Subdivisions of subcutaneous abdominal adipose tissue and insulin resistance

Subdivisions of subcutaneous abdominal adipose tissue and insulin resistance

This is a very interesting article.  They looked at not only visceral vs. subQ abdominal fat, but differentiated between two types of abdominal SCAT (subcutaneous adipose tissue):  superficial vs. deep.

There is a well described fascial plane within the SAT of the abdomen (18, 28), with the superficial adipose layer possessing compact fascial septa (Camper’s fascia), whereas the deeper layer of adipose tissue has more loosely organized fascial septa (Scarpa’s fascia).  Fat lobules of the two sites also differ. The superficial layer is characterized by small tightly packed lobules, whereas those of the deeper layer are larger and distributed in an irregular manner (28). The thickness of the deep layer appears more variable among individuals and especially in relation to obesity (3). The presence of these fascial planes and differences in histology are well recognized with respect to liposuction, which generally is targeted toward the deep layer (15, 23).  Given the anatomical basis for considering the two layers of SAT in the abdomen different and the ability to delineate the fascial plane utilizing CT (22), the current study was undertaken to examine these adipose tissue depots from a metabolic perspective. The related purpose was to address current controversies regarding the importance of subcutaneous abdominal adipose tissue in relation to IR.

I doubt the screen shot below will show up well, but thought to include them anyway:

Summary of Body Composition:

• Systemic FM (I'll use TotFM) was greater in the obese (obviously) and women compared to men in both groups.  
• Thigh FM (I'll use TFM) and superficial abdominal SAT (I'll use SASAT) was also greater in the women
• Deep abdominal SAT (I'll use DASAT) did not differ between genders
• Obese had 2-3X as much DASAT and visceral adipose tissue (VAT) vs. lean
• VAT was not significantly different between genders but trended towards greater VAT in males 
• DASAT was significantly greater crossectional area than VAT in the obese
• About 3/4's (mean ~76%) of the DASAT is located in the posterior (back), and this partitioning varied within a relatively small range (67-87%).  This distribution did not differ in obese v. lean or between genders.
• Anterior and Posterior SASAT is more evenly distributed around the circumference of the abdomen,  being ~55% front /45% back.
• The proportion of SASAT of all abdominal fat (I'll use TAFM)  was 45% v. 41% in lean women vs. obese women
• The proportion of SASAT of all abdominal fat was only 28% in both lean and obese men
• The proportion of DASAT of all abdominal fat did not differ significantly between genders but does appear to trend towards higher levels in men.
• The difference in proportion of DASAT (compared to TAFM) was statistically significant for obese v. lean in both genders:  32% LW, 37% OW, 36% LM, 44% OM
• VAT proportions were as follows:  23% LW, 36% OW, 20% LM, 27% OM.  This was stastically significant for obese v. lean but not for gender.
• VAT was highly correlated with DASAT (r = 0.76),VAT was more modestly correlated with SASAT (r = 0.43).   Statistical Aside:  When two variables are tested for correlation, the closer r is to 1, the tighter the correlation so this is a rather "huge" difference between the two types of SAT and their correlations to VAT.  

Summary of Relationships between Types of Abdominal Adipose Tissue Depots and Metabolic Variables:

• Glucose Rd (a measure of clearance rate) is negatively correlated with TotFM
• Glucose Rd was not significantly correlated with SASAT or TFM
• Glucose Rd was significantly negatively correlated with DASAT and VAT, the strength of this correlation (r) was similar between the two fat depots.
• Combined DASAT & VAT (considered together) were even more strongly correlated (r = 0.68) with decreased glucose Rd than either fat depot considered separately
• Both TotFM (this part is unclear, from the table I think it's TotFM, from the title of the section one might imply total truncal fat) and VAT accounted for 45% of the variance in insulin sensitivity.  Statistical aside:  This statement is related to the degree of correlation (r = correlation coefficient).  Let's use the common example of height and weight which are generally significantly correlated.  If you select an adult at random and measure their height, there will be considerable variability in the result.  If the r for the height weight correlation is 0.7 - made up number - then r^2 = 0.49 and we would say that 49% of the variability in weight is accounted for by its correlation to height. 
• DASAT is independently associated with insulin sensitivity (r^2 = 0.51) when TotFM (again not sure if this was total truncal fat), VAT & DASAT are included in the model.
• Controlling for either TotFM+VAT or TotFM+DASAT, SASAT was not associated with insulin sensitivity.
• The strengths of association for various fat depots and insulin sensitivity were similar to those of glucose clearance and rank:  VAT, DASAT > TotFM, total abdominal SAT > SASAT
• Correlations for glucose and insulin AUC (a measure of total exposure over a defined time period) were weaker.  But they were similar for VAT and DASAT and both greater than SASAT which seems to follow the correlation pattern of TFM
• This pattern was repeated for fasting insulin where VAT and DASAT (r = 0.57 and 0.58 respectively) were significantly greater than for SASAT and TotFM (r = 0.26 and 0.27 respectively)
• Other parameters are shown in the table below.  The pattern continues where SASAT "behaves" more similarly to TFM than do DASAT and VAT which behave similarly.

From the Discussion:

The current study was undertaken to examine the novel hypothesis that superficial and deep depots of subcutaneous abdominal adiposity, defined anatomically by a fascial plane that divides the two depots and differing in histological characteristics (22), might also differ in regard to their association with insulin resistance.  The findings clearly indicate that strong differences do exist. Superficial SAT manifests a powerful relation to plasma leptin but a weak association with insulin resistance, and in these and other respects, it follows a pattern observed for thigh subcutaneous adipose tissue, a depot generally regarded as a weak determinant of insulin resistance. In contrast, the deep subcutaneous adipose tissue of the abdomen manifests a robust relation to IR and other key aspects that define the insulin resistance syndrome (e.g., blood pressure, fasting insulin, and lipids); moreover, it does so in a pattern nearly identical to that observed for visceral adiposity. Therefore, from the perspective of understanding body composition and insulin resistance, these results indicate that it is not accurate to ‘‘lump’’ these two differing adipose tissue depots into a single category, but instead it may be useful to ‘‘split’’ the depots in accord with the anatomic demarcation of the fascial plane (18).

From a personal standpoint, I find this somewhat reassuring as I'm pretty sure that my "central adiposity" is of the superficial variety.  Therefore the shift from its former location (thighs/butt) to the belly may well not have any negative health implications as the behavior of this fat is metabolically similar to that of the depots from where it shifted.