Background
With development of the economy and acceleration of urbanization, combined with aging of the population, the incidence of cardiovascular disease (CVD) has continued to increase over the last 30 years in China [
1]. CVD was the top cause of death among Chinese adults in 2016, ahead of tumors and other diseases [
2]. A number of studies have revealed that dyslipidemia is an important modifiable risk factor with a key role in CVD. Therefore, early screening and effective control of lipid levels can reduce the morbidity and mortality of CVD, which has useful social value [
3,
4]. The prevalence of dyslipidemia in the general population of China aged 18 and older has increased from 18.6% in 2002 to 40.4% in 2012 [
5,
6]. Without timely and effective control, the rate of dyslipidemia will continue to rise, leading to a heavy burden of CVD. Therefore, it is important to identify the potential influencing factors of dyslipidemia, to manage this condition and reduce the burden of CVD.
The occurrence and development of dyslipidemia frequently involves a long-term, continuous process [
7,
8]. Various factors may play different roles in the entire development process of dyslipidemia. Previous studies on dyslipidemia and associated factors have been conducted in different provinces and regions [
5,
9,
10], but large-scale investigations in northern China are scarce. The Inner Mongolia Autonomous Region is located in the north of China and includes 12 cities. Owing to its varying geographical features from east to west, the lifestyle and health problems of Inner Mongolia are representative of northern China. The prevalence and mortality of CVD are at higher levels in the region [
2,
11]. However, as an important risk factors for CVD, there are few recent comprehensive epidemiologic studies on dyslipidemia in Inner Mongolia. Our current study was designed to systematically obtain current data on the prevalence of dyslipidemia and to explore its potential influencing factors, to provide clues for the prevention and control of dyslipidemia in northern China.
Methods
Study population
Participants in our cross-sectional study were recruited from Inner Mongolia, using a convenience sampling strategy to select six sites (Hohhot, Wuhai, Chifeng, Erdos, Hulun Buir, and Xingan League) during 2015–2017. Each site was selected according to geographical, economic, and ethnic distribution factors. Participants were enrolled if they were aged 35–75 years and lived in one of the selected regions for at least 6 of the previous 12 months. Of 70,380 enrolled participants, 5252 (7.5%) were excluded because of missing data for serum lipids or body mass index (BMI). The study was approved by the ethics committee of Fuwai Hospital Chinese Academy of Medical Sciences (approval number: 2014–574). Written informed consent was obtained from each enrolled participant.
Data collection and measurement
The investigation included baseline information, physical examination, and laboratory testing. All investigators underwent professional training before the study. Information on demographic and social characteristics (such as age, sex, ethnicity, residential region, marital status, educational level, and family income), medical history (such as hypertension and diabetes), and lifestyle (such as smoking and alcohol consumption) were collected by interview where conducted at local community health service or door-to-door visits to use a standardized questionnaire.
Two consecutive blood pressure measurements were taken and the mean blood pressure value of the two readings was used. For each participant, blood pressure measurement was performed on the right upper arm after 5 min of rest, with the participant in a seated position, using an electronic sphygmomanometer (Omron, HEM-7430). Fasting blood samples were collected after at least 10 h of overnight fasting. Venous blood specimen was collected in Vacutainer tubes containing ethylenediaminetetraacetic acid (EDTA). Fasting plasma glucose (FPG) was analyzed enzymatically using an autoanalyzer (BeneCheck,PD-G001–2). Serum total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), triglycerides (TG) were measured by an automatic biochemical analyzer (Cardiocheck PA). All laboratory equipment was calibrated.
Height and weight were measured, with participants wearing lightweight clothing and no shoes, to the nearest 0.1 cm and 0.1 kg, respectively. BMI was computed as weight (kg) divided by the square of height (m2). Waist circumference (WC) was measured at the level of the navel using a tape measure, to the nearest 0.1 cm.
Definitions
According to 2016 Chinese guidelines for the management of dyslipidemia in adults [
6], participants were defined as having dyslipidemia if they had one or more of the following conditions: TC ≥6.22 mmol/L (240 mg/dL), LDL-C ≥ 4.14 mmol/L (160 mg/dL), HDL-C ≤ 1.04 mmol/L (40 mg/dL), TG ≥2.26 mmol/L (200 mg/dL), or if they were taking anti-dyslipidemia medication. Hypertension was considered with systolic blood pressure (SBP) ≥140 mmHg or diastolic blood pressure (DBP) ≥90 mmHg or reported use of antihypertensive medication [
12]. The diagnosis criterion of diabetes mellitus was FPG ≥7.0 mmol/L or having received treatment for diabetes [
13]. Participants with BMI ≥28 kg/m
2 were diagnosed with obesity, and central obesity was identified as WC ≥90 cm in men and WC ≥85 cm in women [
14]. Smoking was defined as participants who smoked at least one cigarette per day during the past 12 months. Drinking was defined as drinking alcohol at least one time per month in the past 12 months.
Statistical analysis
We described continuous variables using mean ± standard deviation (SD) or median and interquartile range (IQR), and the Student t-test or Wilcoxon rank sum test was used to compare differences. Categorical variables were shown as proportions and compared using a chi-squared test. The age-standardized prevalence was calculated according to the China 2010 census. To explore potential influencing factors of dyslipidemia, multivariate logistic regression analyses were used to calculate odds ratios (OR) and 95% confidence intervals (CIs). All analyses were conducted using SAS version 9.3 (SAS Institute, Cary, NC, USA). Two-sided P < 0.05 was considered statistically significant.
