Diabetes is a metabolic alteration, characterized by the increase in glucose levels in the blood (hyperglycemia), which is caused by a defect (in its entirety or partial) in the secretion or in how a hormone works: insulin. Insulin is produced in a cluster of cells in the pancreas: the islets of Langerhans.
Glucose (popularly known as sugar in the blood) rarely reaches more than 100 mg/dl on awakening (fasting blood sugar level), even after eating foods high in sugar or fat. Glucose is the principal food of our blood cells. Stability in glycaemia (or blood sugar levels) depends on an extraordinarily fine and sensitive regulation mechanism.
When someone without diabetes ingests any food, sugars in it are absorbed into the intestine and your bloodstream, which tends to increase blood sugar levels. This change is immediately detected by the cells in the pancreas which produce insulin and which respond by rapidly secreting this hormone. Similarly, insulin acts as a key that opens the doors into muscle, fat tissue and liver cells, enabling glucose entry for it to be metabolized and produce energy. This allows organs to keep functioning, thus decreasing blood-sugar levels.
This is a very quick process which prevents blood-sugar levels from increasing. In a person with diabetes, insulin production is so low that it affects the whole regulation mechanism: an increase in blood-sugar levels is not followed by a sufficient increase in insulin production, thus glucose can’t penetrate cells and continues to increase.