Good Lord, the advice your brother’s doctor gave him is 40 years out of date.
Body fat doesn’t cause diabetes, body fat is a SYMPTOM of diabetes.
Gary Taubes in his book “Why We Get Fat” explains the process in a way just about anyone can understand.
1) Your body starts to maintain too much insulin in the blood (there are a number of reasons for this, including diet, but this is where it starts). Because there is too much insulin in your blood your body starts storing food as fat instead of burning it (one of insulin’s jobs is to push fat into the fat cells). You also quit burning body fat (one of insulin’s other jobs is to prevent you from burning body fat—there’s a good reason for this, but the point to remember from this is that when there is excess insulin in the blood your body is incapable of releasing fat from the fat cells into the bloodstream to be burned).
2) Because of the excess insulin in the blood, your cells increase their resistance to insulin.
3) Because your cells start to become insulin resistant, your pancreas starts to produce MORE insulin in attempt to over come the cell’s insulin resistance. The excess insulin in your blood causes you to store more food as fat instead of burning it, and prevents you from burning the fat already stored in your fat cells (you get fatter).
4) Steps 2 & 3 repeat over and over again in a race between the cells (which become more and more insulin resistant) and your pancreas (which pumps ever increasing levels of insulin in your blood in an attempt to overcomes the cells resistance).
5) Eventually, the pancreas becomes so overworked that it fails, and starts to lose the ability to produce insulin.
I recommend you get Taubes book (while not a book specifically about diabetes, it explains the process that causes it, and the root causes of why the excess insulin is produced in the first place), then I recommend you find a doctor that has read a medical book published after the ‘70s.
I hate to tell you but a lot of people who lose weight lose the diabetes symptoms. This is not new.