Introduction

According to a recent World Health Organization (WHO) report, of 56 million deaths globally in 2012, 38 million (approximately 68 %) resulted from non-communicable diseases (NCDs), which have become a serious global public health burden [14]. Generally, cardiovascular disease (CVD) is the most prominent worldwide. Almost all NCDs are associated with both environmental and genetic risk factors [5].

Epidemiology studies, especially in migrant groups, can help identify the influence of environmental and genetic factors on CVD risk. Studies of migrant populations can compare the CVD characteristics of a population in their original location, and trace a migrant group through several generations in the new host country [6, 7]. Migration may lead to substantial changes in the natural environment, lifestyle, eating habits, physical activity, education, income, and medical care of migrants and their offspring, possibly resulting in major changes in epidemiological patterns of CVD and risk factors [8]. Comparison of the changes in CVD prevalence, mortality, and risk factors among immigrants and native people can also provide a unique view of the role of environmental and genetic factors on CVD development.

China has the largest population in the world, comprising about one-fifth of the global population, and has also become the one of the largest migrant-exporting countries. During the past century, more than 50 million ethnic Chinese moved overseas [9, 10]. The epidemiological pattern of CVD in mainland China has been clearly documented, and is characterized by a relatively low prevalence of coronary heart disease (CHD), but a high prevalence of stroke [1114]. It is still unclear whether the epidemiology pattern and risk factors of CVD among Chinese immigrants have changed or not. Several reviews on CVD in migrants have focused on immigrant groups that came from different countries [8, 1517]. There is still a lack of review focus on the information summary about the epidemiological features of CVD status among Chinese immigrant groups. This systematic review aims to focus on the epidemiology of CVD specifically in Chinese immigrant groups, and to compare the difference in the major CVD risk factors including diabetes, hypertension, dyslipidemia, obesity and smoking between Chinese immigrants and mainland Chinese or other ethnic groups in the new host country.

Methods

Search methodology

The PubMed database was used for searching for previously published studies from 1966 to the present time. The following search terms were used: ‘non-communicable disease’, ‘cardiovascular disease’, ‘stroke’, ‘cerebrovascular disease’, ‘coronary heart disease’, ‘ischemic heart disease’, ‘myocardial infarction’, ‘diabetes’, ‘hypertension’ AND ‘epidemiology’, ‘prevalence’, ‘mortality’, ‘risk factor’ AND ‘migration’, ‘immigration’, ‘emigration’, ‘immigrant’, ‘emigrant’ AND ‘China’, ‘Chinese’. The above search was limited to papers published in English. The China National Knowledge Infrastructure (CNKI) database was also used to search for papers published in Chinese using the same key words. A manual search of the reference lists of the articles and reviews was conducted to identify further key publications.

Study inclusion criteria

Studies were included if they met the following criteria: (1) the study presents original data on the prevalence or mortality of CVDs, such as CHD, stroke, or their related risk factors (diabetes, hypertension, dyslipidemia, obesity and smoking); (2) the study population includes first generation immigrants of Chinese origin with or without their offspring; (3) the study contains a comparison group of their own ethnicity in their country of origin, or of other ethnicity in the host country.

Study exclusion criteria

A study was excluded if the sample size is <100, or if it is a duplicate publication. In the latter case, only one article with the most comprehensive data on a particular study is included.

Results and discussion

Study characteristics

The database search identified 341 English papers in PubMed and 45 Chinese publications in the CNKI. After assessing the titles and abstracts, 313 studies met the exclusion criteria, and the full texts of the remaining 73 articles were examined. Sixteen articles met the inclusion criteria, and are included in the systematic review [1833].

Data on 258,474 Chinese migrants are reported in the 16 studies. These surveys were mainly conducted in the United States [21, 22, 24, 29, 30, 32], Canada [18, 20, 23, 27], the United Kingdom [19, 26], and Mauritius [25, 28], and there are also two studies of migrants within mainland China [31, 33].

Stroke

Stroke has become the leading cause of death in China after the rapid changes in health status from 1990 to 2010 [3]. The recent China National Stroke Prevention Project (CSPP) reports that the standardized prevalence rate of stroke is 1.9 %, whereas the mean prevalence of stroke globally is only 0.5–0.6 % [34, 35].

