Ethnicity, Migration and T2DM

With the epidemiological transition and increase in global mobility throughout the last 25 years (Fig 1), the impact of ethnicity, international migration and national rural-urban migration upon the development and progression of type 2 diabetes has gained considerable attention. Particularly in health care systems that are fundamentally based on a principle of equity and equal access to health care, it has been a priority to identify groups with particular susceptibility to disease and try to understand what underlies this particular risk among subpopulations in the society.

Figure 1 - Global number of migrants in 1990, 2000, 2010 and 2013. Adapted from: United Nations, Department of Economic and Social Affairs (2013).
Figure 1 - Global number of migrants in 1990, 2000, 2010 and 2013. Adapted from: United Nations, Department of Economic and Social Affairs (2013).
It is well established that diabetes prevalence differs across ethnic groups and across countries [1][2][3][4][5]. See also the entry on Ethnic aspects of epidemiology. With no common agreement in medical research as to the definition of both ethnicity and of being a migrant, it is challenging to point out the underlying drivers of diabetes prevalence differences. However, work building upon the classic contribution of Kelly West (1925-1980) has demonstrated that genetic-, lifestyle-, environmental- and cultural factors could all be at play. Also national organisation and quality of health care services may influence the relation between ethnicity and disease patterns, although obviously not a direct measure as such.

Unfortunately, there is little evidence from longitudinal population-based studies of incidence of diabetes and complications comparing different ethnic groups and for migrant populations. Most studies have calculated incidence from prevalence data in cross-sectional studies, and moreover, many studies in this field share methodological problems related to selection bias due to under-representation of ethnic minorities.

Ethnic differences in diabetes prevalence

The highest absolute number of people with diabetes is found in the Western Pacific region where 138 million people are estimated to live with diabetes. The highest prevalence of diabetes in adulthood is found in the Middle East and North Africa Region (10.9%) followed by prevalence estimates of 9.6% and 9.2% in the Caribbean Region and in the Central and South America Region [6]. A number of studies have shown that Asian Indian ethnicity is associated with high burden of diabetes compared with western populations [7]. While the absolute numbers with diabetes and prevalence estimates are low in Sub-Saharan Africa, diabetes incidence is expected to increase rapidly in this region with a doubling by 2035 due to growth in wealth and increasing life expectancy.

These ethnic differences in diabetes prevalence presumably occur as a consequence of a rapid socio-economic transition accompanied by decreased rates of physical activity and an unhealthy diet in susceptible populations. Increased susceptibility in some populations appears as a consequence of genetic factors as well as epigenetic factors. Many studies have shown that e.g. persons of Asian Indian origin develop diabetes at a lower level of obesity that Europeans [8][9]. Furthermore, the underlying pathophysiology seems to differ with insulin deficiency rather than peripheral insulin resistances being the main contributor [10].

Migration and diabetes

Figure 2. Conceptual framework of the potential pathways between migration and disease during migration.  Adapted from: Kristiansen et al. Sundhedsmæssige konsekvenser af migration, Ugeskrift for læger 168/36, 2006 (11)
Figure 2. Conceptual framework of the potential pathways between migration and disease during migration. Adapted from: Kristiansen et al. Sundhedsmæssige konsekvenser af migration, Ugeskrift for læger 168/36, 2006 (11)
The relation between migration status and disease pattern is complex as it may be affected by underlying factors related to their country of origin, their new host country, and possibly also by the migration process itself (fig 2 [11]). It is therefore not surprising that people who migrate from their country of origin to a new host country are also commonly found to have a higher diabetes prevalence compared to the population in the new host country [12][13][14][15].

In one of the few prospective incidence studies on ethnicity and diabetes, 2,646 African Americans and 9,461 White middle-aged adults in the US were followed for 9 years and incident self-reported and measured diabetes was measured. African American men and to a lesser degree women had a significantly higher incidence of type 2 diabetes compared with white men and women. Furthermore, the authors showed that the elevated incidence to a large extent was explained Figure 3. Cumulative incidence of diabetes during 20 years of follow-up. Tillin et al, Diabetes Care 36:383–393, 2013 (Image used with permission from American Diabetes Association).
Figure 3. Cumulative incidence of diabetes during 20 years of follow-up. Tillin et al, Diabetes Care 36:383–393, 2013 (Image used with permission from American Diabetes Association).
by modifiable risk factors and in particular adiposity [16]. A 20 year longitudinal follow-up was performed of a total of 839 Indian Asian and 335 African Caribbean first generation migrants residing in the UK and compared with 1354 Europeans. Tillin et al. found an almost 3 fold excess incidence of type 2 diabetes in the Indian Asian population and a more than 2 fold excess incidence among the African Caribbeans. Notably, they found that for women but not for men, the increased type 2 diabetes risk was explained by baseline insulin resistance and truncal obesity [17] (Figure 3).

