This temperature map shows those countries that record public health data on Type 1 diabetes. You can see straight away that the number of people in each country per thousand (the incidence), differs between countries.
In fact, amongst 100 countries worldwide, there is a 350 fold difference in the incidence of Type 1 diabetes. In Europe, there is a ten-fold difference, with the highest being in Finland, then Sardinia. As you might expect, the differences between nations can, in part, be explained by the genetic difference between the different populations. People who move from one country to another are more likely to keep the risk from their country of origin. But, they pass on the risk of the country that they migrated to in their children. In reality, it isn’t easy to make sweeping generalisations for countries. We have talked about genetic differences in ‘snip’ in another blog in this series. The snip PTPN22, which has strong links to Type 1 diabetes, seems to be more common as we move from the south to the north of Europe. That would seem to explain why Type 1 is more common in Scandinavian countries. But PTPN22 is rare in Sardinia. It has a lower incidence there than almost anywhere else in Europe. Yet Sardinia also has one of the highest rates of Type 1 diabetes in Europe. You would think that there would be some mixing with their neighbouring country Italy. Yet Italy has a five times less incidence of Type 1 diabetes than Sardinia. So, it is not straight forward.
To explore if the geographical differences of Type 1 diabetes have any common characteristics, the World Health Organisation did a ten-year study over 105 diabetes treatment centres in 25 countries around the world. The study found that the distance from the equator seems to have an effect. Countries further from the equator having more Type 1 diabetes. Type 1 diabetes is diagnosed more frequently in the winter months and less often in the summer. Boys seem to be more vulnerable to the seasonal effect than girls, and older boys aged 5-14 more than the under 5’s. But there were exceptions in a minority of countries, so it can only be a generalisation.
What this suggests, of course, is that something in our environment, either the country we live in or the season, might be causing genetically vulnerable people to get Type 1 diabetes. There are some
obvious suspects. Could northern countries ( or southern if you are south of the equator) be so dark in their winter months that at the end of the winter, the lack of sunshine is causing someone to develop Type 1 diabetes? This is not going to be the case in Sardinia, of course, but there is some persuasive research that it is vitamin D deficiency that might be causing Type 1 diabetes. Let’s take a look.
The primary source of vitamin D in the body is by the conversion of cholesterol-like products in the skin into vitamin D under the influence of sunlight. It is the ultraviolet light in the sunshine that provides the energy for this process. Other processes in the body convert vitamin D into more active forms. Some critical areas include the cells of the pancreas that make insulin. Vitamin D is needed for the normal release of insulin. This is because when insulin is released from the pancreas, it needs calcium to do so, and vitamin D regulates calcium metabolism. Vitamin D can also be obtained from food or supplements, especially fish. Vitamin D is a fatty substance and can be carried in the fatty tissues. It is thought that diet alone will not provide enough vitamin D, so if you are not getting your vitamin D from sunlight, then you will need supplements.
Vitamin D acts through receptors, and these are present in nearly every tissue in the body. Vitamin D regulates calcium, magnesium, and phosphate. It has a vital role in bone health. It also acts as an anti-inflammatory agent. So, it is easy to see why there has been an interest in vitamin D and Type 1 diabetes. Finland has long, dark winters and one of the highest rates of Type 1 diabetes in the world. A study was done to look at the effect of vitamin D supplements on Type 1 diabetes. This trial followed children from birth and ran for 31 years. The intended dose of vitamin D supplement was 2000 international units (IU) per day. The results were that people who did not take vitamin D supplements at all had ten times more chance of getting Type 1 diabetes than those who took vitamin D regularly. (2 in 1,000 per year compared to 2 in 10,000). The higher the dose of vitamin D that you took, the less chance you were of getting Type 1 diabetes due to vitamin D deficiency. It is an impressive result. But does it prove that Type 1 diabetes is caused by vitamin D deficiency? There seems to be an association for some people. People who took no supplements had what you might interpret as a small risk of 2 in 1000 per year, but ten times greater than those who took the recommended dose. In another study on women who developed diabetes (not Type 1) in pregnancy (gestational diabetes), it was shown that vitamin D supplements improved aspects of their diabetes control.
So, where you are born, and where you move to on the planet, has some associations with your chances of getting Type 1 diabetes. But we can only make generalisations about geography and its effect on Type 1 diabetes. But in some cases, it would seem to play a part.