Considering the huge amount of aging population in China, investigating the age and gender specific prevalence and associated factors of MetS in Chinese middle-aged and elderly population is of great significance. A nationally representative project targeting the aged, CHARLS, helps realize it.
In the present study, the prevalence of MetS in the participants aged 40 years and older was 32.97% based on IDF definition. Under NCEP-ATP III definition, it was 29.75%. According to Li’s report, in 2010–2012, this figure in Chinese adults aged 18 years and older was 24.2% under NCEP-ATP III definition [23]. In He’s study, in the overall population, it was 9.5% in 2002 and 18.7% in 2010–2012 [31]. These figures revealed that the aged had higher odds of MetS, which were in lines with other studies [9, 32]. Based on IDF criteria, the prevalence of MetS was 45.5% in Tunisia, 37.4% in Iran and 53.9% in Gwalior, a city in India [33, 34]. Compared with studies in other countries, prevalence reported in the present study seems lower than these countries, which may be attributed to different races, lifestyles etc. In the aging population, the prevalence was 35% in 40–59 years group and 46.7% in the > 60 years group in the U.S [10]. Prevalence of MetS in aging population were high but still lower than developed countries. In addition, in 2009, this number was 31.5% among people over 35 years old in Jiangsu province, China [35], which indicated that the trend of MetS in aging population did not vary wildly.
It is noteworthy that with aging, the prevalence of MetS descends in males, while ascends in females. This trend is also observed in Korean [36], Indian [34], Spanish [37] etc., which may be partly attributed to hormone secretion. According to Jeenduang’s study, prevalence of MetS was 29.37% in postmenopausal women, while was 16.97% in premenopausal women [38]. Hormones hidden behind menopause may lead to this alteration [39]. However, one previous article performed in U.S. presented an opposite observational result that males had higher prevalence of MetS than females [40]. This finding may be biased by their limited samples, which still needs more evidences.
Furthermore, our study also found that the prevalence of MetS in urban areas was significantly higher than one in rural areas. Sedentary lifestyles in modern cities may account for this finding [41]. Of note, with aging, this upward trend of MetS prevalence in females was not observed in males, especially for those settling in rural areas who displayed a downward trend. This result was in line with previous studies performed in China [17], India [42], sub-Saharan Africa [43] and Mexico [44] but contrary to Lee’s study in the middle-aged Koreans [45]. Study from Javier concluded that men were more susceptible to the urbanization-associated worsening of cardiometabolic health [44], which may partly clarify this disparity. Notwithstanding, the specific reasons remain hazy. Besides hormones and lifestyles stated above, a number of other potential factors may contribute to this difference including household income, educational levels and annual cost of healthcare etc [46, 47].
This disparity of MetS prevalence was also detected in different regions. Regardless of gender, participants had the highest prevalence in the north region and the lowest in the southwest region. According to previous reports [48], it may be linked to dietary nutrient intake. It is well-known that residents in north region mainly eat food made of flour, such as noodles, which is rich in carbohydrates. It was well known that intake of carbohydrate and sodium was closely associated with the increased risk of MetS [45]. Moreover, a diet rich in carbohydrates was thought to be a principal reason of aggravation glucose intolerance [49] and dyslipidemia [50]. Therefore, improving the diet structure seems sensible for those participants.
In this special aging population, a number of relevant factors consisting of age, gender, living areas etc. were identified, as revealed by previous literature [51]. However, two pivotal factors comprising afternoon nap and BUN are little reported elsewhere. On the basis of Maria’s study [52], participants with afternoon nap (< 30 minutes/day) had lower risks of suffering MetS in the overall population, contrary to what we reported. This discrepancy may be due to the different definition of afternoon nap in their study. According to Cao’s study of 27,009 participants (mean age 63.6 years), nappers with longer nap duration had fairly higher rate of hypertension [53]. The design of Maria’s study was limited in the < 30 minutes/day group, which may constrain this conclusion. More importantly, BUN > 20 mg/dl was also determined to be a negative correlate. In Arora’s study, both metabolically healthy obesity (MHO) and metabolically unhealthy obesity (MUO) displayed normal kidney function [54]. However, their target population was children and adolescents. This risk in different population varied, which raised a reminder of renal function in this particular middle-aged and elderly MetS patients.
The present study has several defects. First, although some associated factors were identified here, however, this association still awaits further cohort study to demonstrate in this specific population. Second, this study enrolled the whole aging population instead of one specific gender into analyses, which limited further understanding of its relevant factors by gender. We plan to address this issue in future studies.