Nature and Distribution of Diabetes
Diabetes is a metabolic disease characterized by hyperglycemia resulting from defects in insulin secretion, insulin action, or both (American Diabetes Association 2004).
Classification of Diabetes
Diabetes takes three major forms. Type 1 diabetes results from destruction of the beta cells in the pancreas, leading to absolute insulin deficiency. It usually occurs in children and young adults and requires insulin treatment. Type 2 diabetes, which accounts for approximately 85 to 95 percent of all diagnosed cases, is usually characterized by insulin resistance in which target tissues do not use insulin properly. A third type of diabetes, gestational diabetes, is first recognized during pregnancy. Other rare types of diabetes include those caused by genetic conditions (for example, maturity-onset diabetes of youths), surgery, drug use, malnutrition, infections, and other illnesses.
The Burden of Diabetes
Diabetes affects persons of all ages and races. The disease reduces both a person's quality of life and life expectancy and imposes a large economic burden on the health care system and on families.
Secular Trend and Projections
In 2003, the worldwide prevalence of diabetes was estimated at 5.1 percent among people age 20 to 79 (table 30.1). The prevalence of diabetes was higher in developed countries than in developing countries. In the developing world, the prevalence was highest in Europe and Central Asia and lowest in Sub-Saharan Africa. Some of these variations may reflect differences in the age structures and level of urbanization of the various populations. By 2025, the worldwide prevalence is projected to be 6.3 percent, a 24 percent increase compared with 2003. The largest increase in prevalence by 2025 is expected to be in East Asia and the Pacific, and the smallest in Sub-Saharan Africa. In terms of those affected, the biggest increase in the developing countries is projected to take place among adults of working age.
In 2003, 194 million people worldwide ages 20 to 79 had diabetes, and by 2025, this number is projected to increase to 333 million, a 72 percent increase (table 30.1). The developing world accounted for 141 million people with diabetes (72.5 percent of the world total) in 2003. During the same period, the number of people with diabetes is projected to double in three of the six developing regions: the Middle East and North Africa, South Asia, and Sub-Saharan Africa.
Diabetes-Related Mortality and Disability
The death rate of men with diabetes is 1.9 times the rate for men without diabetes, and the rate for women with diabetes is 2.6 times that for women without diabetes (W. L. Lee and others 2000). Premature mortality caused by diabetes results in an estimated 12 to 14 years of life lost (Manuel and Schultz 2004; Narayan and others 2003). Cardiovascular disease (CVD) causes up to 65 percent of all deaths in developed countries of people with diabetes (Geiss, Herman, and Smith 1995).
The World Health Organization (WHO) estimates that, in 2001, 959,000 deaths worldwide were caused by diabetes, accounting for 1.6 percent of all deaths, and approximately 3 percent of all deaths caused by noncommunicable diseases. More recent estimates by WHO suggest that the actual number may be triple this estimate and that about two-thirds of these deaths occur in developing countries (WHO 2004). Within the developing regions, most deaths caused by diabetes occurred in East Asia and the Pacific and the fewest in Sub-Saharan Africa (table 30.1).
Diabetes-related complications include microvascular diseases (for example, retinopathy, blindness, nephropathy, and kidney failure) and macrovascular diseases (coronary heart disease, stroke, peripheral vascular disease, and lower-extremity amputation). Those complications result in disability. In the United States, a much higher proportion of people with diabetes than of people without diabetes have physical limitations: 66 percent compared with 29 percent (Ryerson and others 2003). Disabilities are even more pronounced among older people (Gregg and others 2000).
The World Health Organization estimated that, in 2001, diabetes resulted in 19,996,000 disability-adjusted life years (DALYs) worldwide. More than 80 percent of the DALYs resulting from diabetes were in developing countries (table 30.1). East Asia and the Pacific had the largest burden, and the Middle East and North Africa had the smallest burden. DALYs resulting from diabetes increased by 250 percent worldwide from 1990 to 2001 and by 266 percent for low- and middle-income countries (Mathers and others 2000).
