There are several ways to answer the question of whether or not high fructose corn syrup, also known as HFCS, causes type 2 diabetes.
First, we know there is a striking correlation between the dramatic rise of type 2 diabetes since the 1970's and the increased consumption of high fructose corn syrup (HFCS) in the United States and other industrialized nations.
Contrary to the commercials paid for and put out by the Corn Refiners Association, high fructose corn syrup and table sugar are NOT alike, i.e. the phrase the commercial says, "sugar is sugar" is DEAD WRONG.
Table sugar is metabolized very differently than high fructose corn syrup. Fructose itself is a natural sugar and is the primary and natural sugar found in fruit but it is found in MUCH lower quantity in fruit than the amount of high fructose corn syrup found in sodas and many processed foods such as snack foods, sauces, condiments, and ketchup. Plus, the chemical structure of high fructose corn syrup (HFCS) is highly altered by industrial processes and behaves very differently in in the human body than does natural fructose (see below).
There is also a difference in the way sucrose or "table sugar" and natural fructose is metabolized. When fructose is eaten, it is immediately shuttled off to the liver. The liver immediately converts the fructose to fat which is probably one of the main reasons belly fat is on the rise. On the other hand, if you eat a small amount of sucrose it is immediately burned for energy. It is not stored as fat unless you eat too much.
Many large scale studies have linked this increase in high fructose corn syrup consumption to the rise in insulin resistance, i.e. type 2 diabetes. For example, a 2008 study by a group of scientists from the University of Florida at Gainsville studying non-alcolholic fatty liver disease found, "...the increased consumption of high fructose corn syrup, primarily in the form of soft drinks, is linked with complications of the insulin resistance syndrome." Most of these large scale studies incorporate data from the National Health Institute which is a mega database.
Similar links between the development of type 2 diabetes (insulin resistance) and the intake of high fructose has been demonstrated in laboratory animals through highly controlled experiments. Of course, we can't officially conduct experiments directly on human beings but in effect we have been doing so since the 1970's when there was a dramatic increase in the consumption of high fructose corn syrup across the vast majority of the population and it has been increasing ever since. I think the mega data speaks for itself. If you look at a graph, the increase in the consumption of high fructose corn syrup follows right along with the rise of type 2 diabetes - it is a perfect match.
In table sugar (sucrose), i.e. cane sugar or beet sugar, the fructose molecule is tightly bonded to the glucose molecule to form the compound sucrose molecule. However, in high fructose corn syrup the fructose molecule and the glucose molecule are NOT tightly bound together so the fructose heads directly to the liver to be converted to fat. So, don't let anyone fool you. Although two sugars may be called the same thing, "sugar" as the misleading commercials say, they may not be chemically the same at all. If they are different chemically, they are metabolized very differently.
There are many other effects of the artificially produced high fructose corn syrup that could be contributing to its ill effects but we just don't understand them fully yet. For example, it has been shown that beverages sweetened with high fructose corn syrup contain high levels of reactive carbonyls. These are highest in carbonated beverages but they are also found in fruit juices, teas, and other drinks sweetened with high fructose corn syrup.
What is the connection between these reactive carbonyls and diabetes?
Well, so far we know for sure that diabetics have very significantly elevated reactive carbonyls in their blood stream compared to people who do not have diabetes.
Basically, I think any time you monkey around with a natural food, take it into an industrial lab and create an artificial food out of it through a radical chemical and temperature industrial process, you are apt to get a product that is not good for the human body!
If you are truly serious about reversing your diabetes, you need the right information. You need to know what foods you need to completely eliminate from your diet and which ones (including supplements) you need to add to your diet. The best source of information I have found, an incredible comprehensive source, is this one and I highly encourage you to check it out thoroughly.