13.4.3.3 Diabetes in the South Asian diaspora

Increased labour demands in Europe following the Second World War led to a migration of workers from the Indian subcontinent to many parts of Europe. A further wave of migration occurred in the 1960s and 1970s because of political turmoil in East Africa. More recently, technological progress and the need for skilled labour has resulted in migration to different parts of the world, including the USA and Canada. The term ‘South Asian’ broadly refers to people of Indian, Pakistani, and Bangladeshi origin, but those from Sri Lanka and Nepal are commonly also included. Although there is considerable heterogeneity between these subgroups, they share many sociocultural factors.

It is estimated that there are more than 25 million people of South Asian origin living outside the Indian subcontinent. Prevalence of type 2 diabetes in this community is particularly high, and is rising at a faster rate than in any other ethnic group (1). In the UK, South Asians constitute about 4% of the total population and are the largest ethnic minority group. The majority live in inner-city areas, and have a very high prevalence of diabetes. Diabetes is 3- to 6-times more common in this group, with overall prevalence estimated to be around 20%—much higher than observed in the native countries (2). The prevalence of diabetes in urban India is estimated to be about 9%, and, in rural India, around 4%, although this is rapidly changing (see Chapter 13.4.3.2). High prevalence of type 2 diabetes among South Asians has also been reported in other countries in Europe, the USA, and Canada. These prevalence rates also vary within the different subgroups of South Asians, with figures being particularly high amongst people of Bangladeshi and Pakistani origin (3). Notably, diabetes also occurs around 10 years earlier in diaspora South Asians, compared with white Caucasians, and is often associated with established complications at diagnosis.

Both genetic and environmental factors are thought to contribute to this high prevalence of diabetes. Genetic studies in South Asians have been relatively few, and the considerable heterogeneity within the group adds to the difficulties in characterizing this population (4). Polymorphisms associated with increased susceptibility to type 2 diabetes identified in other populations have been replicated in South Asians. Some of these—such as the MC4R (melanocortin 4 receptor) gene, which is associated with obesity and insulin resistance—are more common in South Asians, but no gene specific to this ethnic group has yet been identified (5). The increased prevalence of diabetes may, therefore, be due to more individuals sharing the susceptibility genotype, rather than a novel gene itself. Environmental influence, on the other hand, is much stronger. Features of insulin resistance and the ‘thin–fat’ phenotype are manifest as early as infancy (6). Diets rich in saturated fat and reduced levels of physical activity have been noted in migrant South Asians (7), and contribute to increased levels of obesity, insulin resistance, and diabetes (8). Increased body mass index (BMI), central adiposity, and higher prevalence of diabetes in migrant South Asians have been reported in comparative studies involving natives and migrants. The susceptibility to diabetes, therefore, appears to be proportionate to the degree of environmental exposure.

The health needs of the migrant population are unique, and differ significantly from those of both the native country and the host country. Migration is associated with major changes in the sociocultural environment, which often influence health status (9). The conflict of adherence to old cultural values and the process of integrating with the society in the new country can be stressful and challenging. Differences in lifestyle habits of South Asians and white Europeans are well recognized. Traditional cooking practices, attitudes and barriers to exercise, large families, strictly defined gender roles, and communication difficulties are all known to contribute directly and indirectly to increased risk of diabetes.

People of South Asian ethnicity have the highest rates of cardiovascular disease worldwide, and this population is expected to account for nearly 40% of the global burden by 2020. Standardized mortality rates from cardiovascular disease in UK South Asians are 150% of those observed in local white European populations (10). Diabetes is thought to contribute significantly to this excess risk, with death rates in UK South Asian people with diabetes being 3-times greater than for individuals with diabetes in the white population (11). Similarly, higher rates of cardiovascular disease have been reported in immigrant South Asians in other countries. Cross-sectional studies comparing South Asians with other ethnic groups have identified important differences in risk profiles. Visceral obesity is common and can be present at lower BMI values. Blood pressure and total cholesterol and high-density lipoprotein levels are reportedly lower, while triglycerides and apolipoprotein B:apolipoprotein A-1 ratios are high, and abnormalities of glucose tolerance are more common (12). This ominous combination of insulin resistance and dyslipidaemia may be largely responsible for making South Asians so susceptible to cardiovascular disease. Smoking rates, on the other hand, are lower in South Asians, particularly among women. Bangladeshi males are an exception to this. Various other novel risk factors, such as low adiponectin and high homocysteine, fibrinogen, and plasminogen activator inhibitor 1 concentrations, have been postulated to contribute to the excess risk, but their role has not been proven in prospective studies. Ethnicity itself has been suggested as a possible independent risk factor, but the few studies that have addressed this question have found no such association. Indeed, much of the excess risk observed can be explained by the high burden of known risk factors (13), and optimal control of these factors remains the main therapeutic strategy.

