Changes in hip bone mineral density and objectively measured physical activity in middle-aged women: a 6-year prospective study.

PURPOSE: This study was conducted to determine if physical activity volume (PAv) and intensity (PAi) at baseline influence the likelihood of gaining hip bone mineral density (BMD) over 6 years. DESIGN AND SAMPLE: In a prospective study, the sample was limited to 244 female nonsmokers, ages 35 to 45 years, and was approximately 90% white. SETTING AND MEASURES: PAv and PAi were measured in daily living conditions using accelerometers at baseline. BMD, measured by dual energy x-ray absorptiometry, and several confounding factors were measured in the lab. ANALYSIS: On the basis of BMD change scores, participants were divided into three categories: BMD loss, minimal change, and BMD gain. Risk ratios were used to show the likelihood of BMD gains over time across different levels of PAv and PAi at baseline.
RESULTS: Women with higher PAv were more likely to show improvements in hip BMD from baseline to follow-up than their counterparts, as indicated by the Mantel-Haenszel chi-square (χ(2)(mh)  =  6.1, p  =  .01). Women with high PAv were 2.50 times (95% CI, 1.19-5.24) more likely to experience hip BMD gains than women with low PAv, and women with moderate PAv were 2.20 times (95% CI, 1.08-4.45) more likely. PAi was not predictive of gains in hip BMD. Adjusting for potential confounders had little influence on the results.
CONCLUSIONS: Middle-aged women with moderate or high levels of PAv are more likely to experience BMD gains at the hip over time compared with those who have low levels of PAv. However, PAi does not appear to influence the likelihood of gaining BMD at the hip over 6 years.