In the present study, we investigated the effect of duration of lactation on bone mineral density in postmenopausal women. We found that there was a correlation between the total duration of the lactation period and spine BMD and femur BMD. After dividing the study population into four groups according to breast-feeding duration, the group having the longest lactation period had the lowest spine BMD, spine t score, femur BMD, and femur t score. Additionally, in the groups with lower BMD, age, duration of menopause, age of menarche, and parity were higher but BMI was lower. In multivariate logistic regression analysis, we found that predictors of low femur BMD were age, BMI, age of menarche, and duration of menopause, whereas only BMI was an independent predictor of low spine BMD. Moreover, duration of breast-feeding was not found to be an independent risk factor for low femur BMD or low spine BMD.
The major reproductive events in a woman’s life involve complex hormonal changes. The fluctuations in concentrations of endocrine hormones, especially oestrogen, that occur with pregnancy, breast-feeding and menopause also act as potent influences on Ca metabolism and bone metabolism [9
]. However, the results that have been published from studies on reproductive factors such as parity, age at menarche, time since menopause, reproductive period, and BMD are extremely controversial, with some demonstrating a beneficial effect, while others show a detrimental impact of these factors on bone mass [22
]. In the current study, we have found an independent association between femur BMD and age of menarche and duration of menopause. However, there was not an independent correlation between spine BMD and age of menarche, duration of menopause, parity or the duration of fertility.
Trabecular bone (lumbar spine, os calcis) has been shown to be more sensitive to metabolic changes compared with cortical bone (femoral neck, distal radius) [29
]. Furthermore, there are differences in the timing of bone loss in healthy women, with trabecular bone diminishing with every decade of life but cortical bone levels being similar in the third, fourth, and fifth decades [30
]. As there are essential differences in response to environmental (including hormonal and nutritional) factors between the cortical (at the femoral neck) and trabecular (lumbar spine) part of the bone structure, similar to our study, it would seem more logical that measurements of both cortical and trabecular bone are necessary.
Similar to our study, Sowers et al.
investigated the relation of breast-feeding time and BMD in postpartum women, dividing them into three groups according to their breast-feeding time [31
]. They found that women who breast-fed for at least 6 months had significantly different levels of three bone turnover markers and by 18 months of observation there was no difference in the mean values among the three lactation groups. As a result of their study, they showed that differences in the bone-turnover markers in the lactation period were associated with breast-feeding time and menstrual re-activation, rather than age, physical activity, dietary calcium intake or body size. The complete recovery of spine BMD takes 12 months postpartum, and may extend to 18 months for the loss of femur neck BMD. In a large study, Hadji et al.
investigated the effects of reproductive factors on BMD parameters measured by quantitative ultrasonometry in 2080 postmenopausal women [16
]. They compared women who had never breast-fed with women who had breast-fed and found no difference according to factors affecting BMD such as age, weight, BMI, and time since menopause. Similar to our results, they reported that breast-feeding time had no significant effect on BMD in postmenopausal women. Conversely, in 1486 postmenopausal women, Dursun et al.
demonstrated negative correlations between both the lumbar spine and femoral neck BMD values and breast-feeding time [14
]. Additionally, they found increased frequency of osteoporosis in women with longer duration of total breast-feeding. As a result, they emphasized that total duration of breast-feeding might be an independent risk factor for osteoporosis in postmenopausal Turkish women. However, in their study, when comparing with the group having a shorter breast-feeding time, there were significant differences in factors such as age, time since menopause, and BMI which affect BMD in groups having a longer breast-feeding time. Nevertheless, they did not perform any methods of regression analysis including these factors for detecting independent predictors. On the other hand, we have determined inverse correlations between breast-feeding time and spine and femur neck BMD in bivariate correlation analysis. Moreover, we have shown that the number of subjects with LBM was higher in the group with the longest duration of breast-feeding. Although our results are mainly in agreement with the findings of Dursun et al.
, unlike them, we have found that there was no relation between the bone loss and time of breast-feeding in logistic regression analysis. Consequently, this result obtained from regression analysis has confirmed that there is a complex role of numerous factors which affect BMD apart from feeding time.
These conflicting results in previously reported studies might be explained at least in part by different patient selections (such as age, ethnicity, sample size, inclusion of patients with comorbidities), study design, statistical analyses, sites of bone measured, and different diagnostic methods or criteria. In the present study, we followed strict rules in the selection of the study population and in excluding patients with confounding factors such as HRT use, cancer, pulmonary diseases, and coronary artery disease especially affecting bone mass. Additionally, although bone mass was determined by impractical and/or expensive methods in some studies [16
], we used measurements of bone mass obtained from DXA which are considered the best non-invasive methods of determining density [32
Our analysis was restricted to postmenopausal women aged 40 years and older; therefore, our findings are not generalizable to premenopausal women. In addition, because of the cross-sectional design of the study, we are not sure whether low BMD occurred before the menopause. Another limitation of this study is the lack of measurement of vitamin D levels. Therefore, prospective designed studies with a larger sample size including the measurement of biochemical indices of bone mineral metabolism are needed to evaluate in more detail the relationship between total breast-feeding time and BMD.
In conclusion, the present study is the second to evaluate the association between total breast-feeding time and bone mineral density measured by DXA in a large representative sample of postmenopausal Turkish women. Consequently, we have shown that women with a longer breast-feeding time had significantly lower bone mass of femur or spine. However, breast-feeding time was not found to be an independent predictor of low bone mass. This may result from complex interactions between other factors affecting BMD rather than solely the effect of feeding time.