Risk factors for cardiovascular disease in Canadians of South Asian and European origin: a pilot study of the Study of Heart Assessment and Risk in Ethnic Groups (SHARE)
Sonia S. Anand, MD, MSc
Salim Yusuf, MB, BS, DPhil
Clin Invest Med 1997;20(4):204-10.
[résumé]
From the Department of Preventive Cardiology and Therapeutics, Hamilton Civic Hospitals Research Centre, McMaster University, Hamilton, Ont.
(Original manuscript submitted Dec. 13, 1996; received in revised form May 27, 1997; accepted May 28, 1997)
Reprint requests to: Dr. Sonia S. Anand, McMaster Clinic, 2nd floor, 237 Barton St. E, Hamilton ON L8L 2X2; fax 905 521-1166
Contents
Abstract
Objective: To test the feasibility of recruitment strategies and a 2-hour cardiovascular (CV) health assessment (including laboratory tests and questionnaires) in preparation for a national, population-based study to determine CV risk factors among Canadians of different ethnic origins.
Design: Cross-sectional study of people of South Asian and European origin from Hamilton, Ont., identified by community-based stratified random sampling.
Setting: University-affiliated research clinic in Hamilton, Ont.
Participants: Thirty-one Canadians of South Asian origin and 20 Canadians of European origin 35 to 75 years of age.
Interventions: Subjects attended a clinic at which they completed a health questionnaire, provided fasting and postprandial blood samples, and underwent B-mode carotid ultrasonographic examination as well as anthropometric, nutritional and psychosocial assessments.
Results: The participants of South Asian origin had lived in Canada for 18 years, on average, compared with 48 years for those of European origin. More participants of South Asian origin were married than those of European origin, and fewer smoked or consumed alcohol. Participants of South Asian origin were more likely to have some university education. The prevalence of impaired glucose tolerance was 34.5% in the participants of South Asian origin and 9.5% in those of European origin (p < 0.04). The total cholesterol to high-density lipoprotein ratio was elevated in the participants of South Asian origin (5.1), compared with those of European origin (4.2) (p < 0.05), as was the lipoprotein (a) concentration (log transformed) (5.5 v. 4.6 mg/dL, p < 0.02).
Conclusions: This pilot study revealed intriguing lifestyle and metabolic differences between participants of South Asian and European origin. Those of South Asian origin had a higher prevalence of impaired glucose tolerance, dyslipidemia and elevated lipoprotein (a) concentrations -- factors thought to be associated with premature CVD in this group.
Résumé
Objectif : Vérifier la faisabilité de stratégies de recrutement et d'une évaluation de l'état cardiovasculaire durant 2 heures (y compris les examens de laboratoire et les questionnaires) pour préparer une étude démographique nationale afin de déterminer les facteurs de risque de maladie cardiovasculaire chez des Canadiens d'origines ethniques différentes.
Conception : Étude transversale portant sur des personnes d'origine sud-asiatique et d'origine européenne de Hamilton (Ont.), que l'on a identifiées par échantillonnage aléatoire stratifié communautaire.
Contexte : Clinique de recherche affiliée à une université de Hamilton (Ont.).
Participants : Trente et un Canadiens d'origine sud-asiatique et 20 Canadiens d'origine européenne âgés de 35 à 75 ans.
Interventions : Les sujets se sont présentés à une clinique où ils ont répondu à un questionnaire sur la santé et fourni des échantillons de sang à jeun et après un repas. Ils se sont aussi soumis à une échographie carotidienne en mode-B, ainsi qu'à des évaluations anthropométriques, nutritionnelles et psychosociales.
Résultats : Les participants d'origine sud-asiatique vivaient au Canada depuis 18 ans en moyenne, comparativement à 48 ans dans le cas des participants d'origine européenne. Les participants d'origine sud-asiatique qui étaient mariés étaient plus nombreux que ceux d'origine européenne et moins d'entre eux fumaient ou prenaient de l'alcool. Les participants d'origine sud-asiatique étaient plus susceptibles d'avoir fait des études universitaires. La prévalence d'une déficience de tolérance au glucose s'est établie à 34,5 % chez les participants d'origine sud-asiatique et à 9,5 % chez ceux d'origine européenne (p < 0,04). Le ratio du cholestérol total sur les lipoprotéines de haute densité était élevé chez les participants d'origine sud-asiatique (5,1) comparativement aux participants d'origine européenne (4,2) (p < 0,05), tout comme l'était la concentration de lipoprotéines (a) (transformation logarithmique) (5,5 c. 4,6 mg/dL, p < 0,02).
