In some cases it's legitimately genetic. My grandpa was pretty much King Shitlord and yet he passed away at the age of 68 from type 2 diabetes related complications. My whole family has it and many of them are at healthy weights. And I guess I'm next in line, which is why I've eaten healthy and worked out since forever to mitigate the effects of the disease. It's supposedly very common  in my genetic pool (I'm ethnically Nepali Sherpa mixed with Indian and Bhutanese). Now, that doesn't excuse people who eat their way into diabetes (the vast majority of diabetics) but there are some exceptions to the rule.