Epidemiology of type 2 diabetes
Epidemiology is the study of the prevalence and the incidence of diseases, and few of the non-communicable diseases have shown such a dramatic increase as type 2 diabetes in the last decades. The World Health Organisation estimated that 9% of the world's population had diabetes in 2014, and over 90% of these suffered from type 2 diabetes. Moreover, type 2 diabetes already causes 5 million deaths per year, mostly from cardiovascular diseases, and type 2 diabetes is expected to become the 7th cause of death globally by 2030. Type 2 diabetes is strongly associated with obesity, and as such the major burden is now in the middle-income and developing countries where urbanisation and recent affluence have rapidly changed lifestyles.
Global prevalence of diabetes
Figure 1. Actual and projected global diabetes prevalences in subsequent reports. Note how actual prevalences are usually higher than previous estimates, leading to even higher estimates for the future. In the last decades, reports from various organisations have tried to give reliable estimates of both actual and expected prevalences of diabetes. Unfortunately, older estimates have invariably been outpaced by more recent actual prevalences as demonstrated in the 2011 International Diabetes Federation (IDF) report (see figure 1).
The large population of the Western Pacific (WP) region contributes most to the absolute numbers, while the %prevalence is highest in the Middle East and North Africa (MENA). In fact, in Saudi Arabia the population prevalence is now a staggering 20%. However, all regions now have prevalences exceeding 5% of the population (see table below), and the burden of type 2 diabetes is increasingly felt and recognized internationally.
|Population (20–79 years)||Number of people with diabetes (20–79 years)||Comparative diabetes prevalence (20–79 years)||Population (20–79 years)||Number of people with diabetes (20–79 years)||Comparative diabetes prevalence (20–79 years)||Increase in the number of people with diabetes|
AFR = Africa, EUR = Europe, MENA = Middle East and North Africa, NAC = North America and Carribean, SACA = South and Central America, SEA = South East Asia, WP= Western Pacific
The global burden of diabetes
Through its contribution to cardiovascular disease and mortality, the increasing prevalence of type 2 diabetes has a huge impact on global health. The most recent data of the International Diabetes Federation suggest that each year 5 million deaths are directly attributable to diabetes, which is more than the burden of HIV/AIDS, tuberculosis and malaria combined. Diabetes is also a leading cause of acquired blindness and kidney failure and of lower leg amputations. As a result, global health expenditure on diabetes is estimated at 673 billion dollars (equivalent to 12% of total health expenditure) and this will rise as the pandemic progresses. However, since the risk of type 2 diabetes is to a large extent modifiable through (population level) lifestyle changes, there is a huge potential to slow down and eventually reverse this pandemic.
Risk factors for type 2 diabetes
While most of the focus is on obesity
Figure 2. Prevalence of diabetes by age, as shown in the IDF Diabetes Atlas 2013as a modifiable risk factor, age is in fact one of the main drivers of the occurrence of type 2 diabetes, as shown in figure 2. Other manifestations of disordered glucose metabolism such as Impaired Fasting Glucose (IFG) and Impaired Glucose Tolerance (IGT) are also increasingly prevalent with older age and some estimate that over the age of 80 years the chance of having a fully normal glucose metabolism is about 30% (which intriguingly suggests that disordered glucose metabolism may be part of a normal aging process rather than a disease).
Obesity is the major potentially modifiable risk factor for type 2 diabetes, with an exponential relationship between Body Mass Index and the risk of type 2 diabetes. Visceral (central) adiposity seems to confer most of the risk and explains the increased risk of men, who have a more central adipose tissue distribution, compared to women at similar BMI. It should be noted that obesity itself is mainly a reflection /mediator of various metabolically unfavourable lifestyle factors such as excessive energy intake in relation to reduced phyiscal activity.
Currently, the prevalence of diabetes is approximately the same in man and women, but there are some indications that at the same level of obesity, man have a higher risk due to a more visceral adipose tissue distribution.
While it is hard to dissect cultural lifestyle factors from genetic factors, there seems to be an excess genetic risk of diabetes in certain ethnic groups such as South Asians and the indigenous people of certain regions, such as the Aboriginals in Australia. However, it is not so much the genetic predisposition per se, as the influence of affluence on this genetic predisposition that explains the relatively recent increase in diabetes prevalence in these groups.
Two out of every three diabetes patients lives in urbanized areas, and those in the lower socio-economic classes are diproportionally affected. The reasons for this are still poorly understood, but unhealthier lifestyles may be an important mediating factor. Globally, it is the lower-middle income countries that contribute most to the prevalence of the disease, possible because these are the countries where recent urbanisation and economic growth have most drastically changed lifestyle and longevity.
Physical activity has beneficial effects on glucose metabolism and on the risk of becoming obese. However, at the population level sedentary behaviour (i.e. the total lack of any physical activity) is probably more important as a risk factor than the lack of high-intensity physical activity. Particularly in children, clear associations are found between sedentary behaviours (such as TV viewing) and the risks of becoming obese.
Over the years several dietary factors have been implicated as risk factors for diabetes. It is not surprising that, given the strong relationship between obesity and diabetes, some of the best evidence for prevention of diabetes comes from studies where dietary intervention (mainly caloric restriction) was coupled to increased physical activity. Apart from total caloric intake, certain dietary factors have been associated with the risk of diabetes. Of late, particularly sugar-sweetened beverages are thought to increase diabetes risk. Other factors, such as coffee and fibre intake, are associated with lower risks of getting diabetes. However, dietary studies are notoriously difficult to perform and prone to bias because people with healthy diets tend to exhibit other healthy behaviours as well.
Evidence is accumulating that events in early life, even during the fetal stage, may influence diabetes risks in adulthood. Originally, the focus was on the association between birth weight and risk, nut is has now become clear that much more subtle factors, such as deficiencies of micronutrients in early life, may play a role.
Various environmental toxicants/pollutants have been associated with the risk of type 2 diabetes. While the epidemiological evidence in humans is still inconclusive, animal studies give some credibility to the thought that some of the modern biochemicals may be harmful, and more studies in this field are needed. Interestingly, shift work as well as lack of sleep have also been implicated as risk factors for type 2 diabetes. Whether this is related to the disruption of the biological clock or to stress is not fully clear, but the relationship has repeatedly been shown to exist.
The main hormones involved in stress reactions, cortisol and (nor)adrenalin, are well known for their glucose-raising effect and short-term increases in glucose in responmse to stress are clearly documented. However, as yet there is no consistent epidemiological evidence for a relation between (long-term) stress and the risk of type 2 diabetes.
^ Whiting David R.. IDF Diabetes Atlas: Global estimates of the prevalence of diabetes for 2011 and 2030. Diabetes Research and Clinical Practice. Elsevier BV; 94(3):311-321. Available from: http://dx.doi.org/10.1016/j.diabres.2011.10.029
^ Guariguata L.. Global estimates of diabetes prevalence for 2013 and projections for 2035. Diabetes Research and Clinical Practice. Elsevier BV; 103(2):137-149. Available from: http://dx.doi.org/10.1016/j.diabres.2013.11.002