Unfortunately, there’s no clear-cut answer when it comes to diabetes. Unlike some traits, diabetes doesn’t seem to be inherited in a simple pattern, and there is a lot of misinformation out there about its causes. (Have you ever had to explain that diabetes doesn’t happen because someone ate too much sugar?)
It’s apparent, though, that some people are born more likely to develop diabetes than others. We know that type 1 and type 2 diabetes have different causes, but genetics plays an important role in both types. People with diabetes inherit a predisposition to the disease, then something in their environment triggers it.
Identical twins are proof that genes alone are not enough, however. Identical twins have identical genes; therefore, they should have the same genetic risk for a disease—right? Not necessarily. Research has found that if one identical twin has type 1 diabetes, the other twin will get the disease about 50 percent of the time. For type 2 diabetes, that risk rises to as much as 4 in 5. In both type 1 and type 2, identical twins have a much higher risk of both developing diabetes than non-identical (fraternal) twins, which further supports the fact that genetics is involved.
So what are the causes of type 1 diabetes? Again, we know that genetics is involved. We also know that it’s not just one gene responsible, but many different genes, each of which contributes only a small part of the risk. Scientists have identified a few genes responsible for type 1, but the majority of the genetic risk is still unidentified.
Because of the data about identical twins, we know that type 1 diabetes doesn’t arise solely from genetics. Researchers are eager to find out what environmental factors could also contribute to type 1. Potential triggers that have been studied include:
- Differences in climate—Type 1 diabetes develops more often in winter than summer, and the rate of type 1 is higher in countries with cold climates.
- Bacteria or viruses—Because type 1 is an autoimmune disease, scientists have speculated that an infectious agent, either bacteria or viruses, could cause it.
- Early diet—Type 1 is less common in people who were breastfed and in those who first ate solid foods at later ages.
Identifying environmental triggers has proved very difficult, and so far no single factor has been found that is clearly responsible.
What about risk for children born to people with type 1 diabetes? The overall risk that a child born in the United States will develop type 1 is about 1 in 300 to 400. That risk increases if the child’s parents have type 1, to about 1 in 50 if the mother has diabetes and to 1 in 25 if the father has diabetes. The child’s risk appears to be higher if the parent developed diabetes before age 11.
And what about type 2 diabetes? As with type 1 diabetes, genetics plays a role in type 2. Similar to type 1, identical twins show a high rate of both developing diabetes, approaching 75 to 80 percent, with a lower but still high rate for non-identical twins.
Because the rate of type 2 diabetes is not the same in identical twins, environmental causes are clearly involved in the onset. The primary factors appear to be excess weight gain and a sedentary lifestyle, but other factors may also play a role, such as environmental pollutants and the type of bacteria in the digestive system.
Although excess weight increases the rate of type 2 diabetes, it’s worth remembering that most overweight people don’t have diabetes, and many people with type 2 are of normal weight or only moderately overweight—so again, it’s not clear-cut.
If you have a family history of type 2 diabetes, it may be difficult to figure out whether your diabetes is due to lifestyle factors or genetics. Most likely, it is due to both. But don’t lose heart! Studies show that it is possible for many people to delay or prevent type 2 by exercising and losing weight.
Identifying the genetic and environmental factors responsible for both types of diabetes has proven to be very difficult, but a lot of research continues to be done. We hope for the day when doctors can evaluate an individual’s exact genetic risk for diabetes, making it possible to avoid particular factors that would trigger the disease and help Stop Diabetes®.
For more information about the genetics of diabetes, visit our website or check out the recently published The Genetic Landscape of Diabetes, from the National Institutes of Health. To learn your risk for type 2 diabetes, you can take the American Diabetes Association’s Diabetes Risk Test online at http://diabetes.org/risktest.