It is true that obesity may be less of a cause and more of a symptom of a larger problem related to insulin resistance and other hormonal imbalances that lead to diabetes and other diseases. Some studies related to the ‘insulin hypothesis*’ popularized by the science journalism of Gary Taubes suggest that storing body fat is one of the body’s natural strategies for turning the toxicity of chronically elevated blood sugar levels into something more benign (the other main way of dealing with it being turning it into fuel when we exercise, if we’re the exercising type). This would explain why diabetes usually is preceded by obesity (the idea being it only becomes a problem after storing body fat stops being an effective enough defense) and also might have something to do with thin people with diabetes not living as long as fat people with diabetes.
BMI is as rough and simplistic yardstick anyhow, and may not be very useful with different body types, people with more muscle mass than average, etc.
(‘Insulin hypothesis’ being the notion that excess blood sugar from refined starches and dietary sugar is a main cause of both obesity correlated diseases, as opposed to the ‘lipid hypothesis’ that points to dietery fat and cholesterol as the problem and has been the mainstream for decades.)