Economic Burden of Diabetes
Diabetes imposes large economic burdens on national health care systems and affects both national economies and individuals and their families. Direct medical costs include resources used to treat the disease. Indirect costs include lost productivity caused by morbidity, disability, and premature mortality. Intangible costs refer to the reduced quality of life for people with diabetes brought about by stress, pain, and anxiety.
Direct Medical Costs
Good data on the direct medical costs of diabetes are not available for most developing countries. Extrapolation from developed countries suggests that, in 2003, the direct costs of diabetes worldwide for people age 20 to 79 totaled at least US$129 billion and may have been as high as US$241 billion (table 30.1). In the developing world, the costs were highest in Latin America and the Caribbean and lowest in Sub-Saharan Africa. The direct health care costs of diabetes range from 2.5 to 15.0 percent of annual health care budgets, depending on local prevalence and sophistication of the treatments available (International Diabetes Federation 2003b).
Indirect and Intangible Costs
In developing countries, the indirect costs of diabetes are at least as high, or even higher, than the direct medical costs (Barcelo and others 2003). Because the largest predicted rise in the number of people with diabetes in the next three decades will be among those in the economically productive ages of 20 to 64 (King, Aubert, and Herman 1998), the future indirect costs of diabetes will be even larger than they are now.
Diabetes lowers people's quality of life in many ways, including their physical and social functioning and their perceived physical and mental well-being. With a value of 1 representing the health-related quality of life without illness and 0 representing death, people with type 2 diabetes had a value of 0.77 in the population of the United Kingdom prospective diabetes study (Clarke, Gray, and Holman 2002).
Risk Factors for Diabetes
Risk factors for diabetes vary by disease type.
Type 1 Diabetes
Type 1 diabetes is most likely a polygenic disease, and a number of potential environmental risk factors have been implicated—including dietary factors; breastfeeding; initiation of bovine milk; infectious agents (for example, enterovirus, rotavirus, and rubella); chemicals; and toxins—but the results have been inconclusive (Akerblom and Knip 1998).
Type 2 Diabetes
The risk for type 2 diabetes is higher in monozygotic twins and people with a family history of diabetes (Rich 1990). This finding strongly suggests that genetic determinants play a role, but so far few genes have been associated with type 2 diabetes.
Environmental factors include prenatal factors, obesity, physical inactivity, and dietary and socioeconomic factors (Qiao and others 2004). Exposure to diabetes in utero increases the risk of developing type 2 diabetes in early adulthood (Dabelea and others 2000). Disproportionate growth and low birthweight increase the risk of developing diabetes and insulin resistance. In the postnatal environment, breastfeeding protects against the development of obesity, insulin resistance, and diabetes (Pettitt and others 1997; Young and others 2002).
The strongest and most consistent risk factors for diabetes and insulin resistance among different populations are obesity and weight gain (Haffner 1998): for each unit increase in body mass index, the risk of diabetes increases by 12 percent (Ford, Williamson, and Liu 1997). The distribution of fat around the trunk region, or central obesity, is also a strong risk factor for diabetes (Yajnik 2001). Diabetes risk may be reduced by increasing physical activity. Conversely, a sedentary lifestyle and physical inactivity are associated with increased risks of developing diabetes (Hu and others 2003). Some studies report a positive relationship between dietary fat and diabetes, but specific types of fats and carbohydrates may be more important than total fat or carbohydrate intake. Polyunsaturated fats and long-chain omega-3 fatty acids found in fish oils (Adler and others 1994) may reduce the risk of diabetes, and saturated fats and trans fatty acids may increase the risk of diabetes (Hu, van Dam, and Liu 2001). Sugar-sweetened beverages are associated with an increased risk of diabetes (Schulze and others 2004). High intakes of dietary fiber and of vegetables may reduce the risk of diabetes (Fung and others 2002; Stevens and others 2002).
Increased affluence and Westernization have been associated with an increase in the prevalence of diabetes in many indigenous populations and in developing economies (Rowley and others 1997; Williams and others 2001). Conversely, in developed countries, those in lower socioeconomic groups have a higher risk of obesity and consequently of type 2 diabetes (Everson and others 2002). Surrogates for socioeconomic status, such as level of education attained and income (Paeratakul and others 2002; Robbins and others 2001) are inversely associated with diabetes in high-income countries.