3.3Factors Influencing the Prevalence of Diabetes Mellitus

Age strongly influences the prevalence of diabetes (Fig. 3.1).31,34,26 In most studies prevalence increases with age in both genders.14,21,28,34,37 Not all studies

have supported this consensus.21 The third National Health and Nutrition Examination Study (NHANES III) in the United States showed a peak prevalence for diabetes of 13.2% in those 75 years.28 As a consequence, the prevalence of diabetes is high in residents of nursing and retirement homes.37,38

Ethnicity may influence the prevalence of diabetes, although the effects of confounding variables

are difficult to unravel. In Indonesia, the prevalence rate of diabetes is lowest (1.7%).30 In European and in the US white populations, prevalence rates are 2–10%, whereas in Arab, Israeli, Indian, black,

and Hispanic American populations, the rates are 14–20%.21,30 Among Pacific Islanders of Nauru

and among Pima Indians in the United States, the prevalence rate is 50%.30,37 In the United Kingdom,

Asian men were shown to have a prevalence four times than that of white men and Asian women were shown to have a prevalence two times than that of white women.39 In the United States, the NHANES III study demonstrated that non-Hispanic blacks had a 1.6 times higher prevalence of diabetes than non-Hispanic whites and Hispanic Americans had a

1.9 times higher prevalence.28 The diversity in prevalence rates has been attributed in part to gene– environment interaction. Populations that have been relatively protected from periodic famines (e.g., white Europeans) for 15–20 generations have been hypothesized to have adapted to high-caloric diets and to have low insulin resistance, whereas populations exposed until more recently to regular periodic famines (e.g., Pacific Islanders, Native Americans, Australian Aboriginal peoples, natives of middle eastern desert regions) have retained high insulin resistance and develop DM when exposed to obesogenic environments (Table 3.2).30,40

Socioeconomic status has an inconsistent relationship with prevalence of diabetes mellitus. Lower status has been associated with higher diabetes prevalence in some studies and higher status has been associated with higher diabetes prevalence in others.26,37 The inconsistency does not appear to be

haphazard, however. Across the world, as socioeconomic status increases, so does body mass index (BMI), up to approximately 10,000–15,000 international dollars per person gross domestic product.41 As income rises further, BMI falls. In South Asia, most persons are on the rising limb of this relationship such that rising status correlates with rising obesity and more prevalent diabetes.30 In contrast, in developed countries most people are on the descending limb of income–BMI relationship such that increasing poverty is associated with obesity and secondarily an increased prevalence of diabetes.

A sedentary lifestyle is associated with a higher prevalence of diabetes. Lifestyle changes that occur when groups move from rural to urban environments has been shown to increase prevalence rates in Native Americans, Australian Aborigines, Polynesian Pacific Islanders, South Asian Indians, and East Indians (Fig. 3.3).30,37 Urban–rural gradients

of diabetes prevalence seem to be diminishing in south Asia over time.30

Obesity is an independent risk factor for diabetes, presumably through its effect of increasing insulin resistance of peripheral tissues.37 In the NHANES III study, increasing obesity was associated with increasing prevalence of diabetes after adjusting for other variables.42 Central obesity, in particular, is associated with DM. The associations of a high-fat

diet and sedentary lifestyle with prevalence of diabetes probably occur through effects on BMI.37,43

3.4Epidemiology of Diabetic Retinopathy

Diabetic retinopathy is the most common microvascular complication of diabetes mellitus and the leading cause of blindness in persons from age 20 to 74 in

the United States with 12,000–24,000 new cases of blindness per year.24,44 Data from WESDR indicate

that 1.07 and 1.3% of persons with diabetes develop, respectively, PDR and DME annually, almost all in patients older than 20 years.5 Based on the estimated US diabetic population over age 20 in 2008 of 14,456,000, an estimated 154,679 new cases of PDR and 187,928 new cases of DME will develop annually. Similar facts and estimates apply to other developed countries.19 In the United States in the year 2000, the numbers of persons with DR and vision-threatening DR were 4.1 million and 0.9 million, respectively.2 These numbers are expected to triple by the year 2050 and for those over age of 65 to quadruple.2 The primary factors responsible include increasing numbers of diabetics, increasing longevity of diabetics, and disproportionate increases in growth rates of Hispanics, in whom prevalence of DR is higher than in non-Hispanic whites and blacks.2

3.5 Diabetes and Visual Loss

Diabetes mellitus is associated with increased levels of all cause visual impairment after adjusting for the

effects of age, sex, race, educational level, blood pressure, smoking, and body mass index.9,48 The

relative risk of blindness in persons with diabetes has been reported to be 5.2 times the risk of those without diabetes.48 The prevalences of visual

impairment in adults with and without diabetes vary from 3.8 to 13% and from 1.4 to 2%, respectively.5,8,9 The prevalence rates of visual impairment by diabetes type from WESDR are shown in Table 3.3. The annual incidence of blindness was 3.3 per 100,000 population.5

Incidence rates of loss of vision were obtained in WESDR. Two categories of visual loss examined were doubling of the visual angle and blindness. There were relatively few events for the blindness

outcome and many for the doubling of visual angle outcome, thus for statistical reasons associations between risk factors and outcomes were explored for doubling of the visual angle and not for blindness. The most important results are presented in Tables 3.4 and 3.5. The incidence rates for vision loss are higher for the older onset groups than for the younger onset group. The risk factor analysis is complicated as Table 3.5 illustrates. The most salient point is that increasing age is a risk factor for visual loss in all types of diabetes. In a separate study of African Americans with type 1 diabetes, 6-year incidence rates for doubling of the visual angle and blindness were 13.5 and 0.6%, respectively; increased age was the only demographic risk factor for visual loss.50

WESDR provides information of a population that is predominantly white. In black type 1 diabetics, similar data have been reported. The 6-year incidences of doubling of visual angle and blindness were 9.8 and 0.6%, respectively.50 The risk factors were similar as well, with older age at baseline the strongest demographic predictive variable.50

3.6Prevalence and Incidence of Diabetic Retinopathy

Estimates of the prevalence of diabetic retinopathy need to be understood within the context that

retinopathy occurs in a proportion of persons who have no diabetes mellitus under various definitions of the term. Associations with such fundus changes include hypertension, serum glucose levels within the normal range, BMI, and aging. Estimates of the proportions of persons with this status range from 0.6 to 9.8% depending on

methods of retinopathy detection (higher with photographs than clinical examination).21,51–53

Fiveand ten-year incidences of retinopathy in persons without diabetes have been reported to be 6.0–10% and 6.5% in independent studies, respectively.53–55

Some studies do not distinguish DR by diabetes type. In a population-based study of this type in adults over the age of 49 from Australia, prevalence and 5-year incidence of any DR were reported to be 35.3 and 22.2%, respectively.53 In this study, duration of DM was the only demographic baseline factor associated with risk for retinopathy progression.53 In the Barbados Eye Study of blacks aged 40–84, the prevalence and

4-year incidence of any DR were 28.5 and 30.1%, respectively.21,56 There were no important demo-

graphic risk factors associated with incidence of retinopathy.56