Summary: Relationships between objectively assessed free-living physical activity (PA) and changes in bone health over time are poorly understood in older adults. This study suggests these relationships are sex-specific and that body composition may influence the mechanical loading benefits of PA.

Introduction: To investigate associations of objectively assessed PA and bone health in community-dwelling older adults.

Methods: This secondary analysis of a subset of the Tasmanian Older Adult Cohort study included participants with PA assessed utilising ActiGraph GT1M accelerometers over 7 days (N = 209 participants, 53% female; mean ± SD age 64.5 ± 7.2 years). Steps/day and PA intensity were estimated via established thresholds. Bone mineral content (BMC) was acquired at the total hip, lumbar spine, legs and whole body by DXA at baseline and approximately 2.2 years later. Relationships between PA and BMC were assessed by multivariable linear regression analyses adjusted for age, smoking status, height and total lean mass.

Results: Men with above-median total hip BMC completed significantly less steps per day, but there was no significant difference in PA intensity compared with those with below-median BMC. There were no significant differences in PA in women stratified by median BMC. In women, steps/day were positively associated with leg BMC (B = 0.178; P = 0.017), and sedentary behaviour was negatively associated with leg BMC (- 0.165; 0.016) at baseline. After adjustment for confounders including lean mass and height, higher sedentary behaviour at baseline was associated with declines in femoral neck BMC (- 0.286; 0.011) but also with increases in pelvic BMC (0.246; 0.030) in men and increases in total hip BMC (0.215; 0.032) in women, over 2.2 years. No other significant longitudinal associations were observed after adjustment for body composition.

Conclusions: Associations of accelerometer-determined sedentary behaviour and PA with bone health in older adults differ by sex and anatomical site and are mediated by body composition.

Item Type:	Refereed Article
Keywords:	accelerometer, bone mineral content, bone mineral density, intensity, osteoporosis, physical activity
Research Division:	Medical and Health Sciences
Research Group:	Clinical Sciences
Research Field:	Rheumatology and Arthritis
Objective Division:	Health
Objective Group:	Clinical Health (Organs, Diseases and Abnormal Conditions)
Objective Field:	Skeletal System and Disorders (incl. Arthritis)
UTAS Author:	Aitken, D (Associate Professor Dawn Aitken)
UTAS Author:	Jones, G (Professor Graeme Jones)
UTAS Author:	Scott, D (Mr David Scott)
ID Code:	125140
Year Published:	2018
Deposited By:	Menzies Institute for Medical Research
Deposited On:	2018-03-29
Last Modified:	2018-12-11
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