Concerns about the rising prevalence of overweight or obese populations have focused on the well-documented relationships between obesity and increased chronic metabolic abnormalities1. Given that bone and adipose tissue both originate from the same progenitor mesenchymal cells and function as endocrine organs that may produce hormones, these two tissues serve as a good starting point for research the effect of obesity on bone metabolism2. For many years it was thought obesity, evaluated by Body Mass Index(BMI), was a protective factor for bone mineral density(BMD) at different skeletal sites, attributing to the mechanical effect of body weight on bone and the action of estrogens 3, 4, 5. However, a recent study in Korea involving more than 3000 participants observed that the prevalence of osteoporosis in women with waist circumference (WC) obesity (>80cm) was higher than in women with BMI obesity(>25kg/m2)6. Similarly, several studies reported that visceral adipose tissue(VAT), a regional fat distribution index, was a negative determining factor of bone density and bone strength independently of total body fat in obesity7, 8, 9. Another Chinese study showed in both men and women, fat mass, lean mass, and BMI were positively connected with total BMD, but subcutaneous fat was negatively associated with BMD in post-menopausal women10. The current evidence also showed that increased weight or BMI translates to higher BMD but the parallel increase in adiposity affects bone-regulating hormones that can reduce bone quality9. Thus, the outcomes of bone health related to obesity vary depending on where the excess adipose tissue is distributed in the body. Neck circumference (NC), a surrogate of upper-body adipose tissue distribution, has been used as an index for the adverse risk profile of cardiovascular disease, biochemical components of metabolic syndrome, and bone biomarkers11,12. However, the evidence for the associations of NC with BMD in the Chinese population is not known. It has been proved that obesity complications are more associated with regional body fat distribution, ectopic fat deposition, and cellular infiltration, rather than absolute quantity13. So is the relationship between NC and BMD different from generalized obesity or abdominal obesity (BMI, WC) concerning bone health?

Although bone health could be judged with BMD using dual-energy X-ray absorptiometry(DEXA) as normal bone mass, osteopenia, and osteoporosis, this approach is unsuitable for routine clinical screening in a general population on account of the radiation exposure and high costs. It is critical to find convenient and practicable indicators to predict bone mineral loss. In terms of fat distribution indicators, as an alternative to WC, recent studies have suggested the use of NC, as a simpler, more reliable, and more practical parameter, which is unaffected by being full or hungry, or by respiratory movements and provides more consistent results to indicate upper-body fat accumulation14,15. Thus, this study aimed to evaluate the association between NC and BMD and to determine whether NC alone can predict low bone mass change in Chinese participants.