The prevalence and severity of microvascular complications of diabetes appear to vary significantly between ethnic groups. Understanding the true prevalence of these complications is limited by differences in methodology and the small size of some studies. It is generally thought that South Asians have a particularly high susceptibility to diabetic renal disease and retinopathy, but have lower rates of lower-limb amputations (14, 15). The prevalence of microalbuminuria in migrant South Asians is estimated to be between 30–40%, and may be detectable even at ‘normal’ blood pressures (16). South Asians are reported to have a faster decline in renal function, with a greater demand for renal replacement therapy. This predisposition to microalbuminuria and renal disease is consistent with the high prevalence of cardiovascular disease in the population.

Prevalence of retinopathy varies significantly between South Asian migrants and those in native countries. Retinopathy was present in more than 30% of individuals at diagnosis in the United Kingdom Prospective Diabetes Study (UKPDS), with no difference between the ethnic groups. More recent studies in South Asians living in the UK report a much higher prevalence of retinopathy and maculopathy (around 40%) (17). The prevalence reported from studies in south India is around 17–20%. Reasons for such a high prevalence in migrants, however, are not clear. Although ethnicity may have a role, the severity of retinopathy and nephropathy appears to be determined by factors such as longer duration of diabetes and poor glycaemic and blood pressure control, emphasizing the importance of tight control of risk factors in the South Asian migrant population.

In contrast, diabetes-related amputation rates are much lower amongst South Asians than in white Europeans (15). Neuropathy has been detected in fewer South Asians at diagnosis, and the risk of foot ulceration is estimated to be one-third of that observed in white Europeans. South Asians also have less peripheral artery disease and fewer foot abnormalities, all of which may contribute to the lower risk of ulceration and amputation. Additional factors, such as the role of microvasculature and nerve fibre function that may offer protection against ulceration, are under investigation.

Studies in migrant South Asians with type 1 diabetes have been few, but temporal trends indicate a sharp rise in incidence in recent years. A study of South Asian children in the UK reported a steady rise in the incidence of type 1 diabetes between 1978 and 1998, with an average annual increase in incidence of 6.5% (18). Lower figures in native countries suggest a strong environmental effect for this rise. In contrast, autoimmune diabetes in South Asian adults is less common, and is seen in less than 2% of South Asians with diabetes.

Ethnicity alters the susceptibility to diabetes and its complications, but a major proportion of the risk is still attributable to conventional risk factors. Representation of South Asians in clinical trials is poor, and randomized controlled trials exclusively involving South Asians are scarce (19). Yet there is no evidence that South Asians respond differently to known interventions. In the UKPDS, 10% of the patients were of South Asian ethnicity, and had similar reductions in microvascular and macrovascular complications to other ethnic groups with intensive control (20). More recent trials involving South Asians confirm similar benefits. Consequently, the emphasis of management remains on tight glycaemic control and the treatment of blood pressure and dyslipidaemia. Overall, blood pressure and total cholesterol levels are generally lower in South Asians and treatment targets derived from studies in other populations do not adequately address this difference. There are currently no established ethnic-specific targets, but a target blood pressure of 125/75 mmHg, total cholesterol of 4 mmol/L or less, and an HbA_1c level of less than 6.5% (48 mmol/mol) (based on levels in low-risk rural Indian populations and recent studies) may be considered appropriate.

Despite the recognition that people of South Asian origin represent a high-risk group, traditional models of health care, including pay-for-performance initiatives, have been less successful in achieving better clinical outcomes in this population (21). The United Kingdom Asian Diabetes Study (UKADS), a large, cluster randomized controlled trial in South Asians with diabetes, evaluated a new, culturally sensitive model (22). The intervention, which comprised enhanced nursing support, community link workers, and community-based treatment protocols, resulted in improvements in blood pressure—but, interestingly, had no effect on lipids and glycaemic control. Communication and cultural factors, socioeconomic status, and poor access to health care are some of the impediments to delivering good quality health care. Novel strategies that are effective and economically viable are needed to overcome these barriers.

It is predicted that the prevalence of type 2 diabetes will increase dramatically over the next few years, and that a major proportion of this increase will be in those of South Asian origin. If these estimates are correct, the human and economic costs related to diabetes will be huge. Urgent and innovative interventions are needed to tackle this problem, and will require initiatives both at population and individual level. Early screening, education, and the adoption of a healthy lifestyle remain a key to success, and the role of health professionals in promoting these is crucial. The problem of diabetes in the South Asian diaspora can be summarized as too much disease, higher rate of complications, and too little evidence. Accordingly, the priorities for the future lie in effective prevention strategies, intensive risk factor control, and more participation in clinical trials.