Principal findings
We cross-sectionally analysed cardiometabolic correlates of VAT/SAT ratio (a measure of body fat distribution between VAT and SAT compartments) in the Framingham Heart Study cohort. We observed that in women, the VAT/SAT ratio demonstrated a weak, positive correlation with BMI and waist circumference. Conversely, in men, the VAT/SAT ratio was inversely associated with BMI and waist circumference. In both sexes, the VAT/SAT ratio was a significant correlate of several cardiometabolic risk factors, above and beyond associations with BMI and VAT. These findings suggest that ectopic fat distribution, independent of the absolute fat quantity, is a correlate of metabolic risk.
We have hypothesised that the VAT/SAT ratio is a metric of the propensity to store fat viscerally relative to subcutaneous fat stores. The ‘lipid overflow ectopic fat model’ suggests that surplus energy is physiologically primarily stored in subcutaneous compartments, and that when this depot is dysfunctional, energy can alternatively be deposited in visceral compartments [11]. Failure of adipocyte proliferation and differentiation has been discussed as a possible mechanism of functional SAT insufficiency [12]. Lipodystrophy is characterised by SAT dysfunction leading to marked increases in visceral and ectopic fat storage with a paucity of subcutaneous fat. Visceral fat may cause hepatic insulin resistance by releasing a variety of biologically active molecules, such as NEFAs and inflammatory mediators, into the portal vein system (‘portal vein hypothesis’) [11, 13]. Taken together, the propensity to store energy viscerally as compared with subcutaneously may be both a cause and a consequence of multiple different factors, including insulin resistance, smoking, stress and lack of physical activity, and may also be influenced by glucocorticosteroids and sex hormones [11, 12, 14, 15].
Pharmacological studies suggest that the VAT/SAT ratio may be a determinant of metabolic risk. The VAT/SAT ratio can be lowered by thiazolidinedione (‘insulin sensitiser’) treatment, which expands SAT volume and reduces VAT [16], leading to improved insulin sensitivity despite weight gain [17–19]. Conversely, drugs can also cause lipodystrophy, specifically as a common side effect of HIV protease-inhibitor therapy. This adverse pattern of fat redistribution corresponds to a higher VAT/SAT ratio [20], and is associated with insulin resistance and dyslipidaemia [21]. Hence, even though not associated with weight gain [20] (in fact, often weight loss [22]), protease-inhibitor-induced lipodystrophy demonstrates the metabolic profile of the metabolic syndrome [21].
VAT/SAT ratio, other measures of body fat, and sex differences
We observed a weak positive correlation of VAT/SAT ratio with waist circumference in women (and even weaker with BMI), whereas these correlations were modestly inversely associated in men. In a sample of 62 individuals, Gastaldelli and colleagues observed a weak, but positive, association of VAT/SAT ratio with BMI in both women and men [23]. Our considerably larger analysis supports the notion of a very weak relation between VAT/SAT ratio and BMI, but also suggests a different directionality of this weak association in women and men. Taken together, the VAT/SAT ratio appears to provide information that is independent of standard measures of generalised or abdominal adiposity. Also the relations of VAT/SAT ratio with VAT and SAT differ between sexes: in women, VAT/SAT ratio was to a much larger extent determined by VAT than by SAT, whereas in men, VAT and SAT contributed more similarly to the VAT/SAT ratio.
Generally, associations of the VAT/SAT ratio with cardiovascular risk factors were stronger in women than in men. In women, higher levels of the VAT/SAT ratio were also associated with current smoking, with a more than twofold increase in smoking prevalence from the lowest to the highest VAT/SAT ratio tertile. We have previously reported that women who currently smoke have somewhat higher VAT and lower SAT than non-smokers or ex-smokers [24]. The principles underlying this sex-specific association are not entirely clear, but may be related to a decreased bioavailability of oestrogen in smokers [25]. Oestrogen plays an important role in regulating metabolism and lipogenesis [26]. Also, nicotine increases metabolic rate [27] and inhibits appetite [28], which may result in lower SAT. Alternatively, the association between current smoking and higher VAT/SAT ratio may be mediated by other nutritional or lifestyle factors related to smoking.
