Physical activity throughout adolescence and bone mineral density in early adulthood: the 1993 Pelotas (Brazil) Birth Cohort Study

We observed a positive association between PA at early/mid-adolescence and lumbar spine and femoral neck BMD in early adulthood in individuals belonging to the 1993 Pelotas (Brazil) Birth Cohort. PA at 11 years was positively associated with lumbar spine bone density in girls only when the peak strain was considered. In boys, regression coefficients appear to be higher for PA at 15 years, whereas among girls the coefficients were similar at both ages. Associations were more consistent for femoral neck bone density. With respect to the variation in PA from 11 to 15 years of age, overall, being active or belonging to the highest tertile of PA (or peak strain score) only at 11 years was not associated with bone density among boys. Conversely, in girls, it appearss important to take into account the peak strain score at 11 years for the bone density in the future though the same was not found when only the weekly time spent in PA was considered. Attaining the current recommendations of PA (300 min/week) at 15 years appearss to be positively important for bone density at 18 years in both anatomical sites, regardless of PA status at 11 years of age. However, in PA measurements, considering the peak strain score, belonging to the highest tertile of peak strain or peak strain × min/week at 11 years of age was positively associated with higher bone density at both sites, whereas belonging to the highest tertile of these measurements only at 15 years or in both ages was positively associated with bone density only at femoral neck site.

Longitudinal analyses about this subject are scarce, particularly derived from low- and middle-income countries. Such studies are important because PA patterns and confounders are distinct between rich and poor countries [14]. The possibility of making inferences using three different PA measurements is another strength of the study. However, there is no available information about calcium intake in our sample, impeding us to include it as a confounder in the regression models.

In general, coefficients were higher when PA considered the peak strain as an exposure in boys, whereas we did not observe such trend among girls. The mean PA measured with peak strain score was higher in males than in females, as well as the weekly time spent in PA. However, previous study suggests that boys’ bones are more sensitive to loading than girls’ [15].

Similar prospective studies are scarce. Previous cross-sectional analysis performed by McVeigh et al. [16] in 9-year-old South African children belonging to the “Birth to Twenty” cohort study used similar PA measurements. In both PA measurements, white children in the highest quartile showed greater BMD than children in the lowest quartile of PA measurements. The lack of association in black children was attributed to lower levels of PA among them [16].

Another study carried out with individuals from the Amsterdam Growth and Health Longitudinal Study (AGAHLS) used PA as weighted metabolic score of intensity, frequency, and duration and PA as the sum of peak score only [17]. Different from our results in boys, Kemper et al. did not found any relationship between lumbar spine bone density at 32 years of age and PA peak strain score during adolescence. Conversely, lumbar spine bone density was positively associated with PA measurement without peak strain score during adolescence. When the analysis was performed using PA during adulthood, PA evaluated as the sum of peak strain score showed higher regression coefficient in association with lumbar spine bone density [17].

There is some evidence that bone adaptation is limited to loading regions [18]. This may explain the consistency of findings for femoral neck bone density but not for lumbar spine bone density, independent of the PA measurement. This positive effect of PA on femoral neck bone density is important because hip fractures account for the majority of fracture-related health care expenditure and losses in quality-adjusted life years in men and women over the age of 50 years [19].

The osteogenic benefits of PA are dependent on age and the relative risk of fracture [20] and puberty has significant influences on changes in skeletal mass [4]. A recent systematic review found that PA during adolescence was associated with higher values of bone density in young adulthood [9], when the maximal peak of bone mass is attained, at least in males. A 10 % higher peak bone mass can reduce the risk of fracture by 50 % and delay the development of osteoporosis by 13 years [21, 22].

The time of peak bone gain is approximately 12.5 years in girls and 14.1 in boys. On average, 26 % of total adult bone mass is gained during a 2-year period [23]. Our PA measurements were at 11 and 15 years of age. The last was close to the age of peak bone gain in boys whereas in girls the measurements were before and after average peak bone gain. This may help to explain the consistency of findings in boys, appointing to higher importance for PA at 15 years of age, independent of the PA measurement used. In addition, this may help explain the inconsistency of findings in girls.

We conclude that PA in early and middle adolescence was positively associated with both lumbar spine and femoral neck BMD in early adulthood. Association between PA and bone density was higher for the femoral neck anatomical site and being active at 15 years appears to be more important for increases in bone density at least in boys. Findings in femoral neck showed that active girls at both 11 and 15 years had increased BMD at this anatomical site at 18 years. In boys, the regression coefficients were higher when PA used the peak strain score in the measurement, drawing attention to importance of loading in the PA. These individuals will be monitored, and future assessments will be carried out during adulthood.