A large cross-sectional analysis found that physical activity was associated with higher lumbar spine bone mineral density (BMD) in postmenopausal women. However, the benefits appeared to plateau and were primarily observed in those who were younger and had a normal BMI.

Investigators found a positive association between weekly physical activity, measured in metabolic equivalent of task (MET) hours, and total lumbar BMD, with benefits leveling off at approximately 32 MET-hours per week. Activity beyond that threshold was not linked to additional BMD gains.

Furthermore, the results demonstrated “that the efficacy of [physical activity] in enhancing total lumbar BMD is not universal but appears to be contingent upon specific patient factors,” such as age and BMI, the authors noted.

The study was published in the journal Medicine.

Identifying the Optimal MET Threshold

Although osteoporosis medications remain first-line therapy for postmenopausal bone loss, physical activity is widely recommended as a supplemental intervention. However, the amount of exercise necessary to meaningfully impact BMD in high-risk populations has remained unclear.

To address that question, researchers analyzed data from 9339 postmenopausal women participating in the National Health and Nutrition Examination Survey (NHANES) from 2011 to 2018. 

Physical activity was quantified using MET-hours per week, an objective measure of energy expenditure across during work, leisure, and transportation activities. For example, walking briskly equates to roughly 4.3 METs per hour, while tennis is about 7 METs per hour. Therefore, achieving 21 MET-hours per week would require approximately 4.8 hours of brisk walking or 3 hours of tennis per week. 

Lumbar spine BMD was measured using DEXA.

After multivariate adjustment for demographic, biochemical, and lifestyle factors, physical activity was significantly associated with higher lumbar BMD up to 32 MET-hours per week. Above that threshold, the association was no longer significant, suggesting this represents a plateau after which there are diminishing returns.

Participants in the second- and third-highest tertiles of physical activity had significantly greater BMD increases than those in the lowest tertile.

Age and BMI May Influence Effects

Subgroup analyses revealed further notable differences in threshold effects based on age and BMI. BMD benefits were observed among women younger than 45 years at up to 54 MET-hours/week and among women with normal BMI (< 25) at up to 128 MET-hours/week (P < .0001 for both). No significant associations were found between activity and BMD among middle-aged or older women or those with higher BMI.

The differential effects likely relate to physiological and metabolic factors, first author Peng Wang, MD, of Weihai Municipal Hospital, Cheeloo College of Medicine, Shandong University, China, told Medscape Medical News. 

“Younger postmenopausal women may have more responsive bone remodeling mechanisms to mechanical loading,” Wang explained. “In those with normal BMI, the absence of excessive adiposity may reduce systemic inflammation and hormonal imbalances that can interfere with bone formation.”

By contrast, “higher BMI is often associated with chronic low-grade inflammation and altered loading mechanics, which may attenuate the osteogenic effects of physical activity,” Wang added.

Study Limitations 

Although the study established a potential upper limit for activity-related BMD benefits, it did not address a minimum threshold. Wang noted this requires further investigation, particularly in sedentary populations.

Another limitation is that the NHANES does not track osteoporosis medication use, which could independently influence BMD outcomes.

Despite these limitations, Wang said the findings help define activity thresholds that could inform more individualized exercise counseling. 

“For younger women (under 45) and those with normal BMI, moderate-intensity activity of [up to] approximately 13.5 or approximately 32 hours per week, respectively, may optimize lumbar spine BMD,” he noted. “However, for older or higher-BMI women, physical activity alone may not suffice, and a multifaceted approach — including nutrition, pharmacotherapy, and lifestyle modification — should be emphasized.”

Different Exercises, Different Effects

Commenting on the study, Lauren Burt, PhD, adjunct assistant professor at the McCaig Institute for Bone and Joint Health, University of Calgary, Alberta, said the results reinforce the notion that “bone responds to mechanical loading, and higher activity levels tend to relate to higher BMD.” 

However, she cautioned that MET-hours do not distinguish between bone-stimulating activities, such as jumping or resistance training, and non-osteogenic activities, such as cycling or swimming. 

“The analysis broadens our understanding of overall activity dose but does not answer the critical question of which specific exercise modalities are most effective for improving bone health in postmenopausal women,” Burt said.

She also noted that women under age 45 represent a very small proportion of the naturally postmenopausal population, making the findings less generalizable. 

In a recent systematic review and meta-analysis of six randomized controlled trials, Burt and colleagues reported that strength training was associated with improved areal BMD at multiple sites during the menopausal transition, findings consistent with 2020 osteoporosis management guidelines from the American Association of Clinical Endocrinologists and American College of Endocrinology. 

“Physical activity is beneficial, but not all physical activity is equal,” Burt said. “Postmenopausal women likely need targeted, osteogenic training, including resistance training and moderate-to-high-impact loading, to meaningfully influence BMD.” 

“While overall duration of physical activity supports general health and wellbeing, it may not be sufficient on its own to improve bone health.”

The study authors and Burt reported no relevant disclosures.