Physical Activity, Body Mass Index and Bone Mineral Density
Although bone mineral density (BMD) and body mass index (BMI) are strongly related, moderate physical activity decouples this—increased physical activity is associated with both increased BMD and decreased BMI, improving bone health and other health outcomes.
Langsetmo, L. et al. (2011). Physical Activity, Body Mass Index and Bone Mineral Density—Associations in a Prospective Population-based Cohort of Women and Men: The Canadian Multicentre Osteoporosis Study (CaMos). Bone. 50(1): 401–408. doi:10.1016/j.bone.2011.11.009
Physical activity (PA) is an important modifiable risk factor for both bone mineral density (BMD) and body mass index (BMI). However, BMI is itself strongly predictive of BMD. Our aim was to determine the association between PA and BMD, with consideration of BMI as a potential mediating factor.
The Canadian Multicentre Osteoporosis Study (CaMos) is a population-based prospective cohort study of Canadian women and men. PA was determined from interviewer-administered questionnaires at baseline and Year 5 and summarized as daily energy expenditure in total metabolic equivalents of the task multiplied by minutes/day (MET*m/d). Height, weight, and total hip and lumbar spine BMD were measured at baseline and Year 5. General linear models assessed relationships between PA and BMD, both cross-sectionally (baseline PA with baseline BMD) and longitudinally (average PA and change in PA with change in BMD). BMI was considered as a mediating factor. Potential confounders included age, center, education, caffeine intake, alcohol exposure, smoking history, history of weight-cycling, age at menarche, past use of oral contraceptives, history of > 3 months missed menstruation, menopausal status, and antiresorptive use, as relevant.
The study included 2855 men and 6442 women. PA was inversely associated with BMI at baseline, and an increase in PA between baseline and Year 5 was associated with a decrease in BMI, with 0.41 (95% CI: 0.22, 0.60) kg/m2 loss per 1000 MET*m/d increase (in men) and 0.40 (95% CI: 0.23, 0.57) kg/m2 loss per 1000 MET*m/d increase (in women). BMI was strongly associated with BMD, both cross-sectionally and longitudinally. However, increased PA was associated with a small increase in total hip BMD, 0.004 (95% CI: 0.000-0.008) g/cm2 per 1000 MET*m/d (in men) and 0.003 (95% CI: 0.000-0.007) g/cm2 per 1000 MET*m/d (in women). Average PA was associated with an increase in lumbar spine BMD in women, but not in men; it was not associated with change in total hip BMD in either sex.