Osteoporosis is a major public health problem in developed countries. There are many factors that affect BMD. The relative contribution of LM and FM to BMD remains a contentious issue. Some studies have found a positive association between LM and BMD, and others have shown that FM is an important determinant of BMD 8,11. Distinguishing the role of LM versus FM as a determinant of BMD has clinical relevance; however, determination of the independent effects of LM and FM on BMD is not straightforward because the two measurements are correlated, and depending on the magnitude of correlation, when they are considered in a multiple linear regression model, it is sometimes impossible to differentiate one effect from another. It is well known that obese women have elevated BMD 12,13. This may be partially attributable to the effect of increased circulating estrogen levels derived from extraglandular secretion of androgen in adipose tissue and the increased weight-bearing effect of FM on BMD. Still other studies have found that both FM and LM are significant predictors of bone density, with LM being a more important predictor than FM in premenopausal women and FM being more important than LM in postmenopausal women 14,15. The association between BMD and LM suggests that an increase in physical activity may directly translate into protection against osteoporosis, whereas an association between BMD and FM implies that obesity may have a protective effect against bone loss. Factors that affect BMD do not always generally influence BMD throughout all stages of the life cycle. Some factors have a strong impact on BMD during a certain period of the life cycle and then have a reduced impact on BMD at other stages of the life cycle.
Our study showed that the menopausal state had a strong negative influence on total and regional BMD among Egyptian obese women (Table 1). Total BMD correlated significantly with total LM in premenopausal women and total and regional BMD correlated significantly with the total lean and trunk FM ratio in postmenopausal women (Tables 2 and 3).
Some studies have reported a positive correlation between FM and others have reported a correlation with fat-free mass. We examined the differential effects of FM and LM on BMD. Our results showed that both LM and FM were positively associated with BMD in postmenopausal women, which is consistent with previous studies 4,16,17. Our results disagree with other studies that reported that only lean body mass was correlated with BMD on multiple-regression analysis in postmenopausal women 18,19. Our results showed that body FM was best correlated with BMD in postmenopausal women, in agreement with previous reports 8,11,12. It was initially reported that FM was a major determinant of BMD 9. It is speculated that the loose relationship between lean body mass and BMD of the vertical axis may be partially attributable to the decreased physical activity in postmenopausal women. It appears that lean body mass does not become a significant determinant of BMD in a sedentary life style.
On the basis of these results, we conclude that lean body mass is a significant determinant of BMD in premenopausal women, whereas both LM and body FM are significant determinants in postmenopausal women. Our observation is in agreement with the previous reports showing that body FM has no direct influence on BMD in normal premenopausal women 20,21. However, our observation is not in agreement with the other studies that have reported that body FM does not play a major role in preventing bone loss 9,22. Some previous studies did not differentiate postmenopausal women from premenopausal women in their analysis. DeLaet et al.23, in a meta-analysis of 12 multinational cohorts including nearly 60 000 adult participants, showed that independent of sex, those with BMI more than 25 kg/m2 had significantly lower rates of hip, osteoporotic, and all fractures. Moreover, some studies have reported that femur BMD and geometric strength are greater with overweight in postmenopausal women, but they vary in proportion to LM (mostly muscle) and not to body weight or FM 24. Dytfeld et al. 25 have reported that both FM and LM are determinants of BMD in postmenopausal osteoporotic women, with site-specific differences. It appears that LM and FM have an alternative impact on BMD in premenopausal and postmenopausal women. The impact of lean body mass on BMD of the vertical axis attenuated after menopause. In summary, LM exerts positive effects on BMD in premenopausal obese women, and both FM and LM might contribute to BMD in postmenopausal obese women. The mechanism underlying this relationship is an important question to be addressed in bone biology.
Thus, it remains to be elucidated whether these differences are attributable to aging, menopause, and/or both. Further study is needed to examine this issue.
There are no conflicts of interest.
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