In the current study, we presented an interesting finding that there was a inversely relationship between BMI and baPWV or increased AS in Chinese rural adults with hypertension, whereas WHR or central obesity is positively associated with baPWV and increased AS. To our knowledge, these findings have not been previously described.
Hypertension is often accompanied with overweight or obesity [19]. In our study, We found that the prevalence of overweight, general obesity, central obesity and were 32.62%, 8.58% and 63.85%, respectively. Moreover, PWV was increased in hypertensive patients, and the degree of PWV increase was associated with baseline blood pressure [39]. In our study, baPWV was also associated with most clinical characteristics including SBP and DBP, nearly half (44.01%) of the hypertensive participants have increased AS (baPWV ≥ 18 m/s) (Supplement Table 1 and 2), which has been considered to be an independent factor in indicating the relapse of CVD as well as an irreversible progress of atherosclerosis [11].
Obesity is an accredited dangerous factor for CVD [23]. However, some studies have shown that obesity as defined by BMI has a survival benefits on some specific populations [40]. The phenomenon of the obesity paradox may be related to genetics, cardiorespiratory fitness, beneficial adipose tissue and weaker sympathetic activation [34, 40].
In our study, we found that there was a inversely relationship between BMI and baPWV or increased AS in our crowd. Similarly, Huang et al. [29] and Liu et al. [18] found that there was a negative relationship between BMI and baPWV among male hypertension participants. On the contrary, previous studies showed that a higher BMI was associated with increased PWV in Grade I essential hypertension [15] or obese and non-obese hypertensive patients [17]. Different origins of the study participants might contribute to the study differences. The Liu et al. [18] study was based on 699 male hypertensive patients who were hospitalization or had other complications. The Samir et al. [15] study was based on 114 patients with Grade I essential hypertension who were civil servants. The Huang et al. [29] study enrolled 10 1510 participants, a coal occupation group in labor-intensive enterprise, from 11 hospitals in the Kailuan community, which is most similar to our rural hypertensive crowd. In consideration of that BMI fails to account for body composition [34] and physical habits, whereby rural adults are associated with increased muscle mass (muscle mass is greater than fat mass) and better physical fitness, both of which may account for the inversely relationship between BMI and baPWV or increased AS in our crowd.
WHR, a more sensitive marker for central obesity, may be better indicate risk for atherosclerosis or CVD associated with obesity than BMI [35, 36], as high WHR can reflect both increased visceral fat as well as low gluteal muscle mass (and/or low peripheral fat mass), both of which have been found to be independently associated with cardiovascular disease risk [41, 42]. We demonstrated that WHR or central obesity is positively associated with baPWV and increased AS in our crowd. The result was consistent with similar findings in the general population [25, 27]. For example, the Whitehall II study, a prospective study of 10 308 civil servants, showed that standardized effects of central adiposity on aortic PWV increase was obvious and previous adiposity was associated with aortic stiffening independent of change in adiposity, glycaemia, and lipid levels across PWV assessments [25].
Seidell et al. [43] demonstrated that WHR is highly correlated with visceral fat but not with subcutaneous fat (the latter which in fact may be protective), while waist circumference (WC) was highly correlated with both that. The incorporation of a reference body size may explain why WHR is a better predictor of outcomes than WC, a primitive measurement for abdominal obesity [35].
The lack of consistency between WHR and BMI may reflect that these measures identify different characteristics of obesity (central obesity in case of WHR vs. subcutaneous/total fat in case of BMI). Bouchi et al. [44], in a cross-sectional study with patients with diabetes and non-obese (normal BMI), noticed that increased visceral fat seems to be associated with increased AS based on baPWV. Notably, BMI fails to account for body composition, however, skeletal muscle mass [45] and physical fitness [46] counteract risk with obesity. As Chinses rural adults are associated with increased muscle mass and better physical fitness, thus BMI might not be suitable for the evaluation of obesity and the degree of atherosclerosis may be underestimated by relying on BMI in this crowd, while WHR (central obesity) may be better indicate risk for atherosclerosis.
There are some strengths in our study, which should be considered in the interpretation of results. Firstly, the study was conducted in Wuyuan, a typical county in China, which offered us an exceptional opportunity to examine the association between obesity indicators and PWV in this crowd under a natural state. In addition, although the real-world, multicenter, observational study was susceptible to potential confounding, we used rigorous statistical adjustments to minimize residual confounding.
Limitations
Our study also has several limitations that should be considered. Firstly, the study was performed by cross-sectional design and so provides the association of relationships instead of predictive values of WHR on the progression of arterial stiffness. Secondly, results are limited by the absence of gold standard for measurement of visceral fat, such as computerized tomography (CT) and magnetic resonance imaging (MRI) for quantification of body fat composition. Furthermore, WHR measurements were performed by different individuals, although they were provided with clear instructions. Finally, this study was conducted in Chinese hypertension populations, the generalizability of the findings to other populations remains to be determined.