In this study, BMI and body weight are related to BMD of the lumbar spine and hip, and postmenopausal women with higher BMI and body weight have higher BMD values. Adipocytes are known to be an important source of estrogen production in postmenopausal women[13, 14]. Estrogen inhibits bone reabsorption by osteoclasts[15, 16]. As BMI increases in postmenopausal women[17], increased adipose tissue may lead to increased estrogen production and osteoclast inhibition, leading to increased bone mass[18-20]. Obesity is associated with insulin resistance[21], which is characterized by various abnormalities that may result from high plasma insulin levels[22], including androgen and estrogen overproduction in the ovaries and decreased hepatic production of sex hormone-binding globulin[23]. These changes may lead to increased levels of sex hormones, resulting in decreased osteoclast activity, which leads to increased bone mass and possibly increased osteoblast activity[24]. Therefore, BMD of obese people is higher than that of people with normal weight, reducing the risk of OP and fragility fractures in obese people[25].
A prospective study found that early postmenopausal women with lower BMI lost more bone mass than those with higher BMI[26]. Similar studies have reported consistent findings that lower BMI, body weight is associated with lower BMD[27]. There is also a study showing that obesity has no protective effect on fractures in postmenopausal women and is associated with an increased risk of ankle and thigh fractures[28]. In another study, further multivariate analysis in subjects stratified by body weight confirmed the inverse relationship between bone mass and fat mass after controlling for the mechanical loading effect due to total body weight, indicating that increasing fat mass may not have a beneficial effect on bone mass[29, 30]. The differences between these results may be related to the study design, population as well as sampling methods, which should be further studied and discussed in future research.
The results of our study have important clinical significance. Firstly, it can provide a reference for follow-up research to further study and explore the relationship between BMI, body weight and BMD. Secondly, it can provide a reference for the need for OP prevention in natural postmenopausal women with relatively low BMI and weight.
Although this study's findings could explain the relationship between BMI, weight, and BMD, it has some limitations. A potential source of bias in this study is residual confounding due to risk factors that we could not account for in our analyses , such as socioeconomic status, education level, physical activity level, smoking, alcohol consumption, vitamin D status, sex hormone levels, and nutritional status. To obtain more accurate results, the participants in our study were randomly selected, which should ensure its external validity, and the DXA measured BMD was accurate and reliable, which ensures the internal validity of the study, so selection bias is unlikely to affect the associations between BMI, body weight, and BMD investigated in this study. The generalizability of our findings to clinical practice in the general population may be affected by participant inclusion criteria. Another limitation to our study is that, participants were naturally menopausal women aged 45 to 72, and the results may not apply to broader age groups and to perimenopausal and premenopausal women. Another limitation is that because of the limited number of participants, the small sample size, and the lack of detailed BMI classification, the results cannot definitively explain the relationship between BMI and BMD. Finally, only women were included in the study, and the relationship between BMI, weight, and BMD may be different in men can be further explored in future studies.