Discussion
A recent study showed that the annual number of deaths owing to CVD increased from 2.51 million to 3.97 million between 1990 and 2016 [
2]. CVD has become the leading cause of death in China, particularly in northern China [
15]. Dyslipidemia is a well-established risk factor for CVD morbidity and mortality [
16]. The prevalence of dyslipidemia increased rapidly from 2002 to 2015 [
1]. Identifying the epidemiological characteristics and influencing factors of dyslipidemia is an urgent public health priority. Thus, we carried out the present study of serum lipids, the largest in a representative population of Northern China.
In this study, the age-standardized prevalence of dyslipidemia was 31.2%, and the prevalence rate of abnormal lipid levels was 4.3, 2.4, 14.7, and 17.4% for elevated TC, LDL-C, TG, and decreased HDL-C, respectively. A nationwide investigation showed that the prevalence of dyslipidemia was 34.7% among Chinese adults over age 35 years [
17], which was higher than the results of the current study. However, compared with several regional studies [
18‐
20], the prevalence of decreased HDL-C and elevated TG in our study were at higher levels. Located in northern China, Inner Mongolia has different food culture from other regions. Although the consumption of fresh vegetables and fruit has increased in recent years, the diet of local residents was intake meat and animal fat as the main component [
21]. A recent cohort study revealed that higher fat intake was related to a greater risk of obesity [
22]. Previous studies have demonstrated that the obesity could be related to low HDL-C and elevated TG [
23]. While Inner Mongolia was one of the high-prevalence cluster of obesity [
24]. This could be explained that Inner Mongolia had the higher prevalence of high TG and low HDL-C. Compared to other Western countries, low HDL-C was the major subtype of dyslipidemia in China [
25], and the prevalence of high TG has been shown an increasing trend for Chinese populations in recent years [
1]. This was consistent with the situation that the prevalence of dyslipidemia was largely driven by HDL-C and TG in Inner Mongolia. The epidemiological data showed that HDL-C levels were negatively correlated with the risk of atherosclerotic cardiovascular disease (ASCVD) [
26], and elevated TG was commonly associated with increased CVD risk [
27]. Therefore, monitoring of HDL-C and TG should be strengthened to prevent the incidence of CVD among adults in northern China.
We found that prevalence of dyslipidemia increased with age but was decreased in the age group 65–75 years. Serum lipid abnormalities were more prevalent among men and participants living in urban regions, especially those under age 55–64 years. The prevalence of dyslipidemia, low HDL-C and high TG were decreased with age in men, and the different types of dyslipidemia (except decreased HDL-C) were more prevalent in postmenopausal women. The similar situations that occurred in men among different age groups have been shown in past studies [
28,
29]. Although the reason has not been confirmed, in our speculation, men may be more susceptible to risk factors when they are younger, owing to higher intake of fatty foods, greater exposure to unhealthy lifestyles, and a lack of appropriate health knowledge, resulting in men being more prone to dyslipidemia. Previous studies have demonstrated that the prevalence of elevated TC and LDL-C in postmenopausal women is higher than in their male counterparts [
30], which is consistent with the findings of our survey. Dyslipidemia is more pronounced in older women, perhaps owing to decreased estrogen levels accompanying menopause, which may have an adverse effect on serum lipid levels in postmenopausal women [
31]. As is known, dyslipidemia characterized by elevated TC or LDL-C is considered a major risk factor of ASCVD, and lowering LDL-C levels can significantly reduce the risk of morbidity and mortality owing to ASCVD [
6,
32]. To enhance the management of dyslipidemia, greater focus is needed for men under age 55 years and postmenopausal women.
We found disparities in lipid abnormalities according to ethnicity. The prevalence of decreased HDL-C, elevated TG, and dyslipidemia was higher in Han than in Mongol participants, and multivariate analysis showed that people with Mongol ethnicity had a lower risk of dyslipidemia. This finding differs from those of previous studies [
33,
34]. There are different genetic backgrounds, cultures, customs, and food consumption patterns between the two ethnic groups [
35]. Mongolian tend to eat more meat, milk and dairy products [
34]. Some studies conducted in different countries have found that more frequent consumption of milk and dairy products could decreased the risk of dyslipidemia (elevated TG or declined HDL-C levels) [
36,
37]. High consumption of milk and dairy products might related to the lower prevalence of high TG and low HDL-C levels in Mongolian.
We identified several factors that were associated with dyslipidemia in our study population. Our results that smoking, obesity, central obesity, hypertension, and diabetes can elevate the risk of dyslipidemia has been confirmed in many previous studies [
29,
38]. We also found that current drinkers had a lower risk of dyslipidemia; a similar result was reported by Song [
39]. Epidemiological evidence has revealed that moderate alcohol consumption can have beneficial effects on serum lipids, such as elevated HDL-C and decreased LDL-C [
40], lowering the TG/HDL-C ratio [
41], and that polymorphisms in alcohol-metabolizing enzymes can modify the association between alcohol intake and serum lipids [
42], which may reduce the risk of CVD events [
43]. However, specific levels of alcohol consumed were lacking in our study; therefore, the influence of drinking on serum lipids must be investigated further. It is recommended that people with chronic diseases or unhealthy lifestyles should be continuously monitored and managed, to effectively control dyslipidemia as much as possible.
There are several potential limitations in our study. First, a cross-sectional design rather than prospective cohort was used; thus, only associations rather than causal relationships could be determined. Second, given a lack of detailed data of dietary, alcohol consumption, physical activity, and family history of dyslipidemia, the associations of these with serum lipids could not be examined in the current study. Third, it is difficult to avoid recall bias and unmeasured confounding owing to collection of information using a questionnaire and measurement. Furthermore, a non-representative sample was used in the study. Therefore, it is not fully generalizable to all adults aged 35 years or above in northern China.
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