functioning is the most effective way to reduce the risk of falling,(32
) maintenance of physical activity is most likely important at higher ages when hip fractures start to occur.
Using questionnaires to assess physical activity habits is associated with limitations such as obtainment of, to some extent, imprecise physical activity data. In the present study, present sport activity participation was assessed using a self-reporting questionnaire at both the first study visit (baseline) and the 5-year follow-up visit, which limited the risk of inaccurate reporting caused by long time periods of recalling previous physical activity habits.
The use of a measuring technique, pQCT, which enables investigation of the bone geometric properties and the vBMD, together with the large sample size of men followed longitudinally, constitute major strengths of the present study. In addition, the dropout rate was fairly low (22%) and the followed men were not statistically different in anthropometrics and physical activity habits at the baseline visit, suggesting that the longitudinal cohort was representative of the initial population-based cohort. The investigated population, being mainly whites and only males does not allow us to infer that our results will apply to other populations.
In conclusion, this is the first large study reporting that increased physical activity was related to increased aBMD, trabecular vBMD, and augmented cortical bone size in young adulthood, indicating that physical activity during this age has the ability to optimize the attained peak bone mass, which could affect the risk of developing osteoporosis later in life.