Conclusion : Cette étude pilote a révélé, sur le plan du mode de vie et du métabolisme, des différences intrigantes entre les participants d'origine sud-asiatique et d'origine européenne. Les participants d'origine sud-asiatique présentaient une prévalence plus élevée de déficience de la tolérance au glucose, de dyslipidémie et de concentration élevée de lipoprotéines (a) -- facteurs que l'on pense liés aux maladies cardiovasculaires prématurées chez les sujets de ce groupe.
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Introduction
Coronary heart disease (CHD) is the major cause of death in Canada and most developed countries.1 Several developing countries have experienced substantial gains in life expectancy. The World Bank and World Health Organization project that, by 2020, at least 50% of all deaths due to CHD will occur in these countries, especially in the Indian subcontinent.2 There is compelling evidence that South Asians -- people who originate from India, Pakistan, Bangladesh and Sri Lanka -- are at increased risk of CHD, both in their countries of origin and abroad.3,4 This high risk is not explained by established CHD risk factors, such as hypertension, hypercholesterolemia and cigarette smoking.3 Multiple studies have demonstrated that people of South Asian origin have a high prevalence of glucose intolerance, abdominal obesity and dyslipidemia after exposure to urban lifestyles (Fig. 1).5,6
In preparation for a national longitudinal study to determine cardiovascular risk factors among ethnic populations in Canada (Study of Heart Assessment and Risk in Ethnic Groups, SHARE), we conducted a pilot study in a small sample of Canadians of South Asian and European origin.
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Methods
Recruitment
A sampling frame of people of South Asian origin living in the Hamilton, Ont., and the surrounding region was created by searching for unique South Asian surnames in a compact disc compilation of public telephone directories (CD Canada 95, ProPhone Inc.). South Asian surnames are 95% sensitive and 96% specific for identification of individuals of South Asian ethnic origin.7 All households identified by this method were stratified by postal code, and systematic random sampling was employed to ensure equal sampling among geographic regions. Households with members of European origin were matched to the South Asian households by the postal code. Once a potential household was identified, a letter of introduction was mailed, and followed up by a series of 3 telephone calls 4 days later. Respondents were screened for eligibility. When 2 or more individuals from a household were eligible to participate, the member with the earliest birth date in the calendar year was selected. Consenting participants were invited to make a 2-hour clinic visit.
Eligibility criteria
People of South Asian origin were defined as people whose parents and grandparents had originated from India, Pakistan, Bangladesh or Sri Lanka. People of European origin were defined as individuals whose parents and grandparents originated from a European country. All potential participants were required to be between the ages of 35 and 75 years and to have lived in Canada for at least 10 years. Those with a chronic debilitating illness, such as cancer, renal or liver failure, were excluded.
Clinic visit
All respondents who agreed to participate were required to fast for 12 hours before the clinic visit. After written, informed consent was obtained, fasting blood samples were drawn and a random urine sample was collected. All participants who did not have diabetes mellitus ingested a 75-g oral glucose solution, and repeat blood samples were drawn 2 hours after ingestion of the solution. All participants completed a health questionnaire and underwent standard testing for blood pressure, heart rate, height, weight, and waist and hip circumference.8 A B-mode carotid ultrasonographic examination, a noninvasive method of measuring subclinical atherosclerosis,9 was performed on all participants. A dietary assessment was conducted; this consisted of a single 24-hour recall of dietary intake and instructions for completion of 4-day food records. The visit concluded with the completion of a Life Stress and Satisfaction questionnaire to assess psychological well-being. After the clinic visit, all participants were called at random periods (twice) and asked about their diet during the past 24 hours. As well, participants were required to mail their 4-day food records to the project office. All laboratory results, with a risk-factor profile commentary, were sent to each participant within 2 months of the clinic visit.
Statistical analysis
The SPSS statistical program (SPSS for Windows, Release 6.1.3, SPSS Inc., Chicago, 1995) was used for all statistical analysis. All continuous variables that did not display a normal distribution were corrected by log transformation, when appropriate. Significance testing for categorical variables was performed with the use of chi-squared analysis, and continuous variables were compared with the use of Student's t-test. When indicated, categorical variables were adjusted for age and sex with the use of logistic regression analysis and continuous variables with the use of 1-way factorial analysis of variance, with age and sex as covariates.
