Handbook of Diabetes. Rudy Bilous

Chapter 7 Epidemiology and aetiology of type 2 diabetes

       KEY POINTS

       Type 2 diabetes prevalence is set to increase to around 380 million persons worldwide by 2025, with the highest rates in the Eastern Mediterranean and Middle East, North and South America.

       Rates are also higher in urban compared to rural populations and are increasing dramatically in younger age groups, particularly adolescents.

       Obesity is closely linked to development of type 2 diabetes through its association with insulin resistance, partly mediated by hormones and cytokines such as adiponectin, tumour necrosis factor‐α and possibly resistin.

       A genetic basis has been confirmed by the identification of variants in the transcription factor‐7‐like 2 allele and subsequent development of type 2 diabetes.

       The usefulness of the metabolic syndrome as a predictor of diabetes is still debated. β cell dysfunction is present at the diagnosis of type 2 diabetes and gradually declines with time.

Prevalence rates and global burden

Globally, an estimated 422 million adults were living with diabetes in 2014 (85–95% is type 2). According to the WHO Global Report on diabetes, the global prevalence of diabetes (aged‐standardised) has nearly doubled since 1980, rising from 4.7% to 8.5% in the adult population. This reflects the increasing prevalence of type 2 diabetes risk factors, particularly obesity (Figures 7.1–7.3). In people with type 2 diabetes, 90% are overweight or obese.

Diabetes prevalence has risen faster in low‐ and middle‐income countries than in high‐income countries. The highest prevalence rates are currently in the Eastern Mediterranean and Middle East with North and South America close behind (Figure 7.4 and 7.5). Conversion rates from impaired glucose tolerance (IGT) to type 2 diabetes are 5–11% per annum. In terms of absolute numbers, the Western Pacific region (particularly China) will see almost a 50% increase to 100 million people with diabetes by 2025.

      The largest number of people with type 2 diabetes is currently in the 40–59‐year age group, where numbers will almost reach parity with 60–79‐year olds by 2025.

Figure 7.1 The relationship between body mass index (BMI) and risk of type 2 diabetes in men and women.

There is however considerable variation between IDF regions and within each region. For example, in the Western Pacific, the tiny island of Nauru has a comparative prevalence in 2007 of 30.7%, whilst nearby Tonga has less than half that rate at 12.9%, the Philippines 7.6% and China 4.1% (Figure 7.6).

      Figure 7.2 Rising obesity rates in the USA (2008‐2015) based on self‐reported height and weight and using BMI>30 as a cut‐off. U.S. Obesity Rate Climbs to Record High in 2015, Gallup. © 2016 Gallup, Inc.

Figure 7.3 The rising Prevalence of type 2 diabetes parallels the rising incidence of overweight and obesity. This trend has continued in more recent years. Mokdad AH et al. JAMA, 1999, 282: 1519–1522; Mokdad AH et.al Diabetes Care, 2000, 23:1278–1283.

Figure 7.4 Age‐adjusted prevalence rates of type 2 diabetes in different geographical regions of the world, as monitored by the International Diabetes Federation (IDF). IDF 2017.

      © International Diabetes Federation.

Figure 7.5 The IDF atlas also monitors the rising prevalence of prediabetes (impaired glucose tolerance) in different geographical regions. IDF Diabetes Atlas. © 2017 International Diabetes Federation.

      In Europe, comparative prevalence rates vary from 1.6% in Iceland to 7.9% in Germany, Austria and Switzerland. The UK prevalence rate is 2.9% (age adjusted) and 4.0% (absolute), increasing to 3.5% and 4.6% respectively by 2025.

Diabetes caused 1.5 million deaths in 2012. Higher‐than‐optimal blood glucose caused an additional 2.2 million deaths (Figure 7.7 and 7.8). Of these deaths, 43% occur before the age of 70, and the proportion of deaths attributable to high glucose is highest in low‐ and middle‐income countries.

Urban versus rural

There is a clear trend for rates of diabetes to increase in populations as they move from a rural to an urban existence. The reasons are unclear but probably relate to both decreasing physical activity as well as dietary changes. For example, rural Chinese have a prevalence of type 2 diabetes of 5%, which is less than half the rate of Singaporean Chinese (10.5%). Much larger differences are seen in South Asian, Hispanic, African, and Polynesian peoples (Figure 7.9).

Impaired glucose tolerance

      Comparative prevalence for IGT vary by region with rates almost double those for type 2 diabetes in Africa, but slightly lower elsewhere (Figure 7.5). These differences are almost certainly a reflection of socio‐economic factors as well as a paucity of studies in many African countries where extrapolation is necessary between very different populations. In Europe, the comparative prevalence will increase slightly from 9.1% in 2007 to 9.6% in 2025, representing an absolute change from 65.3 to 71.2 million (UK figures 4.7% to 4.9%, 2.17 to 2.4 million respectively).

Incidence

      The reported incidence rates for type 2 diabetes vary according to the population under study and the year of observation. For white Europid populations, rates of 0.1–1% per annum have been reported. For Hispanic populations in the USA, rates of 2.8% were recorded in the San Antonio Study, which are similar to