Discussion
Abdominal obesity and T2DM are significant risk factors for cardiovascular events in South Asians, at any given BMI.16 In the current study, we report significantly positive correlations of the ABSI and Z_ABSI with SCAT and IAAT volumes, and anthropometric measures in non-obese patients with T2DM. Importantly, we observed superior predictive value of the Z_ABSI index for abdominal adiposity in non-obese patients with T2DM as compared with ABSI and other indices, irrespective of age and gender.
The ABSI correlates with mortality risk associated with abdominal obesity while showing minimal confounding effects of weight, height and BMI.6 In a large study on non-diabetic, Caucasian subjects aged between 19 and 76 years and residing in Italy, Bertoli et al8 reported significant association of the ABSI with all determinants of metabolic syndrome and IAAT thickness, latter measured using abdominal ultrasonography. When ABSI was combined with BMI, the association with IAAT thickness improved with better predictive accuracy for hypertriglyceridemia, low high density lipoprotein (HDL) cholesterol and impaired fasting glucose, even after adjustment for confounders.8 These authors concluded that ABSI was a useful index for evaluating the independent contribution of WC, in addition to BMI, as a surrogate measure for central obesity and cardiometabolic risk. However, the Z_ABSI was not applied in the study by Bertoli et al8 despite a large sample size with data of abdominal adipose tissue determined by ultrasonography. In comparison, the correlations of the Z_ABSI and ABSI with MRI-determined SCAT and IAAT volumes in our study on non-obese Asian Indian patients with T2DM are novel observations, despite a smaller but representative sample size. A study on elderly (mean age: 64±12 years) Japanese patients with T2DM (n, 607), showed that ABSI and Z_ABSI were significantly associated with intra-abdominal fat area (estimated by dual-impedance analyzer and defined as ‘visceral fat area’), and arterial stiffness (estimated by brachial-ankle pulse wave velocity). In this study, ABSI was strongly associated with IAAT thickness but not with BMI.17
The observations of the current study are also similar to the studies mentioned above; however, patients with T2DM in our study are younger and non-obese, and we carried out more detailed and accurate estimation of multiple adiposity depots, skinfolds and body fat as compared with the former studies. Importantly, our study is based on MRI which is superior to bioimpedance (as used by Khalil et al18) and ultrasonography (as used by Gönül et al19) for estimation of adiposity depots, as MRI uses magnetic field and radio waves to accurately quantify abdominal adipose tissue volume.20 Further, previously no research has been done on correlation statistics of these novel anthropometric indices and liver span and pancreatic volume, especially in Asian Indians.
It is important to note that hip circumference was inversely related to the risk of T2DM in a meta-analysis, but conflicting results were shown in other studies.21 Hip circumference is a surrogate measure of gluteal obesity while the hip index (HI) was obtained by transformation of hip circumference using the power law relationship making it unrelated to BMI. A recent study on 687 middle-aged, healthy Chinese subjects showed that the HI was a poor predictor of diabetes risk as compared with hip circumference and WHR.22 We applied both HI and Z_ HI and observed significant negative correlation in cases for HI with volumes of IAAT, RPAT and total IAAT. Interestingly, for Z_HI, significant negative correlations were observed with total IAAT and liver span, but not with RPAT. It is important to note that the HI is an index of gluteal adiposity and the femoral-gluteal adipose is a metabolically static depot due to lower rate of blood flow, lipolysis and fatty acid release as compared with SCAT. In line with the same, in the current study, no significant correlations were noted for HI with all depots of SCAT in cases and controls, as HI is an index of gluteal obesity.23 However, this needs to be investigated further to unravel the functional aspects.
The ARI has been shown to correlate significantly with all determinants of metabolic syndrome as shown in the Third National Health and Nutrition Examination Survey (NHANES III).24 In our study, we noted significant correlations of the ARI and Z_ARI with anthropometric measures, thigh, abdominal and truncal skinfolds but not with liver span, volumes of pancreas, SCAT and IAAT. This would have possibly resulted in low specificity of cut-off value, despite high sensitivity of the ARI in this study.
In our previous study, we had shown significant positive correlation of IAAT and liver span with pancreatic volume in non-obese Asian Indians with type 2 diabetes.13 Quite strikingly, in the current study, none of the above-mentioned anthropometric indices showed significant correlations with pancreatic volume and liver span (surrogate measure of hepatic fat) on MRI, thus undermining the utility of anthropometric indices for detection or prediction of fatty liver or fatty pancreas. This is plausible as pancreatic volume and fatty liver assessment is usually done by ultrasonography or MRI that require precise imaging protocols and in such cases anthropometric indices may not be robust enough to quantify it.25 We acknowledge the limitations of the study observations as MRI could not be performed on a large sample size of cases and controls due to reasons of economic feasibility. However, the study observations are worthy to be validated in other populations with larger sample sizes.
In summary, it is important to identify simple and cost-effective anthropometric measures that correlate with volumes of abdominal adipose tissue in Asian Indians, because of the close association of abdominal adiposity with cardiovascular risk in Asian Indians. For the first time, we show significant correlation of the Z_ABSI index with abdominal adiposity in non-obese Asian Indians with T2DM. The superior predictive accuracy of the Z_ABSI for abdominal adiposity, over ABSI, HI and ARI in this cohort underscores the utility of this index in low-cost clinical settings and epidemiological studies. Clearly, more studies are required to validate this index in general population as well as in patients with diabetes.