Are there any other subjects in any field that you've seen definitively proven cause and effect relationships in science? Have you personally definitively seen it proven in science that electricity is a flow of electrons or that gravity exists or that the world is round?
There is certainly room for spirited debate as to whether widespread poor diets and sedentary lifestyle is causing widespread obesity and it is also seperately causing widespread diabetes OR widespread obesity is directly linked to diabetes itself. That's a fair debate. the reality that it's a disease LARGELY attributable to lifestyle and diet though is inescapable--- as clearly evidenced by different cultures with different diets and lifestyles have dramatically different diabetes rates. Is that true in every single individual case? No, of course not. Is it an insult to those of us with the disease-- no, it shouldn't be. Is it helpful to pretend that diet and lifestyle don't enter the equation because we can't quantify definitive proof of that (or of almost any other reality in the universe)-- no, it's detrimental.
I understand there's a guilt and a shame attached to having a disease that is rightly considered largely preventable. I have and do feel it myself. It's not the fault of the individual though. This is the result of a society that mass produces unhealthy food, makes healthy food unaffordable or otherwise unobtainable and rewards sedentary occupations more highly than it does physically active ones. It's really just that simple. It's a problem in our society and culture, not in one person's pantry.
If we can't accept that, as a culture, that we are doing things wrong and we can and should do things differently, each generation will be more and more doomed. I personally believe we are better than that. We, people with diabetes, can be honest that we can stack the deck in our children's favor instead of against it. We can do better, we can in many cases defeat this disease-- let's join forces and do it instead of staying trapped by our own insecurity.