Because the available glucose can't be used (ie, can't get into most body cells), cells start using stored fat for energy, and when the absence of sufficient glucose is made apparent to the liver (in non-diabetics, by persistent low insulin levels and, in some circumstances in diabetics also), the liver (especially) begins to produce glucose from non-carbohydrate sources. Some amino acids can be converted to glucose so those sources are typically locally available protein. This is the only source in muscle tissue; the liver can also use the 'glycerol backbone' made available in its fat processing. Since neither protein nor amino acids are stored as 'raw material', amino acids used in glucose production must come from protein currently in use.
Under these conditions, fat metabolism cannot be complete because the two processes (use of fat for fuel -- oxidation -- and glucose production from amino acids) interfere with each other. Partial metabolism of lipids results in ketone bodies ('ketones of the body'), some of which are strongly acidic. So far, this is normal -- the body does use fat for fuel as part of its normal operation (called ketosis) and both ketone bodies and free fatty acids can be used by some cells as fuel.
Ketoic abnormality is a matter of scale; if too many keotne bodies are produced, and they are allowed to build up in the blood, acidosis follows. Too many ketone bodies can result from glucose processing abnormalities (in diabetics often due to inappropriate insulin levels) or from lack of glucose at all (in everyone; this is almost always a starvation condition). This is a critical problem, for much of human biochemistry works properly only within a narrow pH range; acidotic conditions are outside this range and if too far outside, or too long maintained, are incompatible with life.
Ketoacidosis generally starts slowly and builds up. The signs often include nausea and vomiting, which can lead to loss of fluids, stomach pain, and deep and rapid breathing. Other common signs are a flushed face, dry skin and mouth, a 'fruity' breath odor, a rapid and weak pulse, and low blood pressure. If the person is not given proper treatment immediately (usually fluids and insulin, sometimes food -- ie, glucose), ketoacidosis can and does lead to coma and even death.
See also: Diabetic coma, Ketone bodies
Original version from the public domain NIH Diabetes dictionary.