Glucose (a type of sugar) is the body's main energy source. But when the body can't use glucose for fuel - like when a person has untreated diabetes - the body breaks down fat for energy instead. When fat is broken down, the body produces chemicals called ketones, which appear in the blood and urine. High levels of ketones cause the blood to become more acidic. This is known as ketoacidosis (it's called diabetic ketoacidosis, or DKA, when uncontrolled diabetes is the cause).
Ketoacidosis is a severe life-threatening condition requiring immediate treatment. Symptoms of ketoacidosis include nausea, vomiting, abdominal pain, rapid breathing, and, in severe cases, unconsciousness.