There are two types of diabetes, Type 1 and Type 2.
Type 1 diabetes used to be called juvenile diabetes or insulin dependent. There was no way to prevent this as the person had an inborn issue and thus had it from childhood. From the minute they are diagnosed they need insulin to keep them alive since they have no ability to make it themselves. These people literally have no insulin making cells left in their body.
Type 2 diabetes was previously known as adult onset diabetes but now with the ever increasing issue of childhood obesity it is just called Type 2. There are a few causes for Type 2. One form is the body makes insulin but the tissue is resistant to it and cannot use it properly. There are meds that help sensitize the tissue to insulin. There are also Type 2s that make insulin but not enough for the amount of mass they have (obesity). For these folks they could actually reverse the diabetes by loosing weight through exercise and life style change, namely eating healthy, low fat/sugar diets.
Type 2 diabetes is increasing dreadfully and it comes down to people eating junk food, not exercising, and obesity.