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Results
Recruitment
The response rate to the initial invitation to participate was 54%, with no significant difference between eligible participants of South Asian and European origin. Thirty-one participants of South Asian origin and 20 of European origin completed the clinic visit between July and August 1995. The contact, eligibility and response rates are provided in Table 1. Data on smoking, education and family history of CHD for nonrespondents, who declined to participate, were collected during the initial telephone call. Twenty-seven percent of the nonrespondents versus 33% of the respondents were smokers; 45% of the nonrespondents versus 52% of the respondents had some university education; 27% of the nonrespondents versus 30% of the respondents reported a family history of CHD; and 100% of the nonrespondents had lived in Canada longer than a year. These findings suggest that there were no major socioeconomic or health differences between the nonrespondents and the respondents.
Demographic characteristics
Intriguing demographic differences were observed between the ethnic groups (Table 2). The mean age and sex distribution in the 2 groups was similar. The participants of South Asian origin had lived in Canada for 18 years, compared with 48 years (a lifetime) for those of European origin (p < 0.001). More of the participants of South Asian origin (97%) were currently married than those of European origin (71%) (p < 0.01). The proportion of the participants of South Asian origin who had ever smoked cigarettes (16%) was significantly lower than that of the participants of European origin (65%, p < 0.0004). Self-reported alcohol use was also lower among the participants of South Asian origin than among those of European origin (47% v. 81%, p < 0.01). The prevalence of previously diagnosed hypertension or diabetes mellitus did not differ significantly between the groups.
Test results
The results of laboratory analysis are provided in Table 3. Fasting and postprandial glucose and insulin levels differed significantly between the groups, as did the homeostatic model assessment (HOMA), an index of insulin resistance calculated from the fasting insulin and glucose levels.10 The proportion of the participants of South Asian origin with impaired glucose tolerance, a precursor to diabetes mellitus, was 34.5%, compared with a prevalence of 9.5% in those of European origin (p < 0.04). A tendency toward elevated triglyceride and low high-density-lipoprotein (HDL) concentrations was observed in the participants of South Asian origin, but no difference in total cholesterol concentrations between the groups was identified. However, the total cholesterol/HDL ratio, a known predictor of future cardiovascular events,11 was elevated in the participants of South Asian origin (5.1) compared with those of European origin (4.2, p < 0.05). An impressive difference in the lipoprotein (a) concentration (log transformed) was observed between the groups (5.5 v. 4.6 mg/dL, p < 0.02), and 50% of the participants of South Asian origin had lipoprotein (a) concentrations greater than 30 mg/dL, compared with 24% of those of European origin (p < 0.003). Although no difference in intimal medial thickness, a measure of subclinical atherosclerosis, was identified by B-mode carotid ultrasonography, a qualitative difference in lumen diameter was reported by our ultrasound technicians. They observed that the participants of South Asian origin appeared to have smaller lumen diameters of their carotid arteries than those of European origin. No difference in body mass index or waist-to-hip ratio was identified.
Dietary analysis
Our qualitative analysis of dietary consumption revealed a trend toward vegetarianism among the participants of South Asian origin (10%) compared with those of European origin (0%). The participants of South Asian origin tended to consume a usual Canadian diet at breakfast and lunch -- including cereals, sandwiches and pasta -- but continued to consume their traditional foods for the main meal of the day. Specifically, the participants of South Asian origin reported greater weekly consumption of high-fat dairy products, salt and fried foods than those of European origin (Table 4).
Psychosocial assessment
The validity and reliability of the Life Stress and Satisfaction Questionnaire was determined in 14 participants of South Asian origin and in 15 of European origin, who completed the questionnaire on 2 occasions 2 weeks apart. Although the numbers were small, interesting trends were observed. It appeared that the participants of South Asian origin experienced more depression, less general well-being and more job stress than those of European origin (Table 5).
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Discussion
The SHARE pilot study was conducted in preparation for a national, population-based epidemiologic study of CVD risk factors in ethnic populations in Canada. This pilot study enabled us to (1) successfully recruit Canadians of South Asian origin from the community, (2) determine the feasibility of a 2-hour clinic visit in which laboratory tests, physical measurements and questionnaires were completed and (3) conduct preliminary validation of study instruments.
Community-based random sampling is more desirable than convenience sampling (i.e., use of volunteers) because it increases the representativeness of the respondents and minimizes the potential biases introduced by such factors as socioeconomic status. Given that our clinic visit took 2 hours to complete, required an overnight fast and was not accompanied by any reimbursement, our respondents likely represented motivated individuals who were concerned about their future health. Nevertheless, no major sociodemographic differences between respondents and nonrespondents were identified.
