| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Departments of Orthopedic Surgery and Basic Medical Science (L.-J.Z., Y.-J.L., J.H., H.-W.D.), School of Medicine, University of Missouri-Kansas City, Kansas City, Missouri 64108; Osteoporosis Research Center (L.-J.Z., P.-Y.L., R.R.R.), Creighton University Medical Center, Omaha, Nebraska 68131; Laboratory of Molecular and Statistical Genetics (H.-W.D.), College of Life Sciences, Hunan Normal University, Changsha, Hunan 410081, Peoples Republic of China; and The Key Laboratory of Biomedical Information Engineering of Ministry of Education and Institute of Molecular Genetics (H.-W.D.), School of Life Science and Technology, Xian Jiaotong University, Xian 710049, Peoples Republic of China
Address all correspondence and requests for reprints to: Hong-Wen Deng, Ph.D., Department of Basic Medical Science, School of Medicine, University of Missouri-Kansas City, 2411 Holmes Street, Room M3-CO3, Kansas City, Missouri 64108. E-mail: dengh{at}umkc.edu.
| Abstract |
|---|
|
|
|---|
Objective: The objective of this study was to reevaluate the relationship between obesity and osteoporosis by accounting for the mechanical loading effects of total body weight on bone mass.
Methods: We measured whole body fat mass, lean mass, percentage fat mass, body mass index, and bone mass in two large samples of different ethnicity: 1988 unrelated Chinese subjects and 4489 Caucasian subjects from 512 pedigrees. We first evaluated the Pearson correlations among different phenotypes. We then dissected the phenotypic correlations into genetic and environmental components with bone mass unadjusted or adjusted for body weight. This allowed us to compare the results with and without controlling for mechanical loading effects of body weight on bone mass.
Results: In both Chinese and Caucasian subjects, when the mechanical loading effect of body weight on bone mass was adjusted for, the phenotypic correlation (including its genetic and environmental components) between fat mass (or percentage fat mass) and bone mass was negative. Further multivariate analyses in subjects stratified by body weight confirmed the inverse relationship between bone mass and fat mass, after mechanical loading effects due to total body weight were controlled.
Conclusions: Increasing fat mass may not have a beneficial effect on bone mass.
| Introduction |
|---|
|
|
|---|
Extensive epidemiological data show that high body weight or BMI is correlated with high bone mass and that reductions in body weight may cause bone loss (2, 3, 4). The basic mechanism underlying this observed correlation remains unclear, although several explanations have been proposed. It is generally accepted that a larger body mass imposes a greater mechanical loading on bone and that bone mass increases to accommodate the greater load. Furthermore, adipocytes are important sources of estrogen production in postmenopausal women, and estrogen is known to inhibit bone resorption by osteoclasts. It has been proposed that increases in adipose tissue with increasing BMI in postmenopausal women result in increased estrogen production, osteoclast suppression, and a resultant increase in bone mass (5). Finally, obesity has been associated with insulin resistance characterized by high plasma levels of insulin. High plasma insulin levels may contribute to a variety of abnormalities, including androgen and estrogen overproduction in the ovary and reduced production of SHBG by the liver. These changes may result in elevated sex hormone levels leading to increased bone mass due to reduced osteoclast activity and possibly increased osteoblast activity (6).
Epidemiological correlation between obesity and bone mass may be explained, in part, by the mechanisms presented previously, but further analysis reveals a much more complex relationship (7). For example, leptin, an adipocyte-secreted peptide that regulates appetite and energy expenditures, is found to have complex effects on bone. A recent study reported that leptin-deficient and leptin receptordeficient mice had increased bone formation and that intracerebroventricular infusion of leptin causes bone loss in leptin-deficient and wild-type mice (8). Based on these observations, it was proposed that leptin may inhibit bone formation (8). In contrast to these findings, however, systemic administration of leptin to leptin-deficient mice and wild-type mice results in increased bone growth, increased skeletal mass, and increased skeletal strength (6).
Earlier epidemiological studies investigating the relationship between obesity and osteoporosis centered on phenotypic correlations between body weight (or BMI) and bone mass and produced the generally accepted view that increased mechanical loading, associated with increased body weight, contributes to increases in bone mass (9). One potential problem with these phenotypic studies is that the correlation between body weight (or BMI) and bone mass may not necessarily represent a correlation between obesity per se and osteoporosis, because it is excessive fat mass rather than total body weight that defines obesity. Body weight is a heterogeneous phenotype consisting of fat, lean muscle, and bone mass. Fat mass accounts for approximately 16 and 25% of total body weight in normal-weight men and women, respectively; the majority of the remaining body composition is lean mass (10). Although several previous studies (6) have used fat mass to assess the correlation between obesity and bone mass, they generally did not adjust for the mechanical loading effects of body weight on bone mass. Thus, conclusions from these studies about the relationship between obesity and bone mass may be confounded by the mechanical loading effects of total body weight on the skeletal system. Therefore, a critical question arises: what is the correlation between obesity per se and osteoporosis? To investigate the relationship between obesity and bone mass fully, it is necessary to control for the mechanical loading effects of total body weight in the analyses.
