 Eric was diagnosed with type 1 diabetes just two days after this photo was taken, at age 18 months. Type 1 incidence is increasing fastest in children under 5. His mother wrote a book, 100 questions and answers about your child's type 1 diabetes. What is incidence?Incidence means the number of people in a certain area that are newly diagnosed with a disease during any one period of time. For type 1 diabetes, incidence is usually expressed as a number per 100,000 people. For example, Finland has the highest incidence of type 1 diabetes in the world, where almost 41 people are diagnosed each year per 100,000 people, and China has one of the lowest, where only 0.1 person per 100,000 are diagnosed each year (Diamond Project Group 2006). Increasing incidence mean that more and more people are diagnosed with a disease each year, even when controlling for population growth, and, consequently, that the risk of developing the disease is increasing. Note that "prevalence" is different from "incidence." Prevalence is a measurement of how many people in a certain population have a disease at one time, and includes anyone who had been diagnosed in past years. Prevalence is a measure of how common a disease is, and incidence reflects the risk of someone getting the disease. Most of the studies cited here measure incidence.
Europe has by far the most complete and reliable data on type 1 diabetes incidence in the world. In the 1980s, European countries set up a Europe-wide type 1 diabetes registry to keep track of newly diagnosed children. By 2000, there were 44 European centers contributing to that registry (called EURODIAB for Europe and Diabetes). This registry includes about 30 million children in most areas of Europe. Another network, the DIAMOND network, is a type 1 diabetes registry that includes 112 centers from 57 countries around the world, including some parts of the U.S. and most of the EURODIAB countries (Soltesz et al. 2007). Data from Africa, Asia, and South America are sparse (Diamond Project Group 2006).
Geographic and ethnic variationsType 1 diabetes incidence ranges from very low in South America and Asia, to very high in Europe, especially northern Europe (Onkamo et al. 1999). Finland and Sardinia, Italy have the highest incidence of type 1 diabetes in the world (Diamond Project Group 2006). Newfoundland and Labrador, Canada (35 diagnosed per 100,000 people) may have just surpassed Sweden (30 per 100,000) for the third highest incidence: incidence levels there approached those of Finland (41) and Sardinia (38) (Newhook et al. 2008). As an example of a country with low incidence, black children in Dar es Salaam in Tanzania have very low incidence of type 1 diabetes, at 1.5 people diagnosed per 100,000 people, which is much lower than rates for black children in the U.S., Virgin Islands, or Cuba. In Tanzania, only one child under age 5 was diagnosed during one 10 year study period (Swai et al. 1993). Yet in developing countries, high infant mortality may also affect incidence rates. Within the U.S., there are ethnic differences in diabetes prevalence. In children under age 20, type 1 diabetes is more common (prevalent) among non-Hispanic whites, followed by blacks, Hispanics, and Asian/Pacific Islanders, and lowest in American Indians. (Among these same children, type 2 diabetes, on the other hand, is more common among American Indians, followed by blacks, Asian/Pacific Islanders, and Hispanics, and lowest in non-Hispanic whites) (Liese et al. 2006).
Genetic variations likely explain some of the differing incidence and prevalence rates among people worldwide. Yet even among ethnically similar populations, type 1 diabetes incidence can vary. For example, Finns have a six times higher incidence in type 1 diabetes than Russians living across the border. The genes that confer a high risk of type 1 diabetes, however, are the same in these populations, implying that environmental factors contribute to the differing incidence rates (Kondrashova et al. 2005). As another example, type 1 diabetes incidence is much higher in Sweden and other Nordic countries than in Lithuania and other Baltic states. Yet the genetic risk of type 1 is similar in all of these countries (Skrodeniene et al. 2010).
The "latitude rule"Many of the countries with high incidence are located closer to the polar areas of the globe, both to the north and the south (Soltesz et al. 2007). Even within countries, latitude can make a difference: one Australian study, for example, found that type 1 diabetes was three times more common (prevalent) in more southerly regions of that country than in northerly regions (Staples et al. 2003). Like all rules, however, there are exceptions. For example, Sardinia, Italy's high incidence of type 1 diabetes does not fit the rule. Variations within countries also do not always correspond to latitude (Soltesz et al. 2007).
Vitamin D, which is produced by the skin when exposed to sunlight, is a possible explanation for this pattern. In a study of 51 regions around the world, Mohr et al. (2008) found that areas with lower levels of ultraviolet B radiation (the main source of vitamin D in humans) had a higher incidence of type 1 diabetes. Vitamin D deficiency appears to be a risk factor for type 1 diabetes, and vitamin D cannot be produced adequately by the skin during the winter in areas closer to the polar regions. Another possibility is that persistent organic pollutants (POPs) play a role. POPs evaporate and migrate to the polar regions of the earth; some can even interfere with vitamin D synthesis (see the vitamin D page).
