The association between fat and lean mass and bone mineral density: The Healthy Twin Study

Abstract

The potential beneficial effects of increased body weight on bone mineral density (BMD) conflict with the adverse effects of obesity on various health outcomes, necessitating more specific evaluations of the association between each body component and BMD. In the present study, we evaluated associations of lean mass (LM) and fat mass (FM) with BMD in a Korean sample consisting of a total of 1782 men and women whose mean (standard deviation) age was 43.2 (12.6) years. They were selected from the Healthy Twin Study, a nationwide Korean twin and family study. BMD, FM and LM were measured using dual-energy X-ray absorptiometry. Quantitative genetic analysis and linear mixed analysis were performed with respect to familial relationships and a wide range of probable covariates. Linear mixed analysis revealed that BMD was positively associated with both FM and LM at each region of BMD measurement (whole body, spine, arms, and legs) in men, premenopausal women, and postmenopausal women. However, the association with BMD was stronger for LM than FM. Both LM and FM had positive genetic correlations with BMD at each region, although the correlation with BMD tended to be stronger for LM than FM. Together, these findings suggest that increased LM, rather than FM, is more beneficial for BMD in the Korean population and warrants further study of the common genetic determinants of BMD and body composition.

Introduction

Bone mineral density (BMD) is a major determinant of the risk of bone fracture, which causes serious morbidity and mortality in the elderly [1]. Body weight has been proposed to be one of the important predictors of BMD [2]. Likewise, greater body weight increases mechanical stress on the skeleton, which stimulates osteogenesis through bone remodeling and leads to an increase in BMD [3].

However, despite the beneficial effects of increased weight on BMD, overweight status increases the risk of type 2 diabetes, cardiovascular diseases, liver disease, and certain cancers of the colon, breast, corpus uteri, and gallbladder [4], [5]. In this regard, it is necessary to further disentangle the relationship between body weight and BMD by evaluating the respective effects of each main component of body weight – lean mass (LM) and fat mass (FM) – on BMD, as the hazardous health effects of obesity are driven mainly by fat tissue [4].

Studies of the relationship between body composition and BMD consistently demonstrate a positive relationship between LM and BMD [6], [7], [8], [9], [10], [11], [12], [13], [14], whereas FM exhibits a range of positive [11], [12], [13], [15], null [7], [8], or inverse [10], [14], [16], [17] associations with BMD. In addition, some studies report the relative importance of LM compared to FM on BMD [6], [9], [11], [12], while other studies report contradictory results [14].

In order to give better insights into their relative contributions, an examination of the shared genetic contributions between components of body composition and BMD should be useful [18], [19], especially given the fact that LM [20], [21], [22], FM [20], [22], and BMD [11], [21], [22] are strongly influenced by genetic makeup. Presently, genetic correlations between body composition and BMD have been evaluated in studies of twins [11], [22] or families [10], [16], and several common genomic regions associated with FM and BMD [18] or LM and BMD [19] have been identified. However, the aforementioned studies were conducted in Western populations whose genetic background as well as lifestyle and prevalence of osteoporotic fracture might be materially different from those of Asian populations [23].

In the current study, we evaluated the associations of FM and LM with BMD using the dataset from the Healthy Twin Study. We evaluated the associations of these factors among men, premenopausal women, and postmenopausal women. Specifically, we examined the association between body composition and BMD with consideration of menopausal status in women because both BMD and body composition change after menopause [24], [25]. Investigating families from the general population irrespective of health status, as well as evaluating various family relationships, enabled us to explore possible shared genetic effects influencing both BMD and body composition.