The main cause for diabetes is lack of insulin in the body. Insulin is the hormone that allows body cells to absorb glucose from the blood. With out insulin, the body cells can't get glucose from the blood. So, why doesn't diabetes cause death by starvation?
Not all cells depend on insulin to take in glucose from blood (e.g. vascular endothelial cells). But the majority do (muscle tissue and fat tissue).
The lack of insulin causes the cell to use auxiliary metabolic pathways to generate its energy :
Energy from nutritional principles is as follows: