Elsevier

Bone

Volume 50, Issue 1, January 2012, Pages 401-408
Bone

Original Full Length Article
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)

https://doi.org/10.1016/j.bone.2011.11.009Get rights and content

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/m2 loss per 1000 MET*m/d increase (in men) and 0.40 (95% CI: 0.23, 0.57) kg/m2 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/cm2 per 1000 MET*m/d (in men) and 0.003 (95% CI: 0.000–0.007) g/cm2 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.

Conclusion

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.

Highlights

► Walking-a cohort mean of 30 minutes per day-was associated with increased hip BMD. ► Men & women had similar physical activity; men did more strenuous exercise and work. ► Five year increases in physical activity relate to increased BMD and decreased BMI".

Introduction

Osteoporotic fractures are a major public health concern. Identification of modifiable factors that can reduce fractures is important for healthy aging and reducing the social, medical and personal costs of fracture. Physical activity (PA) is recognized as a potentially important modifiable factor affecting bone mineral density (BMD), bone accrual and loss, and the risk of fracture [1]. PA directly modulates bone remodeling through mechanical stimuli which result in improvements in both material properties (i.e. increased mineralization) and bone geometry (i.e. increased periosteal diameter and cortical thickness) [2], thereby decreasing the risk of fracture.

PA is often quantified using the metabolic equivalents (MET) of a particular task: most commonly as a total of daily energy expenditure over all activities in MET-minute per day (MET*m/d). PA questionnaires may evaluate occupational activity, leisure and sport activity, or activity in daily life, including household chores and walking. Variations in moderate PA, such as walking, have been shown to be more important in determining daily energy expenditure (measured with doubly-labeled water) than high-intensity activities such as competitive sports [3]. Finally, there is increasing interest in moderate-intensity physical activities such as walking, that are more easily adopted by less active people and that are accessible, safe, and can be continued into old age.

The evidence to support a positive role for PA on bone health and fracture prevention comes mainly from studies among postmenopausal women. Cross-sectional studies of middle-aged and older women have found both significant positive [4], [5], [6] and non-significant [7], [8], [9] associations between PA and BMD. Longitudinal studies have likewise shown mixed results [10], [11]. A meta-analysis of exercise intervention studies has shown that a range of physical activities is associated with at least some reduction in bone loss among postmenopausal women [12]. For men, a review of the literature concluded that PA has bone-positive effects, but this conclusion was largely based on smaller and less representative populations [13]. Thus, evidence is inconsistent on the relationship between PA and longitudinal BMD changes in older women and men, and studies have not included longitudinal measurements in population-based studies of men and women with additional demographic data relevant to public health stakeholders. Demographic information can help to identify groups at risk for which adherence to guidelines can be improved. Another limitation of the existing literature is that only one of the two strongly correlated outcomes of BMI and BMD has typically been considered. Bone loss is frequently concurrent with weight loss [14]. Given the current urgent focus on obesity, it is important to know whether losing weight is necessarily associated with concurrent bone loss, and whether factors such as PA might modify the risk for bone loss with needed weight loss.

We used data from the Canadian Multicentre Osteoporosis Study (CaMos), a population-based cohort study in nine urban Canadian centers, to explore both the cross-sectional and longitudinal associations between PA and BMD, with consideration of BMI as an important mediating factor.

Section snippets

Methods

The recruitment, methods and study design for the Canadian Multicentre Osteoporosis Study (CaMos) have been previously published [15], with further information available at www.camos.org. Briefly, CaMos is an ongoing, prospective, age- and sex-stratified population-based cohort study of women and men enrolled from 1995 to 97, who were at least 25 years old, community-dwelling, and living within a 50 km radius of nine Canadian sites. Households were randomly selected from a list of residential

Results

The characteristics of the CaMos cohort according to sex and overall level of PA are shown in Table 1. Comparison of those with PA above the median vs. those with PA below the median in both men and women showed that most covariates were still quite similar in the two groups with some bivariate associations reaching the nominal level of statistical significance (shown in bold). On average, over all ages, both men and women reported similar total PA, with a mean of 2500 (95% CI: 2484, 2516) MET*m

Discussion

In this population-based five-year prospective study of adult Canadian men and women we found that changes in physical activity (PA) were associated with increases in BMD and decreases in BMI, whereas there was virtually no association between estimated mean levels of PA and changes in either bone or weight parameters. Moreover, an increase in PA was associated with a modest un-coupling of the strong associations between BMD and BMI, so that the increased activity was related to increased total

Acknowledgments

We thank all those participants in CaMos whose careful responses and attendance made this analysis possible. The Canadian Multicentre Osteoporosis Study is funded by the Canadian Institutes of Health Research (CIHR). Over some of its duration, CaMos has also received arms length support from Merck Frosst Canada Ltd., Eli Lilly Canada Inc., Novartis Pharmaceuticals Inc., The Alliance for Better Bone Health: Sanofi-Aventis, Procter & Gamble Pharmaceuticals Canada Inc., Amgen, The Dairy Farmers of

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