In this study, we investigated the association of general and central obesity with bone strength and compared their association with vertebral fractures. We found that obesity indices were positively associated with bone density, but negatively associated with TBS. Among the different combinations of general and central obesity, general obesity measured using body fat percentage without central obesity was not associated with vertebral fractures. Furthermore, central obesity, assessed by waist circumference and waist-hip ratio, was still associated with vertebral fractures, irrespective of general obesity. Therefore, central obesity was more strongly associated with vertebral fractures than general obesity.

Previous studies have shown that general and central obesity indices were positively associated with spinal BMD[[4]and that the correlation coefficient of fat mass was higher than that of other obesity indices, similar to our results. Additionally, we found that ALM was positively associated with BMD and that ALM contributed to BMD in both men and women[[4, 16]. Therefore, maintaining ALM is important for bone health. The National Institutes of Health defines bone strength as the integration of bone density and quality[[17]. TBS is a commonly used non-invasive method for evaluating bone quality. Our results demonstrated that general and central obesity indices were negatively associated with the TBS. In the literature, increased soft tissue over the region of interest (lumbar spine) resulted in differences in the spinal BMD and TBS. Although BMD was affected, it was unlikely to lead to a clinical problem because the change did not exceed the least significant change[[18]. In contrast, increased soft tissue thickness resulted in a lower TBS value[[18, 19]. The correlation with waist circumference was therefore stronger in our study, which was consistent with the results of a previous study[[20]. Shevroja et al. developed a soft tissue-adjusting technique for computing the TBS, and showed that obesity indices were positively correlated with the TBS, indicating better bone quality[[19]. However, accumulating evidence supports the association between obesity and compromised bone quality[[21]. Cohen et al. also reported that central obesity was associated with inferior bone quality, as measured by trans-iliac crest bone biopsy[[22]. Further studies are required to determine which TBS algorithm is the most suitable for evaluating obesity.

Accumulating evidence has challenged the protective effects of obesity against fractures, and the association between obesity and fractures appears to be site-dependent. However, studies investigating the association between general obesity and vertebral fractures have yielded conflicting results. Prieto-Alhambra et al. reported that general obesity, measured using BMI, was not associated with vertebral fractures[[7], whereas Liu et al. found that general obesity, assessed using BMI, was associated with an increased risk of vertebral fractures[[8]. Additionally, different diagnostic measures for assessing obesity may affect its association with fractures. Gandham et al. reported that general obesity, based on body fat percentage, was associated with fractures, whereas obesity measured using BMI was not;[[23]this finding was consistent with our results.

Several studies have investigated the association between general obesity and fractures. However, few have examined the association between central obesity and vertebral fractures. In the Nurses’ Health Study, women aged 30–55 years with central obesity (measured using waist circumference ≥ 108 cm) showed an increased risk of vertebral fractures[[24]. Another prospective study in the South Korean population aged ≥ 40 years reported that central obesity (assessed using waist circumference: men ≥ 90 cm and women ≥ 85 cm) was positively associated with the risk of vertebral fracture[[9]. Similarly, in a prospective study with subjects aged > 50 years in China, central obesity (based on waist circumference: men > 90 cm, women > 80 cm; waist-hip ratio: men > 0.9, women > 0.85) was associated with a higher risk of subsequent vertebral fractures[[25]. In our study, postmenopausal women with both general and central obesity showed the highest likelihood of vertebral fractures and central obesity (measured using waist circumference ≥ 80 cm and waist-hip ratio ≥ 0.85) showed a stronger association with vertebral fractures than general obesity.

Several studies have proposed explanations for the association between central obesity and fractures. First, individuals with obesity, particularly central obesity, may have a higher risk of falling[[26]. Second, central obesity may place a higher burden on the spine and increase impact force despite the protective effect of soft tissue pads[[27, 28]. Third, visceral fat adversely affected bone quality by altering bone-regulating hormone levels, oxidative stress, and inflammation[[21, 22]. Fourth, the distribution of body fat, especially in central obesity, can potentially have different effects on the bone[[2], and compared to general obesity, central obesity may increase the risk of vertebral fracture. The relationship between obesity and fractures is a complex and multifaceted issue, and further studies are required to clarify the underlying mechanisms.

This study had the strength in using various common diagnostic measures for assessing general and central obesity that can be applied in clinical practice. To the best of our knowledge, this study is the first to compare the association of different combinations of general and central obesity indices with vertebral fractures. However, our study had several limitations. First, this was a cross-sectional study; therefore, the causal inferences between obesity and vertebral fractures could not be made. Second, the participants in our study were limited to the Asian population, and obesity was defined by Asian criteria, which may have affected the generalizability of our results. Third, the association between obesity and fractures is site-specific. In our study, we focused only on vertebral fractures, and further investigation of different fracture sites is required to reveal additional relationships. Fourth, in this study, vertebral compression fractures were confirmed using radiological reports. While X-rays are not sensitive enough to detect occult fractures, they are the most accessible tool for assessing compression fractures, especially when compared to CT or MRI, which are less readily available due to higher costs and limited accessibility. Fifth, our study only included postmenopausal women. Therefore, our results may not apply to men or premenopausal women. Further studies involving diverse populations are needed to confirm our findings.