WealthMost countries with high incidence are Westernized, developed countries (e.g., see Diamond Project Group 2006), and even within Europe, incidence is correlated to gross national product (GNP) and other indicators of national prosperity (Patterson et al. 2001). Some studies have found higher incidence of type 1 in wealthier areas or in people with higher socioeconomic status within countries as well, such as Chile (Torres-Aviles et al. 2010) and the U.S. (D'Angeli et al. 2010).
Why would wealth make a difference in type 1 diabetes incidence? Differences in nutrition or lifestyle may play a role. These factors could include high growth rates in early life (see the height and weight page), improved hygiene and fewer infections (see the viruses page), or more milk consumption (see the wheat and dairy page) (Patterson et al. 2001).
Another possibility seldom considered is environmental contamination. A number of toxic chemicals are found in plastics, personal care products, and other conveniences of modern life (see, for example, the pages on bisphenol A and phthalates). In developed countries, exposure to bisphenol A is significant and continuous (Welshons et al. 2006). Historically, chemicals such as PCBs were produced in industrialized countries, and now everyone living in developed countries has PCBs in their bodies (Carpenter 2006). While data from less industrialized countries are scarce, in general, people living in more industrialized countries show higher levels of PCBs and dioxins in their bodies than people in less developed countries. In addition, using a measurement of the total toxicity of multiple persistent organic pollutants, levels are higher in people living in industrialized as compared to less developed countries (Sudaryanto et al. 2005; Tanabe and Kunisue 2007).
Type 1 diabetes incidence is now rising even in countries with historically low incidence, suggesting a catch-up phenomenon. High incidence countries (e.g., Norway), where incidence rose many decades ago, do not all show a continuing increase in incidence. It is too early to say whether these high-incidence countries have reached a plateau (Gale 2002b). Levels of most persistent organic pollutants have declined recently in developed countries. In developing and some former Soviet countries, however, some persistent organic pollutants (like DDT) are still in use, and contamination due to open dumping is also a concern. Levels of some organochlorine pesticides (such as DDT) are now higher in people living in developing countries than in developed nations (Tanabe and Kunisue 2007). Perhaps contamination resulting from industrialization contributed to the rising incidence in many now-developed, high-incidence countries, and other countries, where contamination began later, are now "catching up."
Central and Eastern European countries are one region where type 1 diabetes incidence may be "catching up" to the higher incidence found in Western Europe. Over the period of 1989-1998, Central and Eastern European countries showed the highest rates of increasing type 1 diabetes incidence within Europe. Why? Some environmental factors associated with societal development may play a role (Green et al. 2001). Yet because type 1 diabetes takes a long time to develop, the factors responsible for these rapid increases may have operated before the political changes in those countries (EURODIAB ACE Study Group 2000). Environmental contamination is a serious problem in Central and Eastern Europe, beginning during the communist years, before the political changes (Fitzgerald et al. 1998). ClustersClustering has been found in type 1 diabetes, meaning that some groups of people living near each other during the same time periods show a higher than expected incidence of disease. Some studies have found clusters of children with higher than expected incidence of type 1 diabetes, and a study from the U.K. found clustering in teenagers as well (McNally et al. 2006). Disease clusters imply the involvement of some environmental factor. Infectious disease and environmental contamination are two possible suspects (e.g., see the trichloroethylene page).
Urban vs. rural areasThere is no consistent pattern of type 1 diabetes being more common in either rural or urban areas. Some studies have found higher incidence in rural areas, and some in more densely populated areas. For example, a study from Western Australia found type 1 diabetes incidence to be higher in more urban areas as compared to more rural or remote areas (Haynes et al. 2006). A study from New Zealand found type 1 incidence higher in "satellite urban communities" (Miller et al. 2011). And a study from Northern Ireland has found higher incidence of type 1 diabetes in more remote areas (Cardwell et al. 2006). There are no clear trends in any of these directions.
Seasonal variationsA seasonal pattern in type 1 diabetes diagnosis has been seen in some countries, with more people diagnosed during the winter months. The pattern is most apparent in countries with greater differences in summer vs. winter temperatures. Some studies suggest that people with type 1 tend to be born more often during certain months, but the evidence for this is scarce (Soltesz et al. 2007). A seasonal pattern has also been identified in the appearance of the first autoantibodies in Finland, highest in the fall and winter, and lowest in spring and summer. There is also variation year to year in the timing and height of these antibody peaks (Knip et al. 2005).
These seasonal variations are often attributed to viruses, cold weather, or vitamin D levels. A number of environmental contaminant exposures also vary seasonally, such as air pollutant levels (Hathout et al. 2002), nitrate levels in drinking water (Parslow et al. 1997), and agricultural pesticide use. There may be other explanations as well (e.g., children may get less exercise in cold climates in the winter, leading to increased insulin resistance, or their parents eat Icelandic smoked mutton at Christmas... but you'll have to read about that story on the nitrate and nitrite page). |