Ketones (also called ketone bodies) are substances produced by the body as it breaks down fats for energy, a process called ketosis.

Normally the body obtains the energy it needs from sugars (carbohydrates). A person may begin to break down fats and produce ketones because of:

• High blood sugar levels and insufficient amounts of insulin, preventing the body from using sugar for energy, causing it to break down fat instead.
• Fasting or starvation.
• Prolonged vomiting.
• A diet that is low in sugars and starches (carbohydrates).

Ketones are normally released into the urine. If ketone levels are very high or if the person is dehydrated, ketones may begin to build up in the blood. High blood levels of ketones may cause fruity-smelling breath, loss of appetite, nausea or vomiting, and fast, deep breathing. In severe cases, it may lead to coma and death. In a pregnant woman, even a moderate amount of ketones in her blood may harm the fetus.

Large amounts of ketones in the urine may signal diabetic ketoacidosis, a dangerous condition that is caused by very high blood sugar levels.