However, even the people who think that "Alzheimer's" is "Type-3" diabetes don't believe that all senile dementia is "Type 3 Diabetes" or is diet-induced. Just certain types of it.
But being obese is so unhealthy, and so easy to control (just eat LESS) that to not hold people accountable for it is not good or just.
https://en.wikipedia.org/wiki/Genetics_of_obesity
You can also be thin and have a bad diet, and while you may not be obese your internal systems can still be suffering from damage over the long run.
Something that should be noted: Many folks who are poor don't have access to high quality foods. So they can buy soda but not a salad. And what determines what makes those foods cheap or expensive are often driven by industry lobbying. So much so that when there was attempt to tax soda in NYC there was a great deal of money spent to stop that from going into effect.