Critical Factors for Bone Health in Women Across the Age Span: How Important Is Muscle Mass?

Jasminka Ilich-Ernst, RD, MS, PhD, Rhonda A. Brownbill, MS, RD, Martha A. Ludemann, MS, RD, Rongwei Fu, PhD

Disclosures

Medscape General Medicine. 2002;4(2) 

In This Article

Abstract and Introduction

Objectives/Design: This cross-sectional study of 113 healthy white women, 20-88 years of age, evaluated relationships between bone mineral density (BMD), body composition, calcium (Ca) intake, and physical activity. The analysis was performed in the entire cohort and in groups divided by reproductive/menopausal status (premenopausal, perimenopausal, early postmenopausal, and late postmenopausal).
Methods: BMD and body composition were measured with Lunar DPX-MD densitometer using specialized software for total body, spine, femur, and forearm. Ca intake from food and supplements was assessed by a food frequency questionnaire. Past physical activity and past and present walking were assessed only in the older cohort using modified version of the Allied Dunbar National Fitness Survey for Older Adults.
Results: The results showed significant reduction of both total body BMD and lean body mass (LBM) of 13% and 12%, respectively, with age. LBM was the strongest determinant of BMD in various skeletal sites in the entire cohort and groups. Ca was positively associated with BMD of various regions of hip in the entire cohort and in the youngest and oldest subjects (r ranging from 0.32-0.56, P < .05, in simple regression), but not in perimenopausal and early postmenopausal women. Past activity (sports and recreation) was positively associated with BMD in total body, spine, hip, and forearm (r ranging from 0.26-0.37, P < .05). Various modes of present walking were positively associated with BMD in regions of femur and forearm.
Conclusions: These results reveal the importance of lean tissue acting independently on bone at different skeletal sites in women across age groups as well as the positive effects on BMD of Ca in the youngest and oldest women and life-long engagement in physical activity in older women.

Osteoporosis, a chronic, debilitating condition affecting close to 30 million people, is clinically diagnosed by measuring areal bone mineral density (BMD, g/cm2). This areal BMD is also a parameter most often used to assess bone status and risk for development of osteoporotic fractures. BMD is influenced by many biological and lifestyle factors. One factor that researchers unequivocally agree on is the positive influence of body weight on bone across the age groups. Because body weight includes lean and fat tissue, it is not always clear which of these components has a more dominant role on bone and under what circumstances.

The theoretical explanation of both lean and fat tissue independently influencing bone has a physiologic foundation. Lean tissue exerts osteogenic effects by providing mechanical stress and stimulating the skeleton, whereas fat tissue provides additional estradiol because it is a site for the conversion of androgens to estrogens. In addition, bone tissue itself contributes to body weight, and the colinearity between bone and weight should not be neglected in statistical analyses. Because body weight is such a complex entity, it is important to distinguish between its different components and their possible independent influence on bone at different stages of women's life.

The positive relationship between lean tissue and BMD in both premenopausal and postmenopausal women has been observed by several authors.[1,2,3,4] In those studies, the influence of lean tissue dominated over fat. In addition, some studies have shown a significant relationship between muscle strength and bone mass and suggested a relationship between the loss of muscle and the loss of bone with age.[5] Conversely, some studies have shown a higher influence of fat tissue on bone in postmenopausal women.[6,7] The conflicting findings in evaluations of the influence of these variables on bone, even when the same measurement-sites and instrumentation are used, may be due to the intercorrelation among them, as well as their colinearity with body weight and height. Failing to account for these interrelationships may result in associations that reach significance only because of the association between independent variables.

The influence of body composition as a component of weight on bone should be evaluated in concert with the overall, or at least some aspect of, physical activity exerted by subjects as it may affect both muscle and fat tissue. Current evidence supports the notion that resistance exercise helps the maintenance and attainment of bone mass in men and women across the age span, but particularly in the younger population. However, the effect of less strenuous everyday activities (as part of a regular lifestyle) on bone mass and the effect of past activity on current bone status, especially in older individuals, are not well established.

A few studies investigating the independent association of past and/or present physical activity with bone mass, and including lean and/or fat tissue, have led to inconsistent results.[1,7,8,9] Some studies suggest walking and stair climbing are appropriate recommendations for maintaining bone mass in elderly persons,[10,11] while others recommend more vigorous forms of activity, eg, volley ball.[12] Even less is known about the effect of past physical activity on the current bone mineral status in elderly.[13] Obviously, when evaluating factors affecting bone mass, calcium (Ca) intake should be taken into account. However, the majority of studies assessing the relationship between physical activity and bone mass in women have not considered Ca intake.

The purpose of this study was to evaluate the relationship between bone mineral status and body composition, habitual Ca intake, and physical activity in women across the age span. Specifically, we examined the following: (1) the relationship between BMD of different skeletal sites and height, body weight, lean/fat tissue, and Ca intake in healthy premenopausal and postmenopausal women; (2) past involvement in physical activity and past walking (from age 18) in relation to current BMD and lean/fat tissue in postmenopausal women; and (3) whether moderate-intensity current physical activity (such as different modes of walking) is beneficial to BMD and lean tissue in postmenopausal women.

The study is distinct because of the comprehensive approach in evaluating the BMD of various skeletal sites, the precise identification of explanatory variables and analysis of their simultaneous effect on bone, and the assessment of past and present physical activity encompassing common activities of everyday living and their association with BMD.

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