Associations between body composition and bone density and structure in men and women across the adult age spectrum

Abstract

Background/purpose

The objective of this study was to identify independent associations between body composition and bone outcomes, including cortical structure and cortical and trabecular volumetric bone mineral density (vBMD) across the adult age spectrum.

Methods

This cross-sectional study evaluated over 400 healthy adults (48% male, 44% black race), ages 21–78 years. Multivariable linear regression models evaluated associations between whole-body DXA measures of lean body mass index (LBMI) and fat mass index (FMI) and tibia peripheral quantitative CT (pQCT) measures of cortical section modulus, cortical and trabecular vBMD and muscle density (as a measure of intramuscular fat), adjusted for age, sex, and race. All associations reported below were statistically significant (p < 0.05).

Results

Older age and female sex were associated with lower LBMI and muscle strength. Black race was associated with greater LBMI but lower muscle density. Greater FMI was associated with lower muscle density. Cortical section modulus was positively associated with LBMI and muscle strength and negatively associated with FMI. Adjustment for body composition eliminated the greater section modulus observed in black participants and attenuated the lower section modulus in females. Greater LBMI was associated with lower cortical BMD and greater trabecular BMD. FMI was not associated with either BMD outcome. Greater muscle density was associated with greater trabecular and cortical BMD. Associations between body composition and bone outcomes did not vary by sex (no significant tests for interaction).

Conclusions

These data highlight age-, sex- and race-specific differences in body composition, muscle strength and muscle density, and demonstrate discrete associations with bone density and structure. These data also show that age-, sex- and race-related patterns of bone density and strength are independent of differences in body composition. Longitudinal studies are needed to examine the temporal relations between changes in bone and body composition.

Introduction

Body composition varies according to age, sex, and racial background, and contributes to the normal variability in bone structure and bone mineral density (BMD) among adults. Aging is associated with loss of muscle mass and quality, and age-related muscle loss is accompanied by fat gain in older adults [1]. It is well established that low body mass index (BMI) is a risk factor for osteoporosis and fracture [2]; however, studies of the independent contributions of lean mass and fat mass to bone health have yielded conflicting results. For example, an early dual energy X-ray absorptiometry (DXA) report from the Health, Aging, and Body Composition study (Health-ABC) of men and women, ages 70 to 79 years, concluded that lean mass and fat mass were both positively associated with BMD, however, the associations varied according to sex, measurement site, and the index used to adjust for bone size [3]. A subsequent Women's Health Initiative study reported that femur BMD and cross-sectional area (CSA) were greater in women with higher BMI and values scaled in proportion to lean mass but not fat or total body mass [4]. More recent studies suggested differential effects of subcutaneous versus visceral fat mass. The majority of studies employing computed tomography (CT) measures of fat distribution demonstrated that visceral adipose tissue was negatively associated with BMD and bone structure [5], [6], [7], [8].

The “functional muscle–bone unit” approach posits that bone adapts to the mechanical forces to which it is subjected in order to keep the bone strength at a constant set point [9]. A recent peripheral quantitative computed tomography (pQCT) study in men ages ≥ 65 years, enrolled in the Osteoporotic Fractures in Men Study (MrOS) demonstrated that leg power and physical activity were positively associated with bone size and estimates of bone compressive strength [10]. Muscle metabolism may also affect bone health; fatty infiltration of muscle was associated with a higher risk of fracture in Health-ABC participants, independent of BMD, muscle CSA, and muscle strength [11], [12]. These data support the concept that differences in lean mass and muscle quality between subjects of varying age, sex, race, and total fat mass play a critical role in determining epidemiologic associations between these variables and bone outcomes, including fractures.

Prior studies of the functional muscle–bone unit and the impact of adiposity on BMD and bone structure were largely limited to elderly or adolescent participants, were frequently restricted to males or females, and usually did not examine race differences. A recent study demonstrated positive associations between skeletal muscle mass and bone density and structure at multiple skeletal sites; however, the cohort was > 96% white, and the study did not include measures of adiposity [13]. To our knowledge, no prior studies examined measures of volumetric BMD, cortical structure, body composition, muscle strength, and muscle density across the age range from young adults to the elderly in a multiethnic sample.

This cross-sectional study in 500 adults, ages 21 to 78 years included DXA measures of whole body and regional lean and fat mass, tibia pQCT measures of muscle area, muscle density, trabecular and cortical volumetric BMD and cortical structure, and dynamometric measures of isometric muscle strength. The objectives were to (1) determine the effects of age, sex, and race on lean body mass, muscle strength, and muscle density; (2) determine the effects of age, sex and race on trabecular and cortical BMD, cortical section modulus (a summary measure of cortical dimensions); and (3) examine associations between body composition and bone outcomes.