VAT/SAT ratio and cardiovascular risk
Several studies have investigated the correlation of either VAT or SAT with cardiometabolic risk, and many have concluded that VAT is a stronger correlate of cardiometabolic risk than SAT [3, 4, 11]. However, little is known regarding how the relative distribution of fat between visceral and subcutaneous compartments is associated with cardiometabolic traits. Some smaller studies have previously reported a positive association of VAT/SAT ratio with cardiovascular risk factors [23, 29–31]. In their paper based on 62 individuals, Gastaldelli et al [23] reported that VAT/SAT ratio was associated with fasting glucose (whereas SAT or VAT alone were not). Conversely, VAT alone was associated with fasting insulin and insulin sensitivity, whereas VAT/SAT ratio was not. Notably, these analyses were conducted after pooling sexes. Miyazaki and DeFronzo [29] reported that, in a sample of 36 men with type 2 diabetes, the VAT/SAT ratio correlated with endogeneous glucose production during insulin clamp (a measure of hepatic insulin resistance), whereas VAT or SAT alone did not. In contrast, total glucose disposal (a measure of peripheral insulin sensitivity) correlated inversely with VAT and SAT, but not with VAT/SAT ratio. He et al [30] observed that, in a sample of 437 individuals, the VAT/SAT ratio was independently associated with cardiometabolic risk factor clustering in women (n = 197), whereas in men VAT was a stronger correlate of cardiometabolic risk than the VAT/SAT ratio. In a sample of 13 spinal cord injury patients, Gorgey et al reported an inverse association of VAT/SAT ratio with fasting insulin [31]. A recent investigation by Kim et al [32] demonstrated that, in a Korean population, a higher SAT/VAT ratio (i.e. the inverse of our trait) was associated with lower prevalence of the metabolic syndrome, higher HDL-cholesterol, lower triacylglycerol and lower fasting glucose, even after adjustment for BMI. Our data is consistent with this study, and extends these findings to a large white population. We furthermore demonstrate that the VAT/SAT ratio is a correlate of cardiometabolic risk above and beyond BMI and VAT. These observations suggest that the propensity to store energy in visceral vs subcutaneous fat compartments is a correlate of metabolic dysregulation, independent of overall obesity and absolute visceral fat mass. Of note, our study was not designed to demonstrate that VAT/SAT is a stronger correlate of cardiometabolic risk than VAT alone. In fact, when we evaluated VAT and VAT/SAT jointly, VAT was associated with several cardiometabolic traits independently of the VAT/SAT ratio and, in particular, was associated with some traits that were not independently related to VAT/SAT ratio.
Strengths and limitations
Several strengths of our study warrant mention. We used a highly reproducible, highly specific CT-derived volumetric assessment of fat depots rather than anthropometric surrogate measures of body fat composition. The design of the Framingham Heart Study with regular examinations in the dedicated on-site participants clinic assures high quality of clinical and biochemical data. Limitations of our study are the observational design, precluding any definite causal inference. Furthermore, our study cohort is middle-aged and primarily white. Although our findings are similar to observations made in an Asian population [32], generalisibility to other age groups and ethnic groups remains to be shown. Also, we only assessed HOMA-IR as a measure of insulin resistance. However, more sophisticated methods such as hyperinsulinaemic euglycaemic clamp were not feasible in our large community-based cohort. Last, as hip measurements are not part of the Framingham study protocols, we were not able to compare VAT/SAT ratio with waist/hip ratio.
Conclusion
We demonstrate in a large community-based sample that VAT/SAT ratio—a measure of relative body fat distribution—is a unique correlate of metabolic risk, independent of BMI and VAT. These findings suggest that the propensity for ectopic, relative to subcutaneous, fat deposition is associated with increased cardiovascular risk, independent of overall obesity and absolute visceral fat mass.