Effects of Physical Activity and Sedentary Behavior on Bone Density in Healthy Adult Women

Abstract

Physical activity promotes bone health, and recommendations often encourage moderate to vigorous intensity physical activity (MVPA) for maximum benefit. Here we test the effects of MVPA along with low‐intensity physical activity and sedentary behavior on bone density of the tibia and radius in two samples of healthy adult women. We hypothesized that increased time in low intensity physical activity and decreased sedentary behavior would be associated with increased bone density.MethodsWe used an off‐the shelf activity monitor (FitBit One) to measure physical activity in a sample of healthy women from the Mogielica Human Ecology Study Site in rural Poland (n=76) and Polish‐American women from Illinois (n=40). All women were between 18–46 years of age, non‐smoking, not pregnant, not breastfeeding, and not on hormonal contraception. All women provided written informed consent prior to study participation (UIUC IRB#13856). Cortical bone density was measured using quantitative ultrasound (Sunlight MiniOmni; BeamMed, Inc.) in a subset of women (Polish n=46; Polish‐American n=15) at the distal 1/3 point of the radius and the midpoint of the tibia. We used t‐tests for examining differences between populations and Spearman rank correlations to test for associations between physical activity measures and bone.ResultsThe two samples did not have significant differences in time spent in MVPA (p=0.3) or average total daily steps (p=0.6), but the Polish women spent significantly more time active at low intensity (p<0.001; Polish = 338±88 minutes per day; Polish‐American=194±72 minutes per day) and significantly less time sedentary (p<0.001 ; Polish=1078±93 minutes per day; Polish‐American=1189±123 minutes per day). The Polish women had higher bone density in the radius (p<0.001), but not the tibia (p=0.2). We found that density of the tibia was not associated with any of our measures of physical activity. Density of the radius was negatively correlated with time sedentary (ρ= −0.36, p=0.006) and time in MVPA (ρ= −0.33, p=0.012), but positively correlated with total time active (ρ=0.39, p‐0.002) and time active at low intensity (ρ=0.44, p<0.001).ConclusionsThese two samples of healthy adult women have different patterns of physical activity, with rural Polish women spending more time active at low intensity and less time sedentary than Polish‐American women. We also found that our strongest correlation between bone density and physical activity was related to time spent active at low intensity. Our results highlight the need to accurate measuring physical activity at all intensities in order to truly understand the health effects of habitual physical activity performed by women.Support or Funding InformationThis material is based on work supported by NSF#1317140, BCS‐1732117, BCS‐1650839, and DGE‐1144245. It was also supported by Wenner‐Gren #084918 & #089812, American Philosophical Society Lewis & Clark Fund for Exploration & Field Research, UIUC Dept of Anthropology Summer Research Fund, Beckman Institute CS/AI Award, UIUC Graduate College Dissertation Travel Grant, and Grant‐In‐Aid of Research from Sigma Xi.