It is truly a fascinating but a bit morbid thing. Let’s start with this before and after picture of a young girl around the time of the discovery of insulin:
The picture on the left shows no insulin. Insulin is a lot of things in the body. One of it’s main functions is signaling the body about its state of feeding. When you lose insulin your body thinks it’s starving. The girl before insulin may be eating all the time. She is fed. However, her body cannot utilize that food and thinks she is starving. So, her body has burned all its fat and is now actively burning lean muscle tissue to make glucose for her body that already has way too much glucose in the blood.
There are lots of things in the cycle of dying from no insulin. The body acidifies. It over produces these substances called ketones. You lose water and become dehydrated which furthers acidification. It’s all this nasty cycle that leads to a somewhat gruesome death.
Compare that to the girl with injected insulin. Now her body can utilize its food and she can actually start to rebuild tissues and even store some fat. The interesting point here is this fine balance that the body needs. If you give the girl on the right too much insulin, that little belly will begin to grow and she will eventually get obese. Then she may proceed on the type 2 diabetic road. Obesity, heart disease, and many other things are related to too much insulin in the body.