Elsevier

Bone

Volume 64, July 2014, Pages 254-262
Bone

Original Full Length Article
Associations between objectively-measured sedentary behaviour and physical activity with bone mineral density in adults and older adults, the NHANES study

https://doi.org/10.1016/j.bone.2014.04.009Get rights and content

Highlights

  • Repeated exposure to sitting in daily life (objectively measured) is negatively associated with femoral bone mineral density in women.

  • In women, this negative association is seen with longer duration of sitting bouts, as opposed to sitting bout frequency.

  • This negative association with bone mineral density is independent of level of moderate and vigorous physical activity in women.

  • In men, however, time spent in moderate and vigorous activity is positively associated with femoral bone mineral density.

  • Future research should investigate the effect of interventions targeted at reducing sitting time on bone health.

Abstract

Background

Lack of physical activity (PA) is an important modifiable risk factor for bone mineral density (BMD). Time spent in sedentary behaviour (SB), or time spent in non-exercising seated and reclining postures, has recently emerged as a new public health risk, independent of the amount of time someone spends being active. As national surveys report that adults spend on average 8 h per day being sedentary, rising to 10 h a day in older age, it has been hypothesised that a repeated exposure to sitting in modern daily life, whether it is for travelling, working or leisure, might have a deleterious effect on bone health in a way that mirrors the results of studies into the effect of lengthy periods of bed-rest. The aim of this study was to investigate for the first time a) how time spent in SB is associated with bone mineral density (BMD), b) whether this association changes depending on the amount of time spent engaging in different intensity levels of PA, and c) if the pattern of accumulation of SB and long uninterrupted periods of SB are associated with BMD.

Methods

The 2005/2006 National Health and Nutrition Examination Survey (NHANES), is a cross-sectional study of a representative sample of the US population that is conducted biannually by the National Centers for Disease Control. PA and SB were assessed objectively over 7 days using an Actigraph accelerometer and BMD was measured via dual-energy X-ray absorptiometry. In this study, data are presented on four regions of the femur (femoral neck, trochanter, inter trochanter and total femur) and total spine (L1–L4). The associations between BMD, SB and PA levels were examined using multiple linear regressions stratified by gender. In addition, the association between the pattern of accumulation of SB (quantified as frequency and duration of SB) and BMD was also investigated. All models were adjusted for known risk factors associated with BMD. In total, data for 2117 individuals, aged 23–90 + years (males N = 1158), were available to analyse SB and femur BMD and 1942 individuals (males N = 1053) for analysis of SB and spine BMD.

Results

There was no evidence of an association between SB time and hip or spinal BMD in men. For men, time spent doing moderate to vigorous activity (MVPA) and vigorous activity (VIG) was associated with higher total femur and the other hip sub-region BMD. The regression coefficient was BMVPA = 0.306 (95% CI: 0.021–0.591) g/cm2 for each 10 minute increment in daily MVPA. For VIG, the regression coefficient is BVIG = 0.320 (95% CI: 0.058–0.583) but this cannot be interpreted linearly as time spent in vigorous activity was square root transformed. In women, SB was negatively associated with total femur BMD and all sub-regions but not MVPA nor VIG. The regression coefficient for total femur BMD was BSB =  0.159 (95% CI: − 0.241–0.076) g/cm2 for each 10 minute increment spent being sedentary each day. In addition, the duration of SB bouts was deleteriously associated with BMD for the total femur and of other hip sub-regions, but the number of bouts of SB did not have a significant effect. These associations were found to be independent of the amount of MVPA and VIG that women engage in. No associations were found between SB or PA and spinal BMD for either men or women.

Conclusions

These results provide the first evidence that repeated exposure to sitting (SB), measured objectively in daily life, is deleteriously associated with BMD of the total femur and of all hip sub-regions in women, independent of the amount of time women engage in moderate and vigorous activity. This suggests that SB might be a risk factor for bone health in women independent of whether they engage in physical activity. In addition, the duration of SB bouts, rather than their frequency, appears to be deleteriously associated with BMD of the total femur and of all hip sub-regions. Future research should investigate the effect on bone health of interventions which set out to reduce SB and the duration of SB bouts in comparison, and as adjunct, to the promotion of PA. For men, SB is not significantly associated with BMD of the femur or spine and the results appear to confirm that moderate and vigorous activity has a protective effect.

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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