MRI and CT currently provide the best estimates of VAT and IMCL, however, the cost of both modalities and radiation exposure with CT limit their use as screening tools for obesity-related morbidities. Our study shows that specific anthropometric and DXA measurements may serve as cost-effective clinical surrogates for VAT and IMCL. Unlike other measures, PTF and TEFR are better associated with VAT than SAT, and may be reasonable surrogates of VAT in female adolescents. Similarly, these are the first data we are aware of that show that WHR is a good surrogate for IMCL.
Studies in adults have shown variable results in the correlation of anthropometric measures with VAT [12
]. One study [12
] compared BIA and anthropometric data with MRI measures of VAT, and reported that WHR was associated with VAT. In contrast, another study [22
] showed that WC was more strongly associated with VAT than was WHR and that DXA did not offer any advantage over anthropometry for VAT estimation. However, this did not address the issue that these anthropometric measures are associated with both VAT and SAT, and generally more strongly with SAT than VAT.
In children, fat distribution is influenced by gender, ethnicity and puberty. Brambilla et al
] cross validated anthropometry against MRI for assessment of VAT in children by pooling data from seven investigators. They identified WC as the best predictor of VAT in both normal-weight and obese subjects. However, predictors of VAT strongly predicted SAT, and the study again did not determine which anthropometric measure best differentiated VAT versus SAT. In our study, we examined normal-weight and obese girls and show that anthropometric measures, including WC and WHR, are associated with both VAT and SAT and none are specific for VAT. These findings are consistent with those from previous studies [6
]. However, in subgroup analysis, WHR was more strongly associated with VAT than SAT in obese girls, indicating that WHR may be a reasonable surrogate for VAT in obese girls at office visits.
We then evaluated DXA measures as possible surrogates for VAT, and found that total fat, PBF and TF correlated more strongly with SAT than VAT. However, unlike other measures, PTF and TEFR were more strongly associated with VAT than SAT in obese and normal-weight subjects (validated by kappa statistics). Therefore, our data suggest that specific DXA measures may provide clinically useful data for adolescents at risk of obesity-related comorbidities by better differentiating VAT from SAT. This is particularly important given that VAT, not SAT, determines insulin resistance and other morbidities. Additionally, we provide predictive equations for VAT using anthropometric and DXA measures.
IMCL, detected by MRS, is an important determinant of insulin resistance in children and adults [8
]. In conditioned athletes, IMCL increases and serves as a source of reserve fuel for times of exercise. However, in obese individuals, IMCL increases consequent to increased dietary fat and circulating fatty acids. Thus, IMCL is an excellent indicator of energy excess, and predicts insulin resistance. The mechanism whereby IMCL induces insulin resistance is unclear, but may be associated with diacylglycerol (DAG) and ceramide accumulation, lipid metabolites known to inhibit insulin signaling, in myotubes. In vitro
studies indicate that exposure to saturated fatty acids increases DAG and ceramide in myotubes, and inhibits insulin stimulated glycogen synthesis and activation of Akt/protein Kinase B (obligate intermediate in pathway of anabolic metabolism) [24
]. Similarly, consumption of a diet rich in saturated fats increases DAG in myotubes in rodents, with decreased insulin-stimulated glucose uptake [26
]. There are no data that examine whether anthropometric indices can serve as surrogates for IMCL in obese children. We evaluated the usefulness of anthropometry and DXA in predicting IMCL and showed that WHR was the anthropometric measure most strongly associated with IMCL. Of note, WHR was associated with IMCL in obese, but not normal-weight girls.
Limitations of our study include the sample size and the single gender study. This limits the generalizability as fat compartments and anthropometric measures vary between genders consequent to hormonal and physiological differences. However, limiting our subjects to adolescent girls allows for homogeneity of the study group, and provides useful preliminary data to be confirmed in larger studies.