This may reflect not only differences in underlying diabetes risk in the population of origin, but also factors related to migration itself, to social and lifestyle factors and to quality of diabetes care in the immigration countries (Figure 2) [11].

Indigenous people

Diabetes incidence rates are several times higher among Indigenous peoples compared to the general population of countries colonized from elsewhere, and populations such as Australian Aborigines, the Indian and Metis populations of Canada and Arctic Inuit populations are diagnosed with type 2 diabetes at a younger age than western comparison populations. Genetic factors may play a key role. Many Indigenous peoples have survived in relatively small subpopulations and have likely passed through a series of bottlenecks affecting population size.

These populations will be highly genetically differentiated from all large populations, such as Europeans or East Asian, and a strong effect of genetic drift, may suggest that causal variants which are very rare in other populations, could be segregating at high frequency among Indigenous people. For instance, Moltke et al found a common variant that explains more than 10% of all type 2 diabetes cases in Greenland [18]. However, while genetic factors are important in the epidemic of type 2 diabetes among Indigenous peoples, its rapid appearance over a few decades in genetically diverse populations is likely the result of an interaction of local genetic mutations with numerous social stressors and lifestyle factors.

Ethnic differences in diabetes complications

Most studies show that diabetes patients from Asia, Middle East and Sub-Saharan Africa are at higher risk of particularly microvascular complications from eyes [19], kidney and nerve system compared to western populations. This applies also to ethnic minority groups of same origin in western countries from the same regions. E.g. minority ethnic communities with type 2 diabetes in the UK are more prone to diabetic retinopathy, including sight-threatening retinopathy and maculopathy compared to white Europeans [20].

A nationwide Danish study has revealed a higher incidence of coronary heart disease among migrants from the Middle East compared to Danish patients with diabetes, also when adjusting for differences in glycaemic control, cardiovascular risk factors and smoking (figure 4).

Figure 4. Hazard ratio* for coronary heart disease among migrant groups in Denmark compared with native born Danes with Diabetes. Andersen G et al, 2014, Unpublished. * Adjusted for diabetes duration, age, type of diabetes, BMI and smoking
Figure 4. Hazard ratio* for coronary heart disease among migrant groups in Denmark compared with native born Danes with Diabetes. Andersen G et al, 2014, Unpublished. * Adjusted for diabetes duration, age, type of diabetes, BMI and smoking
Some studies point towards a genetic explanation of differences in the underlying complication pathogenesis. However, in the few studies where socio-economic factors and quality of diabetes care have been taken into account, ethnic differences in complication rates have generally disappeared. Lower achievement of treatment goals and lower screening rates for diabetes complications among ethnic minority groups with diabetes have been observed in several studies [21].

The mortality paradox

A paradoxical finding in many studies is that most migrant groups tend to have significant lower mortality rates despite higher incidence of diabetes and long-term complications. The cause of this phenomenon is poorly understood. Lower rates of smoking may counterbalance the increased diabetes-associated mortality and explain much of the paradox. Two theories have been employed; “the salmon bias hypothesis” (a possible tendency for sick immigrants to return to their home country before death) or the “healthy migrant effect” (suggesting migration to be a selective process favouring healthy individuals), but seem not to fully explain this paradox [22]. However, some believe that there is no Mortality Paradox, and that inaccurate counting of deaths among migrants leads to an underestimate of mortality in ethnic minority groups [23].

References

  1. ^ Venkataraman R, Nanda NC, Baweja G, et al. Prevalence of diabetes mellitus and related conditions in Asian Indians living in the United States. Am. J. Cardiol. 2004;94(7):977–980.

  2. ^ Maskarinec G, Grandinetti A, Matsuura G, et al. Diabetes Prevalence and Body Mass Index Differ by Ethnicity: The Multiethnic Cohort. Ethn. Dis. 2009;19(1):49–55.

  3. ^ Agyemang C, Kunst AE, Bhopal R, et al. Diabetes prevalence in populations of South Asian Indian and African origins: a comparison of England and the Netherlands. Epidemiol. Camb. Mass. 2011;22(4):563–567.

  4. ^ Misra R, Patel T, Kotha P, et al. Prevalence of diabetes, metabolic syndrome, and cardiovascular risk factors in US Asian Indians: results from a national study. J. Diabetes Complications. 2010;24(3):145–153.

  5. ^ Kanaya AM, Herrington D, Vittinghoff E, et al. Understanding the High Prevalence of Diabetes in U.S. South Asians Compared With Four Racial/Ethnic Groups: The MASALA and MESA Studies. Diabetes Care. 2014;37(6):1621–1628.

