title: 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).
abstract: BACKGROUND
  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.
  
  
  METHODS
  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.
  
  
  RESULTS
  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/m(2) loss per 1000 MET*m/d increase (in men) and 0.40 (95% CI: 0.23, 0.57) kg/m(2) 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/cm(2) per 1000 MET*m/d (in men) and 0.003 (95% CI: 0.000-0.007) g/cm(2) 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.
  
  
  CONCLUSIONS
  Increased PA is associated with an increase in BMD and a concomitant decrease in BMI. These findings suggest that population-level interventions to increase PA would favorably impact bone and other health outcomes.
authors.affiliation: Canadian Multicentre Osteoporosis Study, McGill University Hospital Centre Research Institute, Montréal, Canada.
authors.forename: L
authors.initials: L
authors.lastname: Langsetmo
authors.forename: C L
authors.initials: CL
authors.lastname: Hitchcock
authors.forename: E J
authors.initials: EJ
authors.lastname: Kingwell
authors.forename: K S
authors.initials: KS
authors.lastname: Davison
authors.forename: C
authors.initials: C
authors.lastname: Berger
authors.forename: S
authors.initials: S
authors.lastname: Forsmo
authors.forename: W
authors.initials: W
authors.lastname: Zhou
authors.forename: N
authors.initials: N
authors.lastname: Kreiger
authors.forename: J C
authors.initials: JC
authors.lastname: Prior
mesh_terms: D015992:Body Mass Index; D015519:Bone Density; D002170:Canada; D015331:Cohort Studies; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D009043:Motor Activity; D010024:Osteoporosis / Q000503:physiopathology*; D011446:Prospective Studies; D012307:Risk Factors; D011795:Surveys and Questionnaires