I’ll throw another thought out there – being T1D does not exempt or prevent a person from having or developing some other metabolic dysfunction that can cause or encourage weight gain. Sure, if T1D is the only problem and the person is paying attention, there are lots of quick and unmistakable signposts along the way to help them halt the progression to obesity. On the other hand, stress, depression, overwork, social pressure, perhaps environmental toxins - even poverty - can result in the selection of many bad habits, fast food, quick snacks, etc., that lead the person down the obesity path. OK, it’s just a few extra units here, a few there… sure, Next doctor appointment, there will be someone there to note the change, but, then blood tests come back, numbers increase, adjustments to insulin regimen (rather than eating habits!) are made, and the downward spiral continues.
It does happen. Growing up, I can remember meeting MANY people with T1D who were morbidly obese… Preventable? Maybe - even probably - but certainly not impossible.