The bone-fat mass relationship: Laboratory studies

Sarcopenia negatively affects hip structure analysis variables in a group of Lebanese postmenopausal women

BACKGROUND

The current study's purpose is to compare hip structural analysis variables in a group of postmenopausal women with sarcopenia and another group of postmenopausal women with normal skeletal muscle mass index. To do so, the current study included 8 postmenopausal women (whose ages ranged between 65 and 84 years) with sarcopenia and 60 age-matched controls (with normal skeletal muscle mass index (SMI)). Body composition and bone parameters were evaluated by dual-energy X-ray absorptiometry (DXA).

RESULTS

Weight, lean mass, body mass index, femoral neck cross-sectional area (FN CSA), FN section modulus (Z), FN cross sectional moment of inertia (CSMI), intertrochanteric (IT) CSA, IT Z, IT CSMI, IT cortical thickness (CT), femoral shaft (FS) CSA, FS Z and FS CSMI were significantly greater (p < 0.05) in women with normal SMI compared to women with sarcopenia. In the whole population, SMI was positively associated with IT CSA, IT Z, IT CSMI, IT CT, FS CSA, FS Z, FS CSMI, FS CT but negatively correlated to IT buckling ratio (BR) and FS BR.

CONCLUSION

The current study suggests that sarcopenia has a negative effect on hip bone strength indices in postmenopausal women.

Osteopetrosis in obese female rats is site-specifically inhibited by physical training

NEW FINDINGS

What is the central question of this study? Clinical studies suggest that obesity 'protects' against osteoporosis. However, these studies used only bone densitometry and assessed only one bone site, which is insufficient to enable conclusions to be drawn about the response of the whole skeleton. Furthermore, the effects of exercise on bone responses in obesity have not been explored previously. What is the main finding and what is its importance? We show that obesity causes osteopetrosis. Therefore, the classical perspective of 'protective effects of obesity' needs to be reviewed, and exercise is an important tool to avoid these alterations and to maintain the homeostasis of bone. A sedentary lifestyle and obesity induce systemic inflammatory responses. Although the effects of physical inactivity on osseous tissue have been well established, the effects of obesity on bone tissue remain controversial. Furthermore, the effects of physical training on bone tissue responses in the presence of diet-induced obesity are unknown. Our aim was to investigate the effects of obesity and physical training at multiple bone sites in rats. Female Wistar rats were divided into the following four groups: (i) control diet, non-trained (C-NT); (ii) high-refined carbohydrate-containing diet, non-trained (HC-NT); (iii) control diet, trained (C-T); and (iv) high-refined carbohydrate-containing diet, trained (HC-T). At 5 months of age, the rats were submitted to daily exercise for 30 min day(-1). After 13 weeks, blood samples, adipose and skeletal tissues were harvested. Two-way ANOVA was applied to detect differences (significance accepted when P ≤ 0.05). The HC-NT group exhibited increased body mass, adiposity, serum leptin, serum insulin, insulin resistance index and concentrations of tumour necrosis factor-α and interleukin-6. Obese rats (HC-NT) exhibited thickening of nasal bones, trabecular bones in the lumbar vertebrae and long bones in a site-dependent manner. The HC-T group exhibited similar adiposity and inflammatory results. Morphological analysis of the lumbar vertebrae in rats fed the HC diet revealed characteristics of osteopetrosis that were inhibited by exercise. In conclusion, the HC diet induced obesity and inflammatory/hormonal alterations and increased the trabecular bone in a site-dependent manner. However, obesity caused osteopetrosis in the lumbar vertebrae, which could be inhibited by physical training. Although exercise inhibited the development of bone alterations, physical training did not inhibit the HC diet-induced obesity responses.

Associations among endocrine, inflammatory, and bone markers, body composition and weight loss induced bone loss

INTRODUCTION

Weight loss reduces co-morbidities of obesity, but decreases bone mass.

PURPOSE

Our aims were to (1) determine if adequate dairy intake attenuates weight loss-induced bone loss; (2) evaluate the associations of endocrine, inflammatory and bone markers, anthropometric and other parameters to bone mineral density and content (BMD, BMC) pre- and post-weight loss; and (3) model the contribution of these variables to post weight-loss BMD and BMC.

METHODS

Overweight/obese women (BMI: 28-37 kg/m2) were enrolled in an energy reduced (-500 kcal/d; -2092 kJ/d) diet with adequate dairy (AD: 3-4 servings/d; n=25, 32.2±8.8 years) or low dairy (LD: ≤1 serving/d; n=26, 31.7±8.4 years). BMD, BMC and body composition were measured by DXA. Bone markers (CTX, PYD, BAP, OC), endocrine (PTH, vitamin D, leptin, adiponectin, ghrelin, amylin, insulin, GLP-1, PAI-1, HOMA) and inflammatory markers (CRP, IL1-β, IL-6, IL-8, TNF-α, cortisol) were measured in serum or plasma. PA was assessed by accelerometry.