Table 1 presents the general characteristics of four studies of stroke prevalence in Chinese immigrants [1821]. Two studies conducted by Anand et al. [18] and Chiu et al. [20] investigated the epidemiologic characteristics of stroke among Chinese immigrants and other ethnic groups living in Canada. Their results show that Chinese immigrants have the lowest prevalence of stroke (0.3–0.6 %), compared with immigrants from South Asia (0.6–1.7 %) and Europe (1.8 %). Another study conducted in England by Zaninotto et al. reports that the age-standardized prevalence of stroke is significantly lower among Chinese men and women (0.9 and 0.3 %, respectively) compared with other ethnic groups, such as Indians (men 1.2 %; women 0.9 %) and Irish (men 1.8 %; women 1.4 %) [19]. Corlin et al. also report similar findings in Chinese immigrants and resident Whites in the United States (1.4 and 6.0 %, respectively) [21].

Table 1 Characteristics of seven studies of stroke prevalence and mortality among Chinese immigrants

Another three studies report stroke-related mortality in Chinese immigrants (Table 1) [2224]. In 2014, Jose et al. published a study of stroke mortality among Chinese, Asian Indian, Japanese, and Whites in the United States [24]. The age-standardized mortality among men and women of Chinese origin is 47.39 and 41.99 per 100,000, respectively, and is markedly lower than in those of Japanese origin (men 56.57 and women 44.82 per 100,000) or in resident Whites (men 50.30 and women 55.26 per 100,000). It is also significantly lower than the stroke-related mortality among Chinese in mainland China (men 139.12 and women 111.62 per 100,000) based on the data of the China Health and Family Planning Statistics Yearbook in 2014 [36].

These studies suggest that after migrating to Western developed countries, the prevalence and mortality of stroke among Chinese immigrants are much lower than the rates in the population remaining in mainland China. The CSPP study reports that 54 % of stroke patients in China are eligible for antihypertensive, statin and antiplatelet medications, but that triple agents are used in only 6.9 % of stroke patients [34]. The REACH Registry enrolled Chinese stroke patients aged 45 years or older in mainland China, Western Europe, and North America, and finds that the use of triple agents is highest among Chinese in Western Europe (53.8 %), and in North America (51.8 %) [37]. Because of the better medical service and availability of stroke drug therapy in Western countries, there is a significant benefit for the primary and secondary prevention of stroke, which is reflected in the marked reduction in the prevalence and mortality of stroke after migration.

Coronary heart disease

In the 2008 National Health Services Survey in China, the prevalence of CHD is 0.77 % (urban 1.59 %; rural 0.48 %) [38]. The World Health Organization MONICA study reports that there is a lower incidence of CHD in China compared with that in Western countries [11, 12].

Of the 16 eligible articles, six report data on the prevalence of CHD in Chinese immigrants (Table 2) [1820, 2527]. The studies of Chiu et al. and Wang et al. analyzed the prevalence of CHD in 3038 and 2008 Chinese immigrants, respectively, and in other ethnic groups living in Canada [20, 27]. The age- and sex- standardized prevalence of CHD ranges from a low level of 3.1–3.2 % in Chinese immigrants to 4.8–5.1 % in Canadians and 4.3–5.2 % in South Asian immigrants. Another study conducted in the United Kingdom by Harland et al. also reveals that the prevalence of CHD is significantly lower in Chinese immigrants than in European men (4.9 vs 16.6 %, P < 0.001) [26]. However, Li et al. find that the prevalence of CHD is several times higher in Chinese immigrants living in Mauritius than in those living in Beijing (24.3 vs 4.0 %), but is similar to the prevalence in Asian Indians and Creoles in Mauritius (24.5 %) [25]. Although the reported prevalence of CHD among Chinese immigrants varies in different studies, all show that CHD prevalence is higher among Chinese migrants than among mainland Chinese.

Table 2 Characteristics of nine studies of the prevalence and mortality of coronary heart disease among Chinese immigrants

Another three studies report CHD mortality in Chinese immigrants (Table 2) [2224]. Fang et al. report that CHD mortality of Chinese immigrants in New York City is substantially lower than that of Whites (men: 61.0 vs 161.7 per 100,000; women: 56.1 vs 143.9 per 100,000), but is significantly higher than that of Chinese in mainland China (men 45.4 and women 46.1 per 100,000) [22]. Another two studies also report similar results [22, 23].

