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A visual look: Does latitude relate to incidence of type 1 diabetes?

Posted on: December 14, 2013   by  Jeff Nicklas

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The International Diabetes Federation (IDF) recently released its sixth edition of their Diabetes Atlas. This atlas includes global data on diabetes, impaired glucose tolerance, and general demographics. The sixth edition updates global data on diabetes from 2011. Among the wealth of data on diabetes, the atlas includes a specific dataset on incidence of type I diabetes.

The IDF used a systematic approach to gathering data. They collected peer-reviewed studies and research as well as gathered information from organizations such as the CDC and WHO. Along with this atlas, the IDF is currently compiling all studies into an online library.

Overall, they found that diabetes is increasing in every country and that 382 million people worldwide have type 1 and type 2 diabetes, up 20 million from just two years ago. For type 1 diabetes specifically, the atlas documents that more than 79,000 children developed the disease in 2013.

Type 1 diabetes (T1D) is an autoimmune disease which usually occurs in early life. What happens is that the body’s immune system attacks the insulin-producing cells in the pancreas (called beta cells). This leads to the body not being able to produce the insulin needed to mobilize glucose into the energy it needs. T1D is sometimes referred to as insulin-dependent diabetes.

Researchers have been interested in a potential link in T1D and vitamin D due to vitamin D’s ability in making the immune system smarter. They’re wondering if getting enough vitamin D may be able to prevent T1D later in life. And, some observational research has shown that vitamin D at an early age may help prevent T1D.

If vitamin D does play some role in the prevention of T1D, might there be a relationship between latitude and risk of T1D, much like we see for latitude and MS?

Researchers have explored the geographical variation of T1D the last five years. A study from 2008 found that incidence of T1D varies, for example, from 0.1 new cases per 100,000 boys in China to 37 new cases per 100,000 boys in Finland. China sits at a latitude of 35 degrees compared to Finland at 64 degrees.

Mohr, S. B. et al. The association between ultraviolet B irradiance, vitamin D status and incidence rates of type 1 diabetes in 51 regions worldwide. Diabetologia, 2008.

When we look at the data compiled from IDF, we can see that there does appear to be some relationship between latitude and T1D.

Google Visualization API Sample

In the Mohr et al. study from 2008, researchers found that incidence rates of T1D were higher in areas at higher latitudes with subsequently lower UVB radiation (p<0.0001).

If we looked at the same data organized by country from closest to the equator to furthest from the equator, we see again that there is a slight trend, the further from the equator, the higher the incidence.

T1D Incidence

This relationship isn’t perfect. Some countries closer to the equator, such as Saudi Arabia, have high incidence rates, whereas some countries farther away from the equator, such as Australia, exhibit relatively low incidence rates. Might genetics play a far greater role than UV exposure?

One possible explanation for the high rates in Saudi Arabia, despite there being an abundance of UV, is that cultural practices — like wearing heavy clothing – lead to low UV exposure, just like at northern latitudes. However, not all Middle Eastern countries have similar high rates as Saudi Arabia (as you can see on the map above).

But if we look at the geography of incidence within a country, where we don’t have to worry about genetics or confounders as much, there does seem to be a UV relationship. When we examine the within country incidence rates for Australia, we get the same pattern: the further away from the equator, the higher the incidence of T1D.

The map represents data collected from the Australian Institute of Health and Welfare in 2008.

This data, too, can be seen statistically. One study, specifically examining latitude gradient within Australia’s territories, found T1D to be positively related with latitude (p = 0.026). There was a strong 3-fold increase in prevalence of T1D moving from the most northern territory to the most southern territory of Australia.

Staples, J. et al. Ecologic Analysis of Some Immune-Related Disorders, Including Type 1 Diabetes, in Australia: Latitude, Regional Ultraviolet Radiation, and Disease Prevalence. Environmental Health Perspectives, 2003.

In summary, there does appear to be a relationship between T1D and latitudinal location. The data from IDF has given us an astounding wealth of information to view diabetes on a global scale.

Geographical variation, vitamin D, and T1D have yet to be conclusively linked; however these patterns suggest that UV and maybe even vitamin D play a role.

Sources

International Diabetes Federation. IDF Diabetes Atlas, 6th edn. Brussels, Belgium: International Diabetes Federation, 2013.

Incidence of Type 1 diabetes in Australian children 2000 – 2008. Australian Institute of Health and Welfare, 2010.

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