" In fact one in 10 people have a ‘normal’ BMI at the time of diagnosis and in contrast around three quarters of people with BMI over 45 do not have type 2 diabetes.
The truth is that we are all individuals, and simple statistical categorisation by BMI is inappropriate. Those of us with a genetic set to live in a body of BMI 25 may well get type 2 diabetes if their weight rises to 28. And those who have normal metabolism with a BMI of 34 may get the disease if they put on weight to say, a BMI of 37. In other words, we all have a personal fat threshold above which mischief will start happening. This has been rather obscured by the present popularity of population level information to drive beliefs about what is relevant for individuals.
But there is a clear bottom line: if a person has true type 2 diabetes, then they have become too heavy for their body."
Prof Taylor
https://inews.co.uk/opinion/type-2-diabetes-reversing-remission-diet-396640
TOFIs, as they have been dubbed, ie thin or normal BMI diabetics can lose weight to lose pancreatic fat in exactly the same way as overweight or obese subjects. Proportionately they have stored too much ectopic fat on their pancreas and that has clogged the beta cells, damaging them and causing them to stop producing insulin. Some nationalities in particular (eg south Asians) have a tendency to store fat on their pancreas and liver when they are only BMI 23 or even below. It's about being too fat for your own body's ability to cope with it, not about overall quantity of fat.