In the present study, using both Chinese and Caucasian populations, we found that body fat mass is negatively correlated with bone mass when the mechanical loading effect of body weight is statistically removed. Our results have important clinical implications because they suggest that interventions or treatments reducing obesity may increase bone mass and thus protect against osteoporosis.
| Subjects and Methods |
|---|
|
|
|---|
The study population consisted of two samples. The first sample was composed of 1988 healthy unrelated Chinese (Han) subjects [878 premenopausal women and 1110 men; age, 27.2 ± 4.5 yr (mean ± SD); range, 19.645.1 yr] who were recruited in the Peoples Republic of China. The second sample came from the United States and consisted of 512 pedigrees with 4489 Caucasian subjects [2667 females and 1822 males; age, 47.8 ± 16.2 yr (mean ± SD); range, 19.190.1 yr]. All the subjects signed informed consent documents before entering the studies. We adopted an exclusion criterion elaborated elsewhere (11). In brief, subjects with diseases, treatments, or conditions that would be apparent and nongenetic causes for abnormal bone mass or fat mass were excluded.
Measurement
BMD (grams per square centimeter) at the lumbar spine (LS) and femoral neck (FN), total body bone mineral content (TB BMC), fat mass, and lean mass (both in kilograms) were measured by Hologic DXA scanners (Hologic Corp., Waltham, MA). PFM is the ratio of fat mass divided by total body weight (i.e. the sum of fat mass, lean mass, and bone mass). For the LS BMD, the quantitative phenotype used here was combined BMD of L1L4. Weight was measured in light indoor clothing using a calibrated balance beam scale, and height was measured using a calibrated stadiometer. BMI was calculated as weight (in kilograms) divided by height (in meters) squared.
In this study, all the subjects completed a nurse-administered risk factor questionnaire to assess information concerning smoking, physical activity, menopausal status, etc. The ages of starting and stopping smoking and the average packs of cigarettes smoked per day were recorded. Study subjects were classified as smokers and nonsmokers (defined as those who never smoked or had stopped smoking for at least 5 yr), with smokers numerically coded as 1 and nonsmokers as 0. For physical activity, the number of episodes of exercise per week was recorded. The regular exercisers were defined as those who exercised at least once per week and were coded as 1; nonexercisers were coded as 0 in the data analyses. Menopause status was coded as 0 and 1 for premenopause and postmenopause, respectively, and was coded as missing for males.
Statistical analyses
For the Caucasian sample, we first identified 1085 unrelated subjects [age, 62.13 ± 10.84 yr (mean ± SD)] from the 512 Caucasian pedigrees by selecting the founders and married-in subjects. Many factors such as age, sex, height, menopause status, exercise, and smoking may have significant effects on both obesity-related phenotypes and bone mass. Their significance was tested, and those significant factors were modeled as covariates in regression models. The selection of covariates was based on whether the variable was statistically significant at the significance level of P < 0.05. Because of the mechanical loading effect of body weight on bone mass, the true relationship between bone mass and fat mass may be confounded. To address this issue, in our regression model, we incorporated total body weight as a covariate to adjust for bone mass. By adopting this model, the mechanical loading effect of body weight on bone mass is eliminated, and the relationship between bone mass and fat mass investigated here is not confounded. In genetic analyses, adjustment for significant covariates can generally increase the genetic signal-to-noise ratio by decreasing the proportion of the residual phenotypic variation attributable to random environmental factors. In our analyses, BMD and obesity-related phenotypes were adjusted for significant covariates such as age, sex, height, menopause status, exercise, and smoking in regression models. The adjusted values were used for subsequent Pearson correlation analysis, and the residuals from the model were used in the data analyses. These analyses were conducted separately in Chinese samples and unrelated Caucasian samples selected from our Caucasian pedigrees.
