It is the hormone insulin that plays a pivotal role in balancing glucose levels in the body. Insulin can inhibit the breakdown of glycogen or it can stimulate the transport of glucose into fat and muscle cells. It can also stimulate the storage of glucose in the form of glycogen. The release of insulin is mainly triggered by food (chiefly food containing absorbable glucose).
If the amount of insulin available is insufficient (insulin insensitivity), or if cells respond poorly to the effects of insulin (insulin resistance), or if the insulin itself is defective, then the cells of the body will not properly absorb the glucose and it will not be stored appropriately in the liver and muscles. The net effect is persistent hyperglycemia, poor protein synthesis, and other metabolic derangements, such as acidosis.
If the glucose concentration in the blood remains high over time, the kidneys will reach a threshold of reabsorption and glucose will be excreted in the urine (glycosuria). In turn, the osmotic pressure of the urine increases and inhibits the reabsorption of water by the kidney, resulting in increased urine production (polyuria) and increased body fluid loss. This leads to a loss in blood volume and in other body compartments, causing dehydration and increased thirst (polydipsia).