The main result to be highlighted is that increases in VAT were strong predictors of reduced spirometric measures, in both sexes, more importantly in women. In order to explore the effects of each abdominal fat compartment independently, multivariate models including VAT and SAT were utilized, since correlation between them was low (r = 0.39 for men and women). As mentioned previously, measures such as WC are more accessible, but cannot differentiate abdominal compartments, which is the main objective of this study. WC and PF results, as other body measures in this same population, can be found in previous publication ; the present findings are in accordance with other previous studies that had investigated central obesity by different measure methods and research settings [20, 21].
Similar results using CT to assess abdominal fat were observed by Park et al. among Korean individuals aged 15 to 85 years : in women, an increase in VAT was associated with reduced PF, while in men the best predictor of reduced lung function was total abdominal adipose tissue, VAT and SAT, after adjustment for height, weight, age, WC, systolic blood pressure and inflammatory markers, not using VAT and SAT in the same model.
We used ultrasound measurements of abdominal as proxies for these abdominal fat masses. While the validity of ultrasound in this specific setting has not yet been determined, some validation studies using the same standardized protocol have found strong correlations between ultrasound and magnetic resonance imaging estimates of abdominal fat in a variety of settings and populations [14, 22, 23]. A strict quality control process was carried out and we could identify consistent sex differences in the distribution of these abdominal fat compartments compared with previous reports using other imaging methods [14, 22, 24,25,26,27].
The role of the adipose tissue is not limited to lipids deposit. There is also evidence of its role as endocrine organ, producing a number of pro-inflammatory molecules and cytokines such as inteleuncine-6 and C-reactive protein, the last found in high levels in the ones with high VAT accumulation and highly related to cardiovascular episodes [28, 29]; VAT has been also known as insulin resistant compartment . On the other hand, SAT has controversial findings regarding its role on the diseases risk. In non-caucasian populations this compartment showed associations with several metabolic changes [31,32,33], but in recent studies it showed a protective effect and lack of association in obese individuals [34, 35].
The two main hypotheses for PF reduction due to excess of abdominal fat are via inflammatory mediators  and/or by mechanical restriction [2,3,4]. Whether the reduced lung function is due to one or the other cannot be completely clarified in our study, we can just have clues that the mechanical restriction seems to be more important, since reduction of both parameters, FEV1 and FVC, were observed.
VAT has been associated to several cardiometabolic risk factors, metabolic syndrome and systemic inflammation . However, two previous studies [3, 11] investigated abdominal fat compartments using CT and reduced lung function, but neither of them supported the inflammation hypothesis: no association was found between inflammatory markers and PF  or persistence of the association between abdominal adiposity with lower PF after controlling for some inflammatory makers in the adjusted statistical model .
SAT showed an inverse association with PF in previous publications [3, 8, 11]. In our study, we observed positive associations between SAT and PF after adjustment and this result must be interpreted carefully due to the lack of plausibility that SAT increment can be beneficial to PF. In our population, the mean SAT is still low at 30 years old and could lead to an absence of influence on reduction of PF parameters. Also, SAT accumulation has been associated with a normal metabolic profile, while VAT accumulation, considered ectopic, is associated with an altered metabolic profile due to habits such as smoking and low physical activity . This might explain this SAT positive trend, mainly after adjustment for VAT and total fat mass. Only one study we are aware of , using CT in an elderly population, did not find association concerning VAT, SAT and PF, although they had find an inverse association between PF and central obesity measured by dual-energy X-ray absorptiometry.
Some strengths of our study should be mentioned. We used a large general population sample, with information for several confounding factors. This allows us power to detect the associations and extrapolate results to similar healthy populations. Also we had rigorous quality control in spirometry and ultrasound measures, a method that allows a good estimation of adiposity compartments (VAT and SAT) despite the literature on adiposity measured by US and FP is scarce.
On the other hand, we did not perform any static lung volume measures, such as total lung capacity or expiratory reserve volume; we have just spirometric measurements, more accessible in this research scenario than methods based on plethysmography or inert gas dilution. Although spirometric and traditional anthropometric measurements were performed several times during cohort follow-ups, abdominal ultrasound measurements were carried out only at the 30th year follow-up allowing just cross-sectional analysis between abdominal fat compartments and PF outcomes preventing us to infer causality.