Ketones are compounds produced for energy use when the body runs out of glucose or carbohydrates; or when levels of glucose are excessively high but cannot be used or stored as glycogen in the liver, such as in type 1 diabetes mellitus.
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When your body isnt getting adequate energy (i.e., calories) from dietary carbohydrates or stored glycogen, your glucose levels fall. Then, you begin breaking down protein and the stored fat in your muscles and liver, a reactive process known as ketosis.
As the liver breaks down free fatty acids for energy, ketones may form as a byproduct of metabolism and serve as a secondary source of fuel. The brain, in particular, relies heavily on ketones for energy during times of reduced food intake.
A separate condition known as diabetic ketoacidosis is also associated with ketones, but typically only in a person with type 1 diabetes mellitus. Though glucose levels are high, the body cannot use the glucose because there is no insulin around. Thus, the body perceives that it is in a starving state.