Ketones, or ketone bodies, are acids your body makes when it’s using fat instead of glucose for energy. Your body gets most of its energy from glucose, a sugar that mainly comes from carbohydrates in your diet. If you don’t get enough energy from glucose, your body breaks down fats for energy instead. The breakdown releases ketones that travel through your bloodstream. The ketones fuel your body’s tissues and eventually exit through your urine.

It’s normal (and safe) to have some ketones in your blood because it’s normal for your body to use its backup energy source — fats — when there’s not enough usable glucose. But too many ketones can make your blood acidic and toxic. This is called ketoacidosis. If you have diabetes, too many ketones can cause diabetes-related ketoacidosis (DKA), which is life-threatening without treatment right away.