Obesity has long been considered to be “protective” against osteoporosis, largely due to mechanical loading and adipocyte-derived hormones. More recent evidence implicates the accumulation of abdominal and visceral fat with bone loss in middle-aged men. Obesity dysregulates the GH/IGF-1 and HPG axes, both of which are important regulators of bone homeostasis. Testosterone, which promotes bone mineral density (BMD) and muscle mass, is often reduced in obese men, whereas estrogen levels increase proportionally to body weight.
A recent study sought to determine hormonal and body composition predictors of bone microarchitecture in young and healthy men with abdominal obesity. The authors hypothesized that obese men with significant abdominal adiposity would have impaired bone microarchitecture and strength and increased bone marrow fat associated with dysregulation of the GH/IGF-1 and gonadal steroid axes. This cross-sectional study evaluated 35 men mean age 33.8 years and mean body mass index 36.5 kg/m2.
A high degree of visceral adipose tissue was found to be inversely associated with mechanical properties of bone, whereas bone marrow fat was inversely associated with cortical parameters. Free estradiol was positively correlated with total bone density and free testosterone was positively associated with trabecular thickness yet inversely associated with trabecular number.
The authors concluded that visceral adipose tissue and bone marrow fat are negative predictors and muscle mass is a positive predictor of bone microarchitecture and mechanical properties in obese men. Testosterone, estradiol, and GH are positive determinants of trabecular microarchitecture, and IGF-I is a positive determinant of cortical microarchitecture. This supports the notion that VAT is detrimental to bone and that decreased GH and testosterone, characteristic of male obesity, may exert deleterious effects on microarchitecture, whereas higher estradiol may be protective. Limitations of this study include a small number of test subjects, cross-sectional study design (limits ability to prove causality), and since the study was limited to obese men, the resultant data cannot be extrapolated to normal-weight men or women. Larger studies across male and female populations are necessary to draw more substantial conclusions regarding these relationships.
Reference: Bredella MA, Lin E, Gerweck, AV, et al. Determinants of bone microarchitecture and mechanical properties in obese men. J Clin Endo Metab 2012;97(11):4115-4122.