We further dissected phenotypic correlations into genetic and environmental components by performing quantitative genetic variance decomposition analyses in the whole Caucasian sample (4489 subjects) using the program SOLAR (www.sfbr.org/solar/). The bivariate quantitative genetic analysis is a powerful method to assess directly the shared genetic and environmental effects by measuring the degree of genetic and environmental correlations between pairs of traits. It reestimated and decomposed the total phenotypic correlation
P into the components due to genetic correlations (
G) and environmental correlations (
E) (12) that are shared among bone mass, lean mass, and fat mass. The signs of
G and
E indicate the directions of action of shared genetic and environmental effects on osteoporosis and obesity. To estimate the genetic correlations
G and environmental correlations
E for pairs of traits, one should first model the bivariate phenotype of an individual as a linear function of the individuals trait measurements. Then the matrix of kinship coefficients is generated, expressing relationships among all pairs of individuals in the pedigree. From standard quantitative genetic theories, the phenotypic variance-covariance matrix and its genetic and environmental components are then obtained. From these matrices,
G and
E are estimated directly, which was detailed elsewhere (13). We tested the significance of both
G and
E between any pair of traits by comparing the likelihood for the restricted model in which each of these parameters was constrained to zero to the likelihood for the general model in which all parameters were estimated. In this study, the significant level refers to P
0.05 in a statistical test. In the statistical analyses, outliers that were 4 SD away from the respective averages were excluded. Natural log transformation was performed for variables that did not follow the normal distribution.
We further investigated the relationship between PFM and BMC in subgroups of 10-kg strata of body weight for the total 4489 Caucasian subjects. A linear mixed model was used with age, height, smoking, exercise, and menopause status modeled as covariates. Family relationships were modeled in the mixed model as random effects. Least squares mean of the TB BMC stratified by PFM in subgroups of 10-kg strata of body weight in the whole Caucasian sample was plotted. In this study, only strata with 100 or more persons were included for each gender to achieve maximum statistical power.
| Results |
|---|
|
|
|---|
|
|
|
G) and environmental (
E) correlations between lean mass and weight-adjusted bone mass were significantly positive (P < 0.01). In contrast to these findings, fat mass and PFM were negatively associated with weight-adjusted bone mass both genetically and environmentally (P < 0.01). This is consistent with the results of our Pearson correlation (phenotypic correlation) analyses presented in Table 3
|
|
| Discussion |
|---|
|
|
|---|
The negative genetic correlation
G between fat mass and weight-adjusted bone mass suggests that fat and bone mass share some common genetic factors and molecular pathways that appear to have opposite effects on fat vs. bone mass. This finding is consistent with our current understanding of the differentiation of adipose tissue and the skeleton. Adipocytes and osteoblasts originate from a common progenitor, pluripotential mesenchymal stromal cells, and their differentiation is regulated through the peroxisome proliferator-activated receptor (PPAR)-
pathway (18). Activation of PPAR-
drives the differentiation of mesenchymal stromal cells toward adipocytes over osteoblasts (19). A set of factors common to osteogenesis and adipogenesis determines the entry of mesenchymal stromal cells into different functional stages (20). For example, bone morphogenetic protein and retinoic acid may cooperate to induce osteoblast differentiation of preadipocytes (21). Molecular genetic studies have also identified some candidate genes that have common effects on osteoporosis and obesity. Such genes include IGF-I, IGF-II, LEPR, NPY, VDR, ER-
, AR, TGF-ß1, IL-6, TNF-
, TNFR2, ApoE, and PPAR-
. For instance, the Pro10 allele in the TGF-ß1 gene was found to reduce the risk of both obesity (22) and osteoporosis (23).
Our current study also found a negative environmental correlation between fat mass and weight-adjusted bone mass. This implies that fat and bone mass share some environmental factors, which may alleviate the risk of both obesity and osteoporosis. Several lines of evidence support our observations here. For instance, physical exercise may prevent body fat accumulation while increasing bone mass (24). Milk and tea are believed to be beneficial for the prevention of both osteoporosis and obesity (25). Milk is a good source of highly absorbable calcium, and increased milk intake may increase peak bone mass in puberty and slow bone loss and reduce the incidence of osteoporotic fracture in the elderly (26). Studies also show that high calcium intake may promote weight or fat loss (27), although long-term trials are needed to confirm such observations.
Our results may have important clinical implications. First, a person may gain bone mass without having to gain fat mass. Second, medical interventions or lifestyle modifications may be favorable for the protection of both osteoporosis and obesity. Indeed, a few studies have suggested such effects. For instance, menopause is associated with increased bone loss, increased fat mass, and decreased lean mass. Hormone replacement therapy has been proven to be efficient in attenuating bone loss in postmenopausal women (28) and reversing menopause-related obesity and loss of lean mass (29). Leptin, a key factor regulating appetite and body weight, has been shown to influence bone mass (30). Treatment of children with congenital leptin deficiency with recombinant leptin leads to increased bone mass (31) and a sustained reduction in weight, predominantly as a result of a loss of fat (30, 31). However, Ducy et al. (8) and Takeda et al. (32) reported that leptin may decrease bone formation through the sympathetic nervous system, indicating that the effect of leptin on bone mass is complex (33). Other medical interventions have also been shown to have adverse effects on health, leading to both osteoporosis and obesity. For example, osteoporosis and obesity are the two main side effects of treatment with GnRH agonists, agents that are used for treating nonmetastatic prostate cancer (34).
