Original Full Length ArticleAssociations between objectively-measured sedentary behaviour and physical activity with bone mineral density in adults and older adults, the NHANES study
Introduction
Osteoporotic fractures are a major public health issue and a growing concern with an ageing population. Loss of bone mass is part of the normal ageing process, [1], [2] and is determined by factors such as gender [3], ethnicity [4], [5] and genetics [6]. However a number of modifiable health behaviours, such as smoking [7], dietary intake [8] and exercise [9], also influence the rate of bone mineral density (BMD) loss and the development of osteoporosis in older age. To reduce the risk of fractures and their associated cost to society, as well as the impact on an individual's health and quality of life, it is important to identify modifiable risks associated with poor bone health. It has long been recognised that long periods of enforced inactivity, reduced weight bearing and muscle loading, such as bed rest [10], [11] and time spent in reduced gravity [12], [13], change the bone turnover and mineral homeostasis. After only a few weeks the bone loss is equivalent to that which would be expected in a decade of normal ageing [14]. These studies suggest that bed rest and weightlessness have two distinct effects on bone health. Firstly, there is a direct physiological increase in bone resorption [10] and secondly, there is an indirect effect through the decreased stimulation of bone formation [15], [16], caused by the lack of physical activity (PA), which modulates the amount and quality of bone that is produced [17].
The effect of long periods of reduced weight-bearing such as enforced bed rest and weightlessness in space might seem irrelevant for the majority of the population. However modern societal and technological changes have dramatically increased the amount of time spent in low impact and reduced weight-bearing postures in everyday life [18]. Sedentary behaviours (SBs), defined as the time spent in sitting or reclining postures and involving low energy expenditure [19], are ubiquitous in modern lifestyles during leisure time, work and transportation [20]. National surveys show that adults spend on average 8 h of the waking day being sedentary [21], rising to 10 h in older age [22], with 67% of older adults spending more than 8.5 h a day sitting [23]. Temporal patterns of SB show that the majority of this sedentary time is accumulated in a small number of long uninterrupted bouts, some of which last several hours [24].
It is conceivable that repeated exposure to reduced weight-bearing activity during daily life, especially in long continuous bouts, might have a similar effect on bone metabolism that single bouts of prolonged bed-rest or weightlessness have. The hypothesis is that detrimental effects of SB on bone health could stem from two possible factors: a lack of musculature activation and unloading of bone structure as in reduced gravity [25]. Recent studies in adolescents suggest that there is, indeed, an association between bone mineral density and time spent in sedentary pursuits such as watching TV and spending time sitting in front of a computer [26], [27], [28]. Furthermore, this association was found to be only partially counteracted by the engagement in oesteogenic physical activity throughout the rest of the day. In a study of white older women aged 65 and over, the risk of fracture was found to double where those individuals spent 4 h or less standing per day, while the fracture risk was only 30% lower for those who walked regularly [29]. However, this study used self-reported measures of sitting and lying time which are imprecise and may affect estimations of the effect of SB on bone health [30]. To date, a potential association between objectively-measured SB and bone health in adulthood has not been investigated.
We used data from the National Health and Nutritional Examination Survey 2005–06 (NHANES) to explore the cross-sectional associations between SB and BMD.
The objectives of this study were to examine: a) whether total time spent in SB is associated with BMD of the hip and lumbar spine, b) if the effects of SB on BMD are compounded when accumulated in long uninterrupted bouts and c) whether this association changes depending on the amount of time spent at different PA intensities.
Section snippets
Study
The National Health and Nutrition Examination Survey (NHANES) is a cross-sectional study conducted annually by the National Center for Health Statistics at the Centers for Disease Control and Prevention (CDC). It uses a complex, multi-stage probability design to obtain a representative sample of the USA civilian non-institutionalized population over a two-year cycle. Details of the surveys and NHANES methods are available from the CDC website. The measurement of BMD via dual-energy X-ray
Results
A sample average for the total amount of time and the pattern of SB, together with the average time spent at different physical activity intensities, is presented per gender in Table 1.
Results of modelling the relationships between of SB, PA and femur BMD are shown in Table 2 for men and Table 3 for women.
Discussion
In this cross-sectional study the amount of time spent engaging in SB is negatively associated with BMD of the femur in women, independently of the amount of time they spent doing moderate and vigorous activity. This effect was not seen in men.
This appears to corroborate reports of an increased risk of osteoporotic fractures that are associated with higher self-reported sitting times in older women [29] and a negative association between SB and femur BMD found in female adolescents [27], [28].
Conclusion
This study reports first evidence that objectively measured sedentary time is negatively associated with bone mineral density of the femur region in women. This association appears independent of the level of PA and is not attenuated by the engagement in moderate and vigorous activity during the rest of the day. There is also an association between LIPA and BMD. Collinearity between LIPA and SB time does not allow for checks for independence between the two, however, the strength of this
Acknowledgments
The authors would like to thank R. Troiano and the NHANES team for their work in making the NHANES data available. The authors also would like to thank J. Saunders, Z. Tieges for proof reading this manuscript.
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