Review articleInsulin resistance syndrome (metabolic syndrome) and obesity in Asian Indians: evidence and implications☆
Introduction
Higher cardiovascular mortality1, 2, 3 and higher prevalence of metabolic syndrome4, 5, 6, 7 have been recorded in Asian Indians settled in various countries as compared with other ethnic groups. Urban India is in a “second stage” of epidemiologic transition, accumulating a high burden of non-communicable diseases.8, 9 Recent reports have indicated that prevalence rates for coronary heart disease (CHD) and diabetes mellitus have risen sharply in India.10, 11, 12
Because Asian Indians have settled in several countries, it is important that they are labeled uniformly for academic research. People belonging to heterogeneous ethnicities from various countries in the Indian subcontinent (India, Pakistan, Sri Lanka, Bangladesh, Nepal, etc.) have been classified as “South Asians” and the observations have been generalized to Asian Indians.13 Other ambiguous terms such as “Asians” and “Asian and Pacific Islanders,” encompassing a wide range of ethnic groups, have been commonly used. In the following discussion we use the descriptive terms as used in the quoted studies; otherwise, the term “Asian Indians” is used.
The migrant Asian Indian populations have been increasing in several countries. In the United Kingdom, South Asians are the largest minority ethnic group.14 In Canada, Asian Indians constitute the second largest migrant ethnic group, having increased by approximately 25% during the previous 5 y.15 In the United States Asian and Pacific Islanders number 10.9 million, approximately 4% of the population in 1999.16 Asian Indians constitute approximately 12% of this population and are one of the fastest growing ethnic minorities in the United States.17 However, relatively less research has been done on many non-communicable disease-linked and ethnic-specific lifestyle factors of Asian Indians. An understanding of the issues presented in the following discussion thus will help physicians and dieticians in designing and executing proper preventive and management strategies for insulin resistance syndrome-related disorders in Asian Indians.
A literature search has been done by using the terms insulin resistance, hyperinsulinemia, metabolic syndrome, obesity, and hyperlipidemia in Asian Indians, South Asians, and Asians in the medical search databases Pubmed (National Library of Medicine, Bethesda, MD, USA), from 1966 to May 2003, and Current Contents (Institute for Scientific Information, Thomson Scientific, Philadelphia, PA, USA). A manual search of the relevant quoted references was also carried out from the retrieved articles. Databases, non-indexed publications, and Web sites of reputed medical research institutions and government health agencies in India (Indian Council of Medical Research, New Delhi; National Institute of Nutrition, Hyderabad: Department of Science and Technology, Ministry of Science and Technology, Government of India, New Delhi; and Ministry of Health, Government of India, New Delhi) were also researched. We also tabulated measures of obesity in several ethnic groups (Table I) from the relevant data. These data may not be strictly comparable; nonetheless, interesting inter-ethnic differences were observed, as discussed later.
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Clustering of risk factors, insulin resistance syndrome, and CHD
Reaven initially proposed that the coexistence of obesity, glucose intolerance, dyslipidemia, and hypertension be termed insulin resistance syndrome (metabolic syndrome, syndrome X).18 The concept of insulin resistance syndrome has been expanded to include many new associations such as procoagulant tendency and endothelial dysfunction. Debate continues as to whether it is the cause or the consequence of atherosclerosis, although several studies have shown a close association.
The Expert Panel of
Atherogenic dyslipidemia
Hyperinsulinemia and consequent insulin resistance are commonly associated with hypertriglyceridemia and low levels of high-density lipoprotein cholesterol (HDL-C).77 In addition to this combination of lipid abnormalities,6, 7, 28, 29, 31, 55, 68, 78, 79, 80, 81, 82, 83, 84 Asian Indians also have high levels of small-dense low-density lipoprotein,56, 85 and lipoprotein (a).86, 87 Interethnic comparison showed higher levels of serum triacylglycerols in adult Asian Indians,88 which manifests at
Heterogeneity of risk factors among Asian Indians and South Asians
Interestingly, average body mass index (BMI) was lower and insulin sensitivity higher in subjects living in India as compared with their siblings living in west London.96 On comparison between Asian Indians living in the United Kingdom and in India with angiographically defined CHD, fasting insulin concentrations were higher in the former.63 Further, differences in body composition and cardiovascular risk profile have been reported among South Asian populations with ancestral origins in India,
Obesity and body fat distribution
Most studies have shown that the prevalence of obesity is 2% to 15% in urban,99, 100, 101 and 0% to 6% in rural populations in India100, 102 by using the current definition (BMI > 30 kg/m2),103 but a higher prevalence of obesity has been seen in migrant Asian Indians.7 Further, there is a peculiar admixture of obesity and malnutrition in economically poor people, constituting “twin burden” of under- and overnutrition.11, 104 An increasing trend of obesity also has been seen in Asian Indian
Recent concepts in insulin resistance: implications for future research in Asian Indians
With the growing interest in the research on insulin resistance and related metabolic diseases, there is a great scope for research in susceptible ethnic groups such as Asian Indians. Clearly, there is a need for international cooperation so that data and resources might be pooled together for multicenter trials.
First, firm guidelines should be established by the World Health Organization and other international bodies for defining cutoffs for various measures of obesity, e.g., BMI, waist
Prevention and control of insulin resistance syndrome in Asian Indians
Appropriate measures should be undertaken early, particularly in high-risk groups such as those with the family history of type 2 diabetes or premature coronary heart disease. The following summarizes the recommendations based on the current knowledge;
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Therapeutic lifestyle changes should be encouraged from childhood. Improvements in obesity, regional adiposity, and insulin sensitivity occur after calorie restriction219 and regular physical activity.219, 220 Change of sedentary lifestyle to
Acknowledgements
The authors sincerely thank Mrs. Vinita Sharma (Ministry of Science and Technology, Government of India) for supporting our several research projects. They thank Dr Bela Shah, Dr. Rakesh Mittal, and Dr. Tooteja (Indian Council of Medical Research, New Delhi, India) for helpful suggestions. They appreciate help from the Ministry of Health, Government of India, the Regional Office of World Health Organization, New Delhi, the Diabetes Foundation (India), and the Center for Human Nutrition, The
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This study was supported in part by a financial grant from Science and Society Division, Department of Science and Technology, Ministry of Science and Technology, Government of India.