We acknowledge that our study is cross-sectional in nature instead of a longitudinal design. Therefore, in this study, the relationship between bone mass and obesity-related phenotypes is descriptive and might be confounded by cohort effects. However, the age range of our Chinese sample is narrow, which may suggest that cohort effects, if any, may be relatively small. Future longitudinal studies will be helpful in clarifying the relationship regarding changes of bone mass and fat mass.
It should also be noted that the statistically significant correlations reported here only imply an overall effect of the shared factors in determination of fat and bone mass. Some individual factors may not follow this correlation. For example, smoking is associated with low bone mass and accelerated bone loss (35), but it may also reduce BMI (36).
An additional concern with this study is that the observed negative correlation between fat mass and weight-adjusted bone mass might be an artifact caused by dual energy x-ray absorptiometry (DXA) measurement. It is likely that heterogeneous distributions of soft tissues could lead to systematic inaccuracies inherent to DXA-derived BMD measurements. Changes of fat distribution can cause alterations in bone measurement without any real change in the skeleton (37, 38). However, in this study, our results are not likely to be biased for the following reasons:
1) A previous analytic and quantitative simulation study indicated that decreasing fat mass by weight change always artificially led to lower BMD, and vice versa (39), which is qualitatively different from our results.
2) Prior studies (37, 38) showed that increasing fat thickness may spuriously decrease total body BMD. The spurious decrease in BMD, if due to DXA measurement, is attributable to the potential spurious increases in both BMC (numerator) and bone area (denominator for BMD) (37, 38). Bone area may have a relatively larger spurious increase than BMC, resulting in a potential spurious decrease of BMD (37, 38). However, in this study, we found a negative, rather than a positive, correlation between fat mass and weight-adjusted TB BMC, which indicates that increasing fat mass is associated with a smaller BMC. This result suggests that our finding is unlikely to be explained by an artifact of the DXA measurement, which leads to larger BMC with increasing fat. Moreover, we tested the relationship between fat mass and bone area in our two large samples. We found that fat mass was negatively correlated with weight-adjusted bone area (Table 5
). This finding is qualitatively different from the positive correlation between spurious change of bone area and change of fat thickness due to DXA measurements as suggested in some earlier studies (38). This result further ensured the robustness of our results against the potential artificial effects of DXA measurement.
|
| Acknowledgments |
|---|
| Footnotes |
|---|
Disclosure Statement: L.-J.Z., Y.-J.L., P.-Y.L., J.H., and H.-W.D. have nothing to declare. R.R.R. consults for Merck, Lilly, Wyeth, Procter & Gamble Pharmaceuticals, Amgen, Roche, GlaxoSmithKline, Novartis, NPS Pharmaceuticals, and Allelix.
First Published Online February 13, 2007
Abbreviations: BMD, Bone mineral density; BMI, body mass index; DXA, dual x-ray absorptiometry; FN, femoral neck; LS, lumbar spine; PFM, percentage fat mass; PPAR, peroxisome proliferator-activated receptor; TB BMC, total body bone mineral content.
Received March 14, 2006.
Accepted February 1, 2007.
| References |
|---|
|
|
|---|
insufficiency enhances osteogenesis through osteoblast formation from bone marrow progenitors. J Clin Invest 113:846855[CrossRef][Medline]This article has been cited by other articles:
![]() |
M Bustamante, X Nogues, L Mellibovsky, L Agueda, S Jurado, E Caceres, J Blanch, R Carreras, A Diez-Perez, D Grinberg, et al. Polymorphisms in the interleukin-6 receptor gene are associated with bone mineral density and body mass index in Spanish postmenopausal women Eur. J. Endocrinol., November 1, 2007; 157(5): 677 - 684. [Abstract] [Full Text] [PDF] |
||||
![]() |
Weight Loss May Benefit Bone Density DOC News, July 1, 2007; 4(7): 18 - 19. [Full Text] |
||||
![]() |
Z.-H. Tang, P. Xiao, S.-F. Lei, F.-Y. Deng, L.-J. Zhao, H.-Y. Deng, L.-J. Tan, H. Shen, D.-H. Xiong, R. R. Recker, et al. A Bivariate Whole-Genome Linkage Scan Suggests Several Shared Genomic Regions for Obesity and Osteoporosis J. Clin. Endocrinol. Metab., July 1, 2007; 92(7): 2751 - 2757. [Abstract] [Full Text] [PDF] |
||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Endocrinology | Endocrine Reviews | J. Clin. End. & Metab. |
| Molecular Endocrinology | Recent Prog. Horm. Res. | All Endocrine Journals |