In six studies reporting the prevalence or mortality of both CHD and stroke, the epidemiological pattern of CVD changes among Chinese immigrants [1820, 2224]. Chinese people traditionally have a very high level of stroke, but a low prevalence of CHD compared with other races, but after migrating to Western developed countries, the prevalence and mortality of stroke among Chinese immigrants are much lower than the rates in the population of mainland China [1114]. For CHD, however, Chinese immigrants have a higher prevalence and mortality of CHD than mainland Chinese, but these rates are still lower than the rates in immigrants of other ethnicities. These studies support the hypothesis that the shift in the epidemiological pattern of CVD among Chinese immigrants might be ascribed to the changes in the living environment and related lifestyle factors after immigration.

Risk factors

Diabetes mellitus

In this review, 9 of the 16 selected articles present data on diabetes prevalence or mortality in Chinese immigrants as shown in Table 3 [1921, 23, 2731]. Two studies of Kandula et al. and Veeranna et al. both using the data of the MESA study in the United States, reveal that the prevalence of diabetes among Chinese Americans was 12.7–13.3 % during 2000–2002 [29, 30]. Whereas, the Inter ASIA study by Hu et al. in 2001 using the same diagnostic criteria, reports that the age-standardized prevalence of diabetes is only 5.49 % in mainland Chinese, which indicates that the prevalence of diabetes in Chinese immigrants is higher than that in mainland China [39, 40].

Table 3 Characteristics of nine studies of diabetes mellitus prevalence and mortality among Chinese immigrants

However, other two studies in Canada show that the prevalence in Chinese Canadians is 4.26–4.30 %, and a study of Zaninotto et al. in the United Kingdom also reveals that the age-standardized prevalence of diabetes is only 4.0 % for men and 3.9 % for women in Chinese immigrants [19, 20, 27]. Given the discrepant results among studies, they might be ascribed to the different methods for diabetes diagnosis used to estimate the prevalence of diabetes. Diabetes in these studies was all defined by participants who self-reported without blood glucose testing, which may significantly underestimate the prevalence of diabetes among Chinese immigrants [19, 20, 27].

Diabetes is often asymptomatic in its early stages, and can remain undiagnosed for several years until significant complications appear [4143]. A national study by Yang et al. in 2010 reveals that the prevalence of undiagnosed diabetes reaches up to 60.7 % in China (men 61.3 %, women 59.8 %) [44]. Another study in the United Kingdom by Unwin et al. in 1997 also shows that the Chinese immigrants have a higher prevalence of undiagnosed of diabetes compared with Europeans living in United Kingdom (62.5 vs 57.7 %) [45]. The high prevalence of undiagnosed diabetes is probably to be linked to the low prevalence of diabetes in Chinese immigrants. It also indicates the lack of diabetes awareness and shortage of medical treatment in both Chinese immigrants and mainland Chinese. Therefore, early diagnosis and treatment for diabetes are highly needed for the prevention of diabetes.

Hypertension

Hypertension is one of the most common CVDs, and is the most important and modifiable risk factor for CHD and stroke [46]. From data of the Chinese National Nutrition and Health Survey, the overall prevalence of hypertension is estimated as 18.8 % among Chinese aged ≥18 years [47]. Table 4 lists eight studies of hypertension prevalence or mortality among Chinese immigrants [1921, 24, 27, 30, 32, 33]. The Health Survey for England reports that the age- and sex-standardized prevalence of hypertension is lowest in Chinese immigrants (men 20.2 %; women 25.0 %), higher in Irish immigrants (men 27.7 %; women 25.3 %), and highest in Black Caribbean immigrants (men 31.5 %; women 33.6 %) [19]. Moran et al. also find that the age-adjusted prevalence of hypertension is 35.2 % in Chinese immigrants living in the United States, and is similar to that in native Whites (36.1 %) [32]. They further show that for people born outside the United States, each 10-year increment in residence in the United States is associated with a higher prevalence of hypertension. Compared to participants living in the United States for 10 years or less, the adjusted odds ratio of hypertension is 1.26 (95 % confidence interval 1.05–1.51) for participants who lived in United States over 30 years after adjusting for age, gender, socioeconomic status and other risk factors of hypertension.

Table 4 Characteristics of eight studies of hypertension prevalence and mortality among Chinese immigrants

These studies suggest that the prevalence of hypertension among Chinese immigrants is significantly higher than in mainland Chinese. Moreover, Hsu et al. report that 52 % of Chinese immigrants comply with antihypertensive medication, and the MESA study shows that only 39 % of immigrant Chinese with hypertension have adequate treatment and control of the condition [48, 49].

