In the present study, we observed that relatively large neck circumference was associated with the prevalence and incidence of NAFLD among postmenopausal women with normal BMI. Higher neck fat accumulation was also associated with metabolic dysregulation and inflammation characterized by insulin resistance, elevated triglycerides, and CRP. Collectively, these findings indicate the importance of excessive neck fat accumulation to predict the development of NAFLD among postmenopausal women with normal BMI.
Epidemiological studies have shown that sex and menopause affect the prevalence and incidence of NAFLD . In women, the relations between visceral fat accumulation and metabolic profile were greater than in men . Furthermore, as age increases and estrogen levels decrease, women after menopause are prone to insulin resistance, hyperlipidemia, and visceral fat accumulation [16, 17], all of which are known risk factors for NAFLD. The robust protective role of estrogens was revealed in epidemiological studies, where the prevalence of NAFLD is lower in premenopausal women compared to men or postmenopausal women . As the main circulating estrogen, 17β-estradiol has effects beyond reproductive health. In insulin-sensitive tissues like skeletal muscle, 17β-estradiol receptor α has a positive effect on insulin signaling and glucose transporter-4 expression , which indicates that 17β-estradiol may directly regulate insulin action. Meanwhile, another study showed that exposure to 17β-estradiol restores insulin sensitivity and glucose tolerance in high fat diet-fed ovariectomized mice . In addition to insulin sensitivity, estrogens can also affect adipose tissue metabolism, energy expenditure, and hepatic glucogenesis . Collectively, the protective effect of estrogen on NAFLD is a combination of multiple factors and its underlying mechanism remains to be further studied.
Previous researches reported that higher BMI, waist circumference, HOMA-IR, elevated triglyceride levels, and reduced HDL-cholesterol levels were associated with NAFLD among non-obese individuals . In our study, neck circumference was significantly correlated with BMI, waist circumference, HOMA-IR, and triglycerides, and negatively correlated with HDL-C, consistent with previous studies [7, 23]. Moreover, the positive association between neck circumference and incidence of NAFLD remained consistent across a wide range of BMI, waist circumference, and HOMA-IR among postmenopausal women with normal weight. Additionally, previous cross-sectional studies have shown that neck circumference was an independent predictor for NAFLD in the general population  and non-obese men . Our strong evidence indicating the relation of neck circumference and incidence of NAFLD even in postmenopausal women with normal BMI may validate their results and provide valuable clues for further studies.
It is well established that obesity is a heterogeneous disorder. BMI is a common screening measure to identify subjects with abnormal body fat distribution. However, BMI cannot provide accurate information about the regional distribution of body fat. Moreover, subjects within the same BMI could have considerable differences in the amount and distribution of regional fat accumulation. Previous studies have demonstrated that differences in body fat distribution result in specific metabolic complications [26, 27]. Thus, neck circumference, a proxy of neck subcutaneous fat, began to show diagnostic value for evaluating metabolic disturbances.
As an alternative measurement of upper-body subcutaneous fat, neck circumference is a great indicator of ectopic fat distribution. Subcutaneous fat in the upper body accounts for a much greater proportion of systemic free fatty acids release and is more lipolytically active than lower body adipose tissue . Large neck circumference means excessive accumulation of subcutaneous fat in neck, which contributes a greater flux of the free fatty acid released into the circulation. Subsequently, elevated free fatty acid contributes to increased synthesis and ectopic deposition of triglycerides, insulin resistance, and inflammation . In addition, increased free fatty acids involved in impaired glucose homeostasis by inhibiting glucose uptake, oxidation, glycogen synthesis, and increasing output hepatic glucose . Concomitantly, excessive free fatty acids could trigger oxidative stress, an early instigator of NAFLD , and endoplasmic reticulum stress which intersects with various inflammatory and stress signaling pathways through unfolded protein response . The excessive free fatty acids release derived from neck subcutaneous adipose might be a potential link between neck circumference and NAFLD.
In our study, neck circumference was positively correlated with C-reactive protein, and negatively correlated with adiponectin. As we best known, the abnormal accumulation of fat is associated with adipose tissue metabolic capacities, endocrine, and immune function, which along with altered lipid mediators, adipokines, pro- or anti-inflammatory cytokines, and impaired signaling pathways that involved in metabolic abnormalities . In addition to being a depot of fat, the adipose tissue is also a highly active endocrine organ, secreting various biologically active molecules, collectively termed adipokines . When adipose tissue expands, the capacity of adipocytes to act as endocrine cells and secrete a variety of adipokines is altered in subjects with NAFLD . These altered kinds and levels of adipokine are associated with dysregulation of triglyceride, fatty acids metabolism, and insulin resistance . Moreover, due to excessive fat accumulation and substantial infiltration of immune cells, a specific crown-like disposition of macrophages around single necrotic adipocytes occurs in subjects with NAFLD . Subsequently, proinflammatory pathways were activated, and a variety of proinflammatory cytokines and chemokines were overflowed that contribute to low-grade inflammation and insulin resistance . In general, adipose dysfunctions, inflammation, and stress partly linking neck obesity to insulin resistance and NAFLD.
Several potential limitations of the current study should also be noted. First, we did not quantify the tissue composition of the neck. The use of neck circumference to assess the neck subcutaneous fat was a convenient and practical way but was unable to quantify the fat accumulation and muscle mass. Hence, the amount and size of subcutaneous adipocyte and muscle fat are unclear. Second, due to ultrasonographic examination was performed to determine the presence of NAFLD, the sensitivity of liver ultrasonography may vary depending on the hepatic fat content. Nevertheless, when performed properly, ultrasonography has been reported to detect as little as ≥ 5% hepatic fat content. Although as discussed above, liver ultrasonography offers several strengths including the non-invasive nature of the test, portability, low cost, and simplicity of use, make it further applicable and acceptable in large-scale epidemiological studies, particularly in developing countries. Third, given the diagnosis of NAFLD was based on ultrasound imaging, NAFLD patients in our study were in at least a moderate stage of the disease. Therefore, in the present study, we were unable to determine the relationship between neck circumference and mild-stage NAFLD.