If you eat a calorie-restricted diet for several days, you will increase the breakdown of your fat stores. However, many of your tissues cannot convert these fatty acid products directly into ATP, or cellular energy. In addition, glucose is in limited supply and must be reserved for red blood cells -- which can only use glucose for energy -- and brain tissues, which prefer to use glucose. Therefore, your liver converts many of these fatty acids into ketone bodies, which circulate in the blood and provide a fuel source for your muscles, kidneys and brain.
Synthesis and Breakdown of Ketone Bodies
Low fuel levels in your body, such as during an overnight fast or while you are dieting, cause hormones to increase the breakdown of fatty acids from your stored fat tissue. These fatty acids travel to the liver, where enzymes break the fatty acids into ketone bodies. The ketone bodies are released into the bloodstream, where they travel to tissues that have the enzymes to metabolize ketone bodies, such as your muscle, brain, kidney and intestinal cells. The breakdown product of ketone bodies goes through a series of steps to form ATP.
Conditions of Ketone Body Utilization
Your liver will synthesize more ketone bodies for fuel whenever your blood fatty acid levels are elevated. This will happen in response to situations that promote low blood glucose, such as an overnight fast, prolonged calorie deficit, a high-fat and low-carbohydrate diet, or during prolonged low-intensity exercise. If you eat regular meals and do not typically engage in extremely long exercise sessions, the level of ketone bodies in your blood will be highest after an overnight fast. This level will drop when you eat breakfast and will remain low as long as you eat regular meals with moderate to high carbohydrate content.
Ketone Bodies and Fasting
If you fast for two to three days, ketone body synthesis will increase progressively along with your blood fatty acid levels. Your muscles will use most of these ketone bodies during these first few days of fasting. Eventually, the level of ketone bodies in your blood will be high enough to enter your brain cells. The ketone bodies reduce your brain's reliance on glucose, and will supply up to two-thirds of your brain's energy requirements. Ketone bodies can increase almost 25 times the typical level during a two-week fast, which helps minimize muscle protein breakdown and maintain blood glucose levels.
Low-Carbohydrate Diet and Ketone Bodies
A low-carbohydrate diet will increase ketone body synthesis and usage as well, although not as much as prolonged fasting would. This chronic low-grade increase in ketone bodies may even benefit your body composition. In a study published in the journal "Metabolism" in July 2002, men who were normal weight men and followed a normal calorie, 8 percent carbohydrate diet for six weeks lowered their body fat and increased muscle mass, even though their ketone body levels were elevated because of the low carbohydrates. Although the cause-and-effect relationship is unclear, the researchers hypothesize that ketone bodies may influence levels of hormones, such as insulin, that are involved in storage and breakdown of body fat and muscle tissue.