Ethnic aspects of epidemiology
A wealth of epidemiological data have shown that the prevalence of both types of diabetes mellitus, type 2 in particular, has increased over recent years and continues to increase at an alarming rate across the entire world. Historically, diabetes first became common in affluent western populations, and then rapidly spread to other populations in response to improved living conditions and adoption of a western way of life (sometimes referred to as "Coca-colonisation"). Environmental changes related to urbanization, obesity and aging are known to play a major role in the diabetes epidemic. In an increasingly multicultural world, it has been possible to compare the frequency of diabetes in the same ethnic group in different environments, and in different ethnic groups sharing the same environment. This experience suggests that diabetes was infrequent in our ancestors and in peoples of all races living a traditional lifestyle. However, in populations exposed to similar environmental conditions, ethnic minority groups are often disproportionately affected by diabetes mellitus and its complications. Genetic differences between populations are likely to be present, but allowance must also be made for social and cultural differences and access to medical care when making such comparisons.
Ethnicity and Type 1 Diabetes
Although the worldwide prevalence of type 1 diabetes is relatively low (<1%), the incidence of type 1 differs between ethnic groups, the risk being greater in those of European origin than in those of African origin, and much less in Asian and Pacific Islanders. There is however a wide range of risk even within Europe, and some non-European populations (e.g. Kuwait) have a high incidence. The relative roles of geography and ethnicity in type 1 diabetes are discussed in more detail in our section on the Epidemiology of type 1 diabetes
Ethnicity and type 2 diabetes
Several studies on ethnic differences in people with type 2 diabetes sharing a western environment have reported higher prevalence and incidence rates among ethnic minorities. In particular, Native Americans and people of South Asian, African, Hispanic and Arab origin are disproportionally affected. In the USA, for example, approximately 14% of Hispanics and 12% of African Americans are affected by diabetes, compared to 7% of non-Hispanic whites. Furthermore, in a European study, a prevalence of 17% was reported in South Asians, as opposed to 8.2% in those of African origin and 4.2 % in Caucasians. Apart from the higher prevalence in South Asians when compared to Caucasians, type 2 diabetes is also known to develop at an earlier age and at lower BMI levels in South Asians than in Caucasians.
The explanation for ethnic differences in the prevalence and incidence of type 2 diabetes is multifactorial, complex, and only partially clear. There seem to be some ethnic differences in pathogenesis. In the UK Prospective Diabetes Study (UKPDS), subjects with type 2 diabetes from three major ethnic groups were included. Most patients (82%) were Caucasians, whereas 10 percent were of South Asian origin and 8 percent were of African origin. Insulin resistance was highest in the South Asians, followed by the Caucasians and Africans. On the other hand, initial beta-cell function was higher in the South Asians than in the Caucasians and Africans. This suggests that, although both insulin resistance and reduced insulin secretion are involved in the pathogenesis of type 2 diabetes, the predominant mechanism may differ between ethnic groups.
There are some explanations for an ethnic difference in insulin resistance and beta-cell function. For example, it is known that certain genetic polymorphisms which predispose to insulin resistance, more frequently cluster in those of South Asians and Hispanic origin than in Caucasians. Furthermore, polymorphisms of the glucokinase gene that may affect the threshold for glucose-stimulated insulin secretion have been described in Caucasians, Hispanics and African–Americans and Japanese.
In addition, obesity occurs more frequently in African and Hispanic populations than in white populations in the USA. However, the risk associated with obesity also differs between ethnic groups. For example, at a given body mass index, South Asians and, to a lesser extent, Hispanics, have a higher degree of abdominal adiposity than Caucasians and Africans, which makes them more prone to developing insulin resistance. Indeed, ethnic differences in fat distribution have been considered the major contributor to the observed excessive prevalence of insulin resistance and type diabetes in South Asians, Hispanics, and Native Americans.
Apart from biological differences, socioeconomic as well as cultural factors are an important consideration in the etiology of ethnic disparity in type 2 diabetes. Ingrained attitudes toward physical activity, obesity, and diet composition may all bear on the expression of a metabolic disorder.
It has been reported that diabetic patients from ethnic minority groups have a higher risk of developing diabetes complications. However, the literature on ethnic disparities in diabetes complications is not consistent. For example, in a review of the literature on ethnic differences in the prevalence and complications among patients with diabetes, studies conducted in the USA found an increased risk for complications in ethnic minorities whereas in many UK studies, ethnic minorities were reported to have lower or equal risk in comparison to Caucasians. The review concluded that end stage renal disease was more common among U.S. blacks and Hispanics and among U.K. Asians. Furthermore, blacks and Hispanics in the U.S. were found to have an increased risk of retinopathy. In contrast, the risk of cardiovascular disease and neuropathy was similar across ethnicities. In a study conducted in the U.K., both micro- and macrovascular complications were more prevalent in newly diagnosed South Asians than in Caucasians.
Despite similarities in weight, diabetes duration and apparently equal access to treatment, minority populations often display worse glycaemic control. This poor control is especially worrisome in light of research that demonstrates that poor glucose control contributes to the risk of micro- and macrovascular complications and mortality.
Several reasons for the disparities in diabetes control between ethnic groups have been proposed. Socioeconomic factors and lifestyle may play an important role. For example, lack of insurance to pay for medications and physician visits may prevent individuals of an ethnic/racial group from receiving treatment and studies from the USA suggest that differences in access to medical care do exist. Furthermore, variation in the intensity of treatment by the health care provider may also exist.
While ethnic differences may exist in the response to various glucose-lowering drugs, whether because of altered pharmacogenetics or possible differences in pathogenesis, there are no studies actually demonstrating a differential response to drugs.
^ Serrano-Rios M, Goday A, Martinez LT: Migrant populations and the incidence of type 1 diabetes mellitus: an overview of the literature with a focus on the Spanish-heritage countries in Latin America. Diabetes Metab Res Rev 15:113-132, 1999
^ Harris MI, Flegal KM, Cowie CC, Eberhardt MS, Goldstein DE, Little RR, Wiedmeyer HM, Byrd-Holt DD: Prevalence of diabetes, impaired fasting glucose, and impaired glucose tolerance in U.S. adults. The Third National Health and Nutrition Examination Survey, 1988-1994. Diabetes Care 21:518-524, 1998
^ UK Prospective Diabetes Study. XII: Differences between Asian, Afro-Caribbean and white Caucasian type 2 diabetic patients at diagnosis of diabetes. UK Prospective Diabetes Study Group. Diabet Med 11:670-677, 1994
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