Our cross-sectional survey found a significant positive correlation between AC and total femoral BMD among American adults. Subgroup analysis and interaction assessment showed that this negative correlation was consistent in all subgroups except age and BMI, including gender, race, physical activities and diabetes status subgroups. It is worth noting that an inverse L-shaped relationship between AC and total femur BMD is found, with a turning point at 35.4cm.
Previous studies have explored the association between AC and BMD[16; 17; 18; 19]. A noteworthy study conducted by Azizeh et al. investigated this correlation in postmenopausal Iranian women and revealed a positive correlation between AC and BMD[16]. Similarly, Zhang et al. conducted a study involving 48 HIV-positive children and adolescents of both genders from southern Brazil. Their findings indicated a significant correlation between AC and not only bone mineral content (BMC) but also BMD[17]. In Japanese women, Tsutomu et al. found that AC displayed a positive correlation with lumbar spine BMD in postmenopausal women[18]. Lin et al. demonstrated that in 48 chronic peritoneal dialysis patients, AC were positive correlation with BMD[19]. In summary, these studies had limitations, such as a selected population, small sample size, or adjusted variables; however, this study avoided these drawbacks. First of all, this study used nationally representative samples from the NHANES database, which resulted in a large sample size. Secondly, since previous studies usually considered the relationship between AC and BMD in women, especially postmenopausal females, this study also considered the potential impact of gender, age, race, BMD, diabetes and physical activity. Thirdly, this study adjusted for more variables that may affect bone density. As expected, this study not only demonstrated the correlation between AC and bone density, but also found an inverted L-shaped relationship between AC and total femur BMD, with a turning point at 35.4cm
The current studies not only reveal a significant association between AC and BMD, but also hold potential therapeutic implications that can greatly benefit doctors. This positive correlation indicates that individuals with a higher AC may also have a higher BMD. As a result, when developing strategies for preventing osteoporosis and obesity, it is crucial for clinical guideline developers to take into account the positive effects of AC on BMD and the advantageous impact of increased physical activity. To ensure the accuracy of our results, we conducted subgroup analysis during our research, thus providing a more precise representation of the dataset. Our findings suggest that regardless of gender, race, physical activity level, or diabetes status, a higher AC is consistently linked to elevated total femur BMD.
In this analysis, it is found that this positive connection changed by BMI and age. Such individuals with ages ≥ 65years old and non-obese, should be vigilant. On the contrary, the lack of gender difference may result from the selection of adults over 50 years old, rather than including all groups, which may have selective bias. Obese individuals with a BMI > 28 kg/m2 have a weak correlation due to increased AC accompanied by excessive fat accumulation leading to bone loss [20].
Our research results also indicate that through further threshold effect analysis, there is an inverted L-shaped relationship between AC and total femur BMD. When AC is less than 35.4cm, there is a positive correlation between AC and total femur BMD. When AC is greater than 35.4cm, the recorded effect size is not significant. Which may mean that AC has a dose-dependent effect on total femur BMD. Normal or moderately high levels of AC are beneficial for bones, while very high levels of AC may not be related to bone health
There are several factors that can explain why subjects with higher AC have higher bone density. First of all, subjects with higher AC have a higher body size and greater mechanical load on bone tissue. At the same time, greater mechanical load can activate bone cells, reduce Bone resorption, and promote bone formation[21]. In addition, AC is also a reflection of the overall nutritional status of human body. Malnutrition can result in bone metabolism disorders and affect bone remodeling [22]. For people with low AC, it is necessary to restore normal weight to improve the low peak bone mass and accelerate bone loss. In addition, individuals with larger AC leads to increased insulin resistance, insulin resistance promotes insulin secretion, and hyperinsulinemia results in increased bone density. Insulin can promote the proliferation of osteoblasts, inhibit the activity of osteoclast, and act as a bone anabolic agent[23]. Finally, it is reported that AC not only reflects the size of regional muscles, but also reflects regional muscle strength[24], which is an important determinant of regional and overall bone density[25].
The advantage of this study lies in the application of large sample analysis from NHANES surveys, with highly reliable and standardized data that can represent American adults. In addition, we stratified the analysis according to gender, age, race, diabetes status, BMI and physical activity, making the results more detailed and reliable. Finally, our research results indicate that there is a positive correlation between AC and total femoral BMD with a point of inflection at 35.4cm. The inverted L-shaped relationship between AC and total femur BMD has not been revealed in previous studies. It is undeniable that our research has the following limitations. First of all, this is a cross-sectional study, so causal relationships cannot be inferred. Further longitudinal study needs strong evidence to solve the causal relationship of these relationships. Secondly, we may not have adjusted for variables that may affect the results, such as calcium intake and dietary intake. Finally, due to the COVID-19 pandemic in 2019, data collection for the NHANES 2019–2020 cycle has not yet been completed