Most of previous studies have focused on the association between betatrophin and obesity or BMI. In the past two years, researchers have begun to turn their attention to the relationship between betatrophin and body fat distribution. Kriebel et al. sampled visceral and subcutaneous fat from patients with or without hepatic steatosis to detect betatrophin mRNA expression, with the resluts that betatrophin mRNA levels were higher in the VAT than the SAT in both groups . Another study found that circulating betatrophin levels have an inverse relationship with SAT expression in lean and obese patients with and without T2DM, suggesting that the local effect of betatrophin on adipose tissue is independent of obesity . These two studies also confirmed that betatrophin is not specifically secreted by liver tissues, as previously reported by Zhang , but is also secreted by visceral and subcutaneous fat. In this study, we used a noninvasive approach to explore the relationship between betatrophin and body fat distribution in patients with different glucose tolerance status. We interestingly found that betatrophin levels correlated positively with VAT/SAT ratio which is consistent with the results of Kriebel , and negatively with lower body adiposity including lower limb and gluteal fat in NGT subjects, but not IGT subjects, indicating that betatrophin levels could be closely associated with body fat distribution in NGT subjects.
Which mechanism plays a role in the emergence of this result? As is well known, the contribution of various adipose tissue deposits varies for the risk of metabolic disease . Visceral adiposity is regarded as be more closed with metabolic diseases such as hypertension, diabetes, and dyslipidemia compared to other tissue fat deposits . Thus, the VAT/SAT ratio which is a metric of relative body fat composition has been proposed to be an independent predictor of death and coronary events . In addition, visceral obesity has also been defined as a predictor of nonalcoholic fatty liver disease (NAFLD) . Unlike visceral adiposity, lower body subcutaneous adiposity which accumulates in the thighs and hips is thought to be metabolically protective. Many studies reported that the accumulation and infiltration of macrophages in adipose tissue to active inflammation could be the underlying mechanism. Obesity-associated adipose tissue inflammation varies between individuals, possibly due to depot-specific differences. In mice and humans, VAT contains a higher percent of proinflammatory M1 macrophages and CD4 Th1 T-cells than in SAT. Pinnick et al. provided an evidence to support an increased macrophage presence in abdominal SAT, whereas no corresponding enrichment was observed in gluteal SAT. Ejarque et al. reported that macrophages can express and secrete ANGPTL8 in their preliminary experiments. Therefore, we speculate that the higher the VAT or VAT/SAT, the more macrophages would accumulate and infiltrate in the adipose tissue, and the more betatrophin would be secreted. The opposite trend could be observed when lower limb fat increased. The speculation needs to be verified in further study.
Betatrophin has recently emerged as an indicator of metabolic disorders, with two separate case-control studies finding that betatrophin levels are elevated in subjects with. metabolic syndrome and hypertension . A Chinese study of non-diabetic individuals found that circulating full-length betatrophin levels are an independent risk factor for coronary artery disease (CAD) and are positively associated with its severity . The homogeneity in the correlation between VAT/SAT ratio, betatrophin, and metabolic disorders may be due to their close association, as observed in this study.
It is therefore reasonable to speculate that the association between abnormal body fat distribution and cardiovascular and metabolic diseases may be partially mediated by betatrophin. Indeed, a growing body of evidence has suggested that body fat distribution is closely related to the inflammatory state of the body . Moreover, VAT accumulation and a higher VAT/SAT ratio may also be associated with increased chronic low-grade systemic inflammation, which could further increase betatrophin synthesis . Correspondingly, elevated betatrophin levels may contribute toward the pathogenesis of dyslipidemia, which is one of the most important risk factors for CAD, while in vitro and in vivo studies have suggested that betatrophin could aggravate hypertriglyceridemia by promoting the ability of ANGPTL3 to bind and inhibit lipoprotein lipase (LPL) . Clinical studies have also confirmed that betatrophin levels are significantly and positively related to TG and LDL-C levels and inversely related to HDL-C levels in children and patients with diabetes  . In addition, betatrophin is positively correlated with age , liver fat content , and blood pressure , all of which are independent risk factors for atherosclerosis and may contribute to the occurrence and development of CAD.
In addition, our findings may explain why previous clinical studies have yielded inconsistent and even opposite betatrophin levels in obese or overweight people . This is likely due to the different baseline characteristics of the populations recruited in these clinical trials, including age, sex, lifestyle, genetics, and gene-environment interactions that can influence body fat distribution . In this study, we also found that correlations between betatrophin and body fat distribution indices only existed in the NGT group, not the IGT group. We speculate that the different glucose metabolic states of the patients could affect their inflammatory state, since increased inflammatory cytokine levels affect betatrophin synthesis  , accordingly the relationship between betatrophin and body fat distribution cannot be observed. It should be noted that the hypothesis requires further elucidation. In addition, other factors such as the presence of cardiovascular disease, hypertension, and dyslipidemia, which are common in IGT group, could affect the expression of this protein, as demonstrated in previous studies . Previous animal experiments have confirmed that betatrophin was a pivotal regulator of plasma triglycerides. Serum triglycerides levels are reduced in ANGPTL8-null mice and increased dramatically in ANGPTL8 overexpressing mice. However, as are inconsistent with the results of animal experiments, many clinical studies including the present study did not find any relationship between betatrophin and triglycerides. So, the adjustment of TG levels cannot affect the results of the nonsignificant relationship between betatrophin and fat body distribution. After adjusting for age, sex, BMI, the results was still the same. The possible reason is that there is no statistical difference in these variables between NGT group and IGT group.
Our study has several limitations. Firstly, food ingestion greatly affects betatrophin levels; however, our analyses were based on single blood betatrophin measurements obtained under fasting conditions, which may not reflect betatrophin levels over time. Secondly, the cross-sectional design of this study allowed us to observe the correlation between VAT/SAT ratio and betatrophin levels but cannot prove causality between the two variables. Thirdly, since no patients with impaired fasting glycemia were enrolled in this study due to its relatively low prevalence, our findings do not fully reflect the metabolic characteristics of prediabetes; however, this does not affect the conclusions drawn from the NGT population. Lastly, the ethic differences in the expression or plasma concentrations of betatrophin should be considered. However, as we know, no related study has ever been published. The present study was carried out just in Chinese population, the results of which cannot directly extend to other ethic population.
In summary, the findings of this study could provide new insights into the possible contribution of betatrophin to the pathogenesis of obesity. We demonstrated that betatrophin levels are correlated with body fat distribution in individuals with NGT, showing a significant positive correlation with VAT/SAT ratio and negative correlation with lower body fat. The gold standard methods for assessing body fat distribution include CT and MRI, which allow the amount of adipose tissue deposited in particular depots to be accurately evaluated ; however, their time-consuming nature and high cost limit their clinical applications . The findings of this study suggest that betatrophin could be a favorable indicator that reflects body fat distribution during the normal stage of glucose intolerance and could be a simple and reliable risk assessment surrogate for CAD and metabolic disease in clinical practice. However, the mechanisms via which this protein affects ectopic body fat distribution remain unclear and further studies are warranted.