Present day African Americans originated in West Africa and came to the New World during the eighteenth-century slave trade. Their present heterogeneous West African, Native American and European genetic background may be the basis for their 1.5-2-fold greater prevalence of diabetes than whites. The majority of diabetes in adults is Type 2 diabetes. Some of the predisposing factors for diabetes in blacks are similar to whites (age, sex, family history), some are different (generalized obesity, impaired glucose tolerance) and for some there are no reliable data (regional obesity, diet, physical activity). The excess prevalence of diabetes in African Americans is unexplained by the known risk factors and may be related to their specific genetic and environmental interactions.

A unique pathophysiological aspect of Type 2 diabetes in African Americans is the presence of insulin-resistant and insulin sensitive variants. In contrast, insulin-resistance predominates in many other populations such as whites and Hispanics. The insulin-sensitive variant compared to the insulin-resistant variant has a lower cardiovascular disease risk. This may explain the paradox among blacks, who, despite a high prevalence of diabetes and hypertension have lower rates of coronary artery disease. The insulin-sensitive variant, characterized by insulin deficiency, may comprise up to 30% of diabetes in some African American groups. Insulin resistance appears to be more importantly linked with the amount of visceral adipose tissue in both African American men and women with diabetes and not with subcutaneous adipose tissue. Serum triglycerides levels are also linked to visceral adipose tissue volume and liver fat content. Thus, the actual frequency of the insulin-sensitive and insulin-resistant subtypes may be related to the degree of visceral adiposity in the particular population which is likely to be both environmentally and genetically determined.

There are unusual clinical variants of Type 2 diabetes among African Americans: among those presenting with severe symptomatic hyperglycemia there is a component of pancreatic beta-cell recovery resulting in long-term remissions (>3 years) off anti-diabetic therapy without marked weight loss. In addition, adults not infrequently present with diabetic ketoacidosis yet have Type 2 diabetes: they are GAD and islet cell antibody negative, insulin-resistant, with relatively decreased insulin levels. They are obese, have family histories of diabetes and a clinical course typical of Type 2 diabetes, requiring diet, oral agents or insulin for control of hyperglycemia.

In contrast to adults, in whom Type 2 diabetes is more common than whites, among children Type 1 diabetes is much less frequent than in whites but more frequent than native Africans, in whom it is uncommon. This is thought to be due to a genetic admixture with European diabetes susceptibility genes. Among children and adolescents there is a distinct maturity-onset-diabetes of youth (MODY): acute hyperglycemic presentation, with and without diabetic ketoacidosis, variable obesity, positive family history of diabetes, absence of autoimmune markers and absence of an absolute requirement for insulin treatment.

The microvascular complications of diabetes, particularly retinopathy, nephropathy and amputations, affect African Americans disproportionately while the rates of macrovascular disease are lower.

The Epidemiology of Diabetes Mellitus. An International Perspective. Edited by Jean-Marie Ekoe, Paul Zimmet and Rhys Williams. © 2001 John Wiley & Sons Ltd.

Prevalence data for African Americans with Type 2 diabetes comes from the National Health Interview Survey (NHIS), an annual population-based interview of physician-diagnosed illnesses (1) and the Second National Health and Nutrition Examination Survey (NHANES II), 1976-80, which screened for diabetes mellitus using the 2-hour oral glucose tolerance test (2,3). NHANES II data should be interpreted with some caution because of the small numbers of African Americans who completed the oral glucose tolerance test compared to whites (n = 351 vs 3348). These epidemiologic data did not distinguish African Americans by their complex genetic background including European, West African and Native American Indian (4,5).

The age-adjusted prevalence of physician-diagnosed diabetes is comparable in African Americans and whites below the age of 45 years, 2-fold greater over age 45 years, and 16% in African Americans over age 75 years (Figure 9A.1). These

Figure 9A.1 Percentage of diagnosed diabetes among US blacks and whites, 1994, NHIS

Source: National Center for Health Statistics (1)

represent large numbers of subjects with diagnosed diabetes (Table 9A. 1). The prevalence of diabetes in adults has increased ~ 4 fold from 1963 to 1990 (Figure 9A.2) with African American women having the highest rates and largest increases (6).

NHANES II, using oral glucose tolerance testing, found nearly half of both African American (and white) individuals with diabetes in the United States are undiagnosed, suggesting a large burden of potential diabetic complications (2,7). Figure 9A.3 shows total rates of diabetes (diagnosed and undiagnosed) by race, sex and age (7). African American women have higher rates at all ages except in the oldest age group of 65-74 years. Figure 9A.4 shows the separate rates of previously and newly diagnosed diabetes and impaired glucose tolerance by age in African Americans and whites (2). The overall age standardized prevalence of diagnosed and undiagnosed diabetes in African Americans is 1.5 times whites.

0 _i_i_i_i_i_i_i_i_i_i_I'''''_i_i_i_i_i_i_i_i_i_i_i

0 _i_i_i_i_i_i_i_i_i_i_I'''''_i_i_i_i_i_i_i_i_i_i_i

Figure 9A.2 Time trends in the percentage of black and white men and women with diagnosed diabetes, US, 1963-90 (NHIS)

Source: National Center for Health Statistics (182)

Table 9A.1 Number of persons (in 1000s) with diagnosed diabetes, US 1992-94 (NHIS)

Age l992 l993 l994 !992-l994 Average

(years) Black White Black White Black White Black White

<45 2l6 l033

45-64 408 2238

65-74 367 l7l0

>75 l55 ll06

Total ll46 6087

304 ll5l 260

578 24l3 740

292 l576 242

l4l ll6l l64

l3l5 633l l406

l086 260 l090

23l4 575 2322

l572 300 l6l9

l053 l53 ll07

6025 l288 6l38

Sources: National Center for Health Statistics (1, 183, 184)

Figure 9A.3 Percentage of the population with diagnosed and undiagnosed diabetes (WHO criteria), by age, sex and race, 1976-80 (NHANES II)

Source: Reproduced from (7) by permission

Source: Reproduced from (7) by permission

Figure 9A.4 Percentage of the population with glucose intolerance—WHO criteria (diagnosed diabetes [solid line], undiagnosed diabetes [dashed line] impaired glucose tolerance [dotted line]) by race and age

Source: Reproduced from (2) by permission

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