RESULTS

Following weight loss, AD intake resulted in significantly greater (p=0.004) lumbar spine BMD and serum osteocalcin (p=0.004) concentration compared to LD. Pre- and post-body fat was negatively associated with hip and lumbar spine BMC (r=-0.28, p=0.04 to -0.45, p=0.001). Of note were the significant negative associations among bone markers and IL-1β, TNFα and CRP ranging from r = -0.29 (p=0.04) to r = -0.34 (p=0.01); magnitude of associations did not change with weight loss. Adiponectin was negatively related to change in osteocalcin. Factor analysis resulted in 8 pre- and post-weight loss factors. Pre-weight loss factors accounted for 13.7% of the total variance in pre-weight loss hip BMD; post-weight loss factors explained 19.6% of the total variance in post-weight loss hip BMD. None of the factors contributed to the variance in lumbar spine BMD.

CONCLUSION

AD during weight loss resulted in higher lumbar spine BMD and osteocalcin compared to LD. Significant negative associations were observed between bone and inflammatory markers suggesting that inflammation suppresses bone metabolism. Using factor analysis, 19.6% of total variance in post-weight loss hip BMD could be explained by endocrine, immune, and anthropometric variables, but not lumbar spine BMD.

Cytokines and Hormones That Contribute to the Positive Association between Fat and Bone

The positive association between body weight and bone density has been established in numerous laboratory and clinical studies. Apart from the direct effect of soft tissue mass on bone through skeletal loading, a number of cytokines and hormones contribute to the positive association between adipose and bone tissue, acting either locally in sites where cells of the two tissues are adjacent to each other or systemically through the circulation. The current review describes the effects of such local and systemic factors on bone physiology. One class of factors are the adipocyte-secreted peptides (adipokines), which affect bone turnover through a combination of direct effects in bone cells and indirect mechanisms mediated by the central nervous system. Another source of hormones that contribute to the coupling between fat and bone tissue are beta cells of the pancreas. Insulin, amylin, and preptin are co-secreted from pancreatic beta cells in response to increased glucose levels after feeding, and are also found in high circulating levels in obesity. A number of peptide hormones secreted from the gastrointestinal tract in response to feeding affect both fat and bone cells and thus can also act as mediators of the association between the two tissues. The current review focuses on results of laboratory studies investigating possible mechanism involved in the positive association between fat mass and bone mass.

Influence of age and gender on associations of body mass index with bone mineral density, bone turnover markers and circulating calcium-regulating and bone-active sex hormones

Although it is well known that body mass index (BMI) and bone mineral density (BMD) are positively correlated, the mechanisms by which adiposity reduces the risk of osteoporotic fractures are not fully understood. The present study was initiated to gain deeper insight into the mechanisms underlying the osteoprotective effect of adiposity, and to assess particularly the relevance that BMI-associated changes in circulating hormone levels could have for the build-up of additional bone mineral density. Using data from a previous study on a large cohort of healthy adult Austrians, we analyzed correlations of BMI with (i) BMD at sites in the lumbar spine and hip region, (ii) bone resorption and formation markers, (iii) circulating levels of vitamin D, parathyroid hormone, testosterone and estrogen, and (iv) rates of daily vitamin D and calcium intake. After adjustment for age, positive correlations between BMI and BMD were highly significant (P<0.0001) at all skeletal sites across the entire study cohort. Associations were stronger in post-menopausal women than in pre-menopausal women and in men. In absolute values, the gain in BMD at the lumbar spine from an incremental rise of BMI in post-menopausal women was 1.5-fold higher than in pre-menopausal women, and three times of that observed in men (P<0.05). Inverse relations between BMI and β-crosslaps were consistently found in men (P<0.01) and in women before and after menopause (P<0.01 and P<0.05, respectively), suggesting that inhibition of osteoclastic bone resorption is responsible at least in part for the positive effect of high BMI on BMD. Sub-group analysis revealed that increasing BMI was associated with a significant fall of testosterone in men (P<0.05), and of 25-(OH)D in pre- and post-menopausal women (P<0.001 and P<0.05, respectively), but with a significant rise in PTH (P<0.01) in women before menopause. Since all these hormonal changes would cause bone loss, this excludes their playing any role in the osteoprotective effect of adiposity.