Discussion
This large-scale population-based study of Chinese children and adolescents provides a unique opportunity to examine the associations between abdominal adipose compartments quantified by DXA and cardiometabolic risk profiles. We found that both high SAT and VAT were positively associated with elevated blood pressure and lipid disorders for both sexes (except for low HDL-C in girls) independent of FMI or BMI. However, their relative contributions differ between sexes. In addition, boys had a stronger association between SAT and most of metabolic risk factors compared with girls, but no sex differences for VAT were observed.
It is well documented that visceral fat accumulation bears adverse effects on cardiometabolic health in adults.6–8 20 21 Several longitudinal studies have indicated that visceral adiposity has been shown to be associated with incident cardiovascular disease after adjustment for clinical risk factors and general adiposity.20 22 23 However, several cross-sectional studies of children and adolescents have demonstrated inconsistent results, with some showing the adverse effect of VAT on metabolic disorders9 10 but not for others.11–13 These inconsistencies may be due to small sample size, focusing only on obese subjects or without adjustment of body fat mass. Our results extended prior findings within the context of a large population-based sample of Chinese children and adolescents. In agreement with findings from studies of adults, we found significant associations of VAT with most of metabolic risk factors, including elevated blood pressure and high-risk lipid profiles, independent of total fat mass in boys and girls. All these findings suggest that VAT plays a pathological role in the development of metabolic disorders in both adults and children.
The majority of studies in adults have shown that SAT is positively associated with obesity-related complications, but the association is weaker than that for VAT. In several studies, however, SAT is not associated with cardiometabolic risk and even exerts a protective effect after accounting for total fat mass.24 25 In contrast, we demonstrated adverse effects of high SAT on all risk factors after adjustment for body fat. In addition, the associations of SAT were even stronger than those for VAT for most risk factors, especially in boys. Similar results have been reported in several prior small studies.11–13 26 27 A recent study of 94 Mexican school children showed magnetic resonance-measured SAT rather than VAT was associated with higher cardiometabolic risk.11 Another small cross-sectional study of 30 overweight and obese prepubertal children reported that insulin sensitivity was negatively correlated with DXA-measured SAT but not with VAT.26 A study consisting of 999 individuals aged 6–90 years compared the association of abdominal fat depots with cardiometabolic traits between youth and adults, showing that abdominal SAT was the most significant predictor of metabolic traits in children and adolescents, whereas VAT was the most significant predictor in adults.12 The difference in the contribution of SAT on cardiometabolic health between youth and adults suggests that adnominal fat depots may differ in their pathogenic significance at different life stages. In the present study, we also found that the SAT area was much greater than VAT area in both girls and boys, suggesting that SAT may contribute to more absolute cardiometabolic risk than VAT in early life.
It should be noted that abdominal SAT can be further divided into superficial and deep compartments. Prior studies have shown that abdominal deep and superficial SATs have different metabolic function and activity28–30 and deep SAT has been more strongly associated with metabolic risk factors superficial compared with superficial SAT.31 32 However, we did not differentiate these two distinct fat depots in the current study, and further studies are required to explore the impacts of these two specific depots of SAT on metabolic health in youth.
Interestingly, we found a sex difference in the association of SAT with lipid risk factors, with boys exhibiting greater association. In line with our findings, a cross-sectional study of 1223 Hispanic/Latino youth aged 8–16 years indicated that the associations between multiple measures (BMI, waist circumference, waist-to-hip ratio, and fat mass percentage) and insulin resistance were stronger in boys than in girls.33 These results were inconsistent with several prior studies of adults demonstrating that SAT and VAT were more strongly associated with adverse risk factors levels in women than in men.6 34 However, we did not find sex differences in the associations of VAT with metabolic risk factors. The underlying mechanism for this sex-difference for SAT is not clear and remains to be elucidated in future studies.
BMI is the widely used method to assess overall adiposity and identify individuals for further assessment of cardiometabolic risk. In adults, VAT but not SAT can improve the identification of cardiometabolic risk.6 However, a recent study from the National Health and Nutrition Examination Survey showed that the use of fat mass does not improve on BMI for the identification of metabolic syndrome in US adolescents.35 In the current study, although the associations of VAT and SAT with multiple cardiometabolic risk factors were independent of BMI, adding VAT or SAT or both on BMI did not improve the identification of most outcome factors. Given that measuring BMI had great convenience and low-cost, BMI remains an appropriate measure for screening obesity in children and adolescents in public health settings. However, we found that the prevalence of elevated blood pressure and lipid disorders showed increasing trends across tertiles of VAT and SAT among individuals with overweight and obesity. Thus, VAT and SAT may be useful to identify those with higher cardiovascular risk among overweight and obese children and adolescents.
The strengths of our study include a large sample size and estimating abdominal fat compartments using DXA. This study also has several limitations. First, our study is cross-sectional and thus causal relationship of VAT and SAT with metabolic risk factors cannot be inferred. Longitudinal studies are required to examine their long-term health impact. Second, we measured adnominal VAT and SAT using DXA rather than MRI and CT. However, studies have shown that DXA-derived VAT performed as well as a clinical read of VAT from a CT scan.19 Third, we cannot distinguish superficial and deep SAT compartments using DXA, and their relative contributions on cardiometabolic risk need to be elucidated in future studies. Fourth, hyperglycemia has been considered as one important cardiometabolic risk factor; however, we did not assess the influence of VAT and SAT on hyperglycemia in current study. Future studies are required to confirm the associations with hyperglycemia. Fifth, data on dietary factors and family history of chronic disease were unavailable in current study, and thus we cannot assess their influences on our observed associations.