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 :
- in muscle, the protein breakdown is stimulated
- liver gluconeogenesis is stimulated by low insulin levels and high amino acid levels in blood and using substrate from muscle and fat tissue
- glucocorticoids stimulate lipolysis
- liver starts ketone body synthesis, which along with medium chain fatty acids prove to be quite a good alternative energy source for the brain
Energy from nutritional principles is as follows:
- Lipids = 9 kcal/g
- Proteins = 4 kcal/g
- Carbohydrates = 4 kcal/g - the most important because they are used as is. Lipids and proteins require energy from other sources to start chemical reactions which yield energy.
Glucose enters your cells via a family of transporters. A primary transporter in muscle and fat cells is known as GLUT-4. Insulin stimulates GLUT-4 to move from the interior of a cell to the cell surface, where the glucose can then bind to the GLUT-4 transporter and enter the cell. However, there are plenty of glucose transporters on the cell surface, even when there is no insulin. In fact, there are enough transporters on the cell surface to allow the cell to get enough glucose to sustain its energy needs. Thus, glucose transport into cells is never truly dependent upon insulin. Insulin enhances the uptake of glucose into cells, but it is not required for it. In fact, when you knock out the insulin receptor in mice so that insulin cannot stimulate glucose uptake into muscle or fat cells (yet you keep the insulin receptor intact on other cells like brain and liver), the animals do not become diabetic and they have normal blood sugars.-https://www.ncbi.nlm.nih.gov/pubmed/15254588
Metabolic tracer studies have allowed us to learn how insulin operates in humans in vivo. When you take a type I diabetic off insulin, blood glucose climbs sharply. However, it's not because glucose can't get into cells. In fact, glucose uptake into cells actually increases. This is because the concentration of glucose in the blood is so much higher than the cellular concentration that glucose must move into the cells (remember, there's already enough glucose transporters on the cell surface even if there's no insulin). So why does blood glucose climb so high? Remember that the amount of glucose in your blood is both a function of how much glucose is entering the blood (the rate of appearance), as well as how much glucose is leaving the blood (the rate of disappearance). In a fasted diabetic without insulin, all of the glucose is coming from the liver. Remember that your liver helps maintain blood sugar levels when you are fasted by releasing glucose; this glucose comes from both gluconeogenesis (the formation of glucose from non-carbohydrate sources, like protein) and glycogenolysis (the breakdown of glycogen stored in your liver). Insulin acts as a brake (a chalone as Dr. Schafer described it) on these processes. Thus, when you do not have insulin, you have runaway gluconeogenesis and glycogenolysis. The high blood sugar in an uncontrolled diabetic is thus caused by overproduction of glucose from the liver, not because glucose can't get into cells.-http://m.bja.oxfordjournals.org/content/85/1/69.short