Dyslipidemia

Elevated serum cholesterol is also one of the most important modifiable risk factors for CVD [46]. The China National Diabetes and Metabolic Disorders Study conducted during 2007–2008, reports that the age- and sex-standardized mean levels of total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), and triglycerides (TG) are 4.72, 2.68, 1.30, and 1.57 mmol/L, respectively, in a general Chinese population aged ≥20 years. The age-standardized prevalence of high TC (≥6.22 mmol/L) is 9.0 % [50].

Studies of Chinese immigrants in Melbourne, Australia and in California, United States finds that the cholesterol level is 5.50–5.59 and 5.30–5.33 mmol/L among Chinese men and women, respectively, and is significantly higher than the level in mainland Chinese (men 4.70 mmol/L; women 4.73 mmol/L) [5052]. The SHARE study further reveals that the mean LDL-C and triglyceride levels among Chinese immigrants in Canada are 3.14 and 1.78 mmol/L, respectively, and also higher than that in mainland Chinese, while the level of HDL-C is lower (1.19 vs 1.30 mmol/L) [18]. These studies suggest that serum TC and LDL-C levels increase in Chinese immigrants, likely related to adverse changes in lifestyle after migrating to developed countries.

Obesity

In general, Chinese have a low prevalence of overweight or obesity. Chiu et al. find that the mean body mass index (BMI) is lowest among Chinese immigrants (22.3 kg/m2), followed by South Asian (24.2 kg/m2), White (25.3 kg/m2), and Black populations (25.5 kg/m2) in Canada [20]. Two other studies also find that Chinese immigrants have a lower adjusted mean BMI and waist circumference than other ethnic groups living in United States [53, 54]. However, there is evidence to suggest that changes in socioeconomic conditions and urbanization in China have resulted in a rapid rise in obesity levels, as in Chinese immigrants [55, 56]. The prevalence of obesity among Chinese immigrants increases with longer duration of residence overseas [57]. In addition, Zhou et al. and Au et al. find that nearly one-third of Chinese American children aged 6–11 years are overweight or obese [58, 59]. Chinese immigrants in the United States carry higher risks of being overweight or having obesity compared with European immigrants. Moreover, Chinese individuals with the same BMI as Whites are at a higher risk of developing CVD and diabetes [60, 61].

The transition in dietary patterns may significantly influence the development of obesity. Lv et al. suggest that Chinese Americans increase the frequency of consumption of Western foods (more fats, sugars and soft drinks), resulting in a reduction in intake of traditional Chinese foods after migration [62]. A recent review published in 2014, reports that Chinese immigrants appear to adopt poor dietary habits and inactive lifestyles since moving to the United States [63]. Their traditional diet of vegetables, meats, and whole grains appears to have been replaced by readily available processed, high fat, and sugary food.

Smoking

The Community Assessment of Freeway Exposure and Health study in the United States finds that Chinese immigrants are less likely to be current smokers or to be exposed to cigarette smoke than American whites [odds ratio (95 % confidence interval) 0.31 (0.16–0.60) and 0.34 (0.18–0.66), respectively] [21]. However, the smoking trends increase with acculturation to the host country. For males, the smoking prevalence increases from 52.9 % in China to 70.0 % in the United States [21, 64]. The California Tobacco Survey and the California Youth Tobacco Survey show that acculturation is significantly associated with smoking onset among Chinese Americans [65, 66]. Although Chinese American adolescents have a lower level of smoking than White adolescents, the prevalence in Chinese Americans continues to rise after 15 years of age, in contrast to a decline in Whites. Thus, tobacco prevention policies, strategies and programs should be targeted to specific immigrant populations and to take acculturation factors into account.

Conclusions

This systematic review and summary of the available published studies regarding the changes of epidemiology of CVD among Chinese immigrants establishes that the prevalence and mortality of stroke among Chinese migrants are much lower than the rates in the native population of mainland China, but there is a higher prevalence and mortality of CHD in Chinese migrants. The prevalence of CVD risk factors is also markedly different in Chinese immigrants with a higher prevalence of diabetes and hypertension, higher serum cholesterol, poorer dietary patterns, and higher prevalence of obesity and smoking when compared with mainland Chinese, indicating that a less healthy lifestyle related environmental factor after immigration may be a major trigger in the adverse CVD status of Chinese immigrants. Therefore, it is very important for policy-makers to pay more attention to specific minority immigrant groups, and to formulate more effective preventive measures to improve the health of these populations.