  6. ^ International Diabetes Federation. IDF Diabetes Atlas, 6th edn. Brussels, Belgium: International Diabetes Federation, 2013. http://www.idf.org/diabetesatlas

  7. ^ Garduño-Diaz SD, Khokhar S. Prevalence, risk factors and complications associated with type 2 diabetes in migrant South Asians. Diabetes Metab. Res. Rev. 2012;28(1):6–24.

  8. ^ Ramachandran A, Chamukuttan S, Shetty SA, et al. Obesity in Asia – is it different from rest of the world. Diabetes Metab. Res. Rev. 2012;28:47–51.

  9. ^ Choi SE, Chow VH, Chung SJ, et al. Do Risk Factors Explain the Increased Prevalence of Type 2 Diabetes Among California Asian Adults? J. Immigr. Minor. Health. 2010;13(5):803–808.

  10. ^ Staimez LR, Weber MB, Ranjani H, Ali MK, Echouffo-Tcheugui JB, Phillips LS, Mohan V, Narayan KM. Evidence of reduced β-cell function in Asian Indians with mild dysglycemia. Diabetes Care. 2013 Sep;36(9):2772-8. doi: 10.2337/dc12-2290.

  11. ^ Kristiansen M, Mygind A, Krasnik A. Sundhedsmæssige konsekvenser af migration. Ugeskr. Laeger. 2006;168(36):3006.

  12. ^ Kristensen JK, Bak JF, Wittrup I, et al. Diabetes prevalence and quality of diabetes care among Lebanese or Turkish immigrants compared to a native Danish population. Prim. Care Diabetes. 2007;1(3):159–165.

  13. ^ Ujcic-Voortman JK, Schram MT, Jacobs-van der Bruggen MA, et al. Diabetes prevalence and risk factors among ethnic minorities. Eur. J. Public Health. 2009;19(5):511–515.

  14. ^ Jenum AK, Diep LM, Holmboe-Ottesen G, et al. Diabetes susceptibility in ethnic minority groups from Turkey, Vietnam, Sri Lanka and Pakistan compared with Norwegians - the association with adiposity is strongest for ethnic minority women. BMC Public Health. 2012;12:150.

  15. ^ Misra A, Ganda OP. Migration and its impact on adiposity and type 2 diabetes. Nutr. Burbank Los Angel. Cty. Calif. 2007;23(9):696–708.

  16. ^ Brancati FL, Kao WH, Folsom AR, et al. Incident type 2 diabetes mellitus in African American and white adults: the Atherosclerosis Risk in Communities Study. JAMA J. Am. Med. Assoc. 2000;283(17):2253–2259.

  17. ^ Tillin T, Hughes AD, Godsland IF, et al. Insulin Resistance and Truncal Obesity as Important Determinants of the Greater Incidence of Diabetes in Indian Asians and African Caribbeans Compared With Europeans The Southall And Brent REvisited (SABRE) cohort. Diabetes Care. 2013;36(2):383–393.

  18. ^ Moltke I, Grarup N, Jørgensen ME et al. A common Greenlandic TBC1D4 variant confers insulin resistance and type 2 diabetes. Nature. 2014 Aug 14;512(7513):190-3. doi: 10.1038/nature13425

  19. ^ Sivaprasad S, Gupta B, Gulliford MC, et al. Ethnic variations in the prevalence of diabetic retinopathy in people with diabetes attending screening in the United Kingdom (DRIVE UK). PloS One. 2012;7(3):e32182.

  20. ^ Davis TME, Coleman RL, Holman RR, et al. Ethnicity and long-term vascular outcomes in Type 2 diabetes: a prospective observational study (UKPDS 83). Diabet. Med. 2014;31(2):200–207.

  21. ^ Lanting LC, Joung IMA, Mackenbach JP, et al. Ethnic differences in mortality, end-stage complications, and quality of care among diabetic patients: a review. Diabetes Care. 2005;28(9):2280–2288.

  22. ^ Norredam M, Agyemang C, Hoejbjerg Hansen OK, et al. Duration of residence and disease occurrence among refugees and family reunited immigrants: test of the “healthy migrant effect” hypothesis. Trop. Med. Int. Health TM IH. 2014;19(8):958–967.

  23. ^ Weitoft GR, Gullberg A, Hjern A, et al. Mortality statistics in immigrant research: method for adjusting underestimation of mortality. Int. J. Epidemiol. 1999;28(4):756–763.

Comments

Nobody has commented on this article

Commenting is only available for registered Diapedia users. Please log in or register first.