Although we studied only a small number of participants, this pilot data revealed striking differences in specific demographic and metabolic profiles between these 2 groups of Canadians. The participants of South Asian origin had an increased prevalence of glucose metabolic abnormalities, dyslipidemia and elevated lipoprotein (a) concentrations compared with those of European origin. Furthermore, the results of the carotid ultrasonographic examinations suggested that there is a qualitative difference in the width of the carotid arteries between the 2 groups. In the national study, currently under way, we are developing methods to standardize the measurements of intimal medial thickness for artery width.
Our dietary analysis revealed interesting qualitative differences in the consumption of high-fat dairy products, fried foods and salt between the 2 groups (Table 4). These data from the pilot study provided information we are now using to create a food-frequency questionnaire specifically for people of South Asian origin.
Although data concerning psychosocial stressors affecting immigrants from southern Asia are limited, we created and performed preliminary validation of a Life Stress and Satisfaction Questionnaire. Although the number of participants was small, we observed that, as a group, people of South Asian origin appeared to experience more job stress, more depression and worse overall emotional health than people of European origin. These exploratory hypotheses, as well as differences in responses between sexes, will be studied in more detail in the national study.
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Conclusion
People of South Asian origin represent one of the largest ethnic groups in Canada; worldwide, they account for about 2 billion people. They have a high mortality rate due to premature CHD that is not fully explained by traditional cardiac risk factors. The results of the national study under way, to which this pilot study was a precursor, will be relevant to the health needs not only of people of South Asian origin in Canada but also of people of South Asian origin in other countries and of other populations (i.e., aboriginal Canadians and people of Hispanic descent) who suffer from so-called "Syndrome X."12 This study will also help resolve fundamental questions about impaired glucose tolerance, hyperinsulinemia, lipoprotein (a), the fibrinolytic system, increased triglyceride concentrations and low HDL concentrations as well as their interrelationship in the development of atherosclerosis in men and women. Although some prevention strategies (e.g., smoking cessation and lowering blood pressure) are expected to be as useful in people of South Asian origin as in those of European origin, others (e.g., decreasing abdominal obesity, lowering glucose concentrations and modifying fibrinolytic activity) may have to be specifically developed for and targeted at this group. This study is an important step toward this goal.
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Acknowledgements
The authors would like to acknowledge the contributions of Tej Sheth and Ronak Kanani.
Dr. Anand is the recipient of a Heart and Stroke Foundation of Canada Research Fellowship and Dr. Yusuf is the recipient of a Medical Research Council of Canada Career Scientist Award.
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References
- Murray CJ. Quantifying the burden of disease: the technical basis for disability-adjusted life years. Bull World Health Organ 1994;72:429-45.
- Lopez AD. Assessing the burden of mortality from cardiovascular diseases. World Health Stat Q 1993;46:91-6.
- Jha P, Enas EA, Yusuf S. Coronary artery disease in Asian Indians: prevalence and risk factors. Asian Am Pac Islander J Health 1993;1:161-75.
- Enas EA, Yusuf S, Mehta J. Prevalence of coronary artery disease in Asian Indians. Am J Cardiol 1992;70:945-9.
- McKeigue PM, Shah B, Marmot MG. Relation of central obesity and insulin resistance with high diabetes prevalence and cardiovascular risk in South Asians. Lancet 1991;337:382-6.
- Bhatnagar D, Anand IS, Durrington PN, Patel DJ, Wander GS, Mackness MI, et al. Coronary risk factors in people from the Indian subcontinent living in west London and their siblings in India. Lancet 1995;345:405-9.
- Anand S, Gerstein H, Yusuf S. Metabolic abnormalities and dietary differences in South Asian Canadians compared to European Canadians -- Study of Heart Assessment and Risk in Ehtnic groups (SHARE) [abstract]. Can J Cardiol 1996;12:82E.
- Young TK, Gelskey DE. Is noncentral obesity metabolically benign? Implications for prevention from a population survey. JAMA 1995;274:1939-41.
- Salonen R, Salonen JT. Determinants of carotid intima-media thickness: a population based ultrasonography study in Eastern Finnish men. J Intern Med 1991;229:225-31.
- O'Rahilly S, Hattersley A, Vaag A, Gray H. Insulin resistance as the major cause of impaired glucose tolerance: A self-fulfilling prophesy? Lancet 1994;344:85-9.
- Grundy SM, Greenland P, Herd A, Huebsch JA, Jones RJ, Mitchell JH, et al. Use of computer simulation to explore analytical issues in nested casecontrol studies of cancer involving extended exposures: methods and preliminary findings. Circulation 1987;40(suppl 2):201S-8S.
- Reaven GM. Role of insulin resistance in human disease. Diabetes 1988;37:1595-607.
