Most medical resources regard ketosis as a pathological state associated with chronic starvation. Glucose is regarded as the preferred energy source for all cells in the body with ketosis being regarded as a crisis reaction of the body to a lack of essential carbohydrates in the diet.
Ketone bodies, from the breakdown of fatty acids to acetyl groups, are also produced during this state, and are burned throughout the body. Excess ketone bodies are excreted in the breath and urine. The brain has a residual need for glucose because ketones can only provide energy when used during aerobic respiration in mitochondria. In the long thin neurons, much of the metabolically active cellular membrane must derive its energy from glucose via anaerobic respiration without the assistance of mitochondria.
When glycogen stores are not available in the cells (glycogen is primarily created when carbohydrates such as starch and sugar are consumed in the diet), fat (triacylglycerol) is cleaved to give 3 fatty acid chains and 1 glycerol molecule in a process called lipolysis. Most of the body is able to utilize fatty acids as an alternative source of energy in a process where fatty acid chains are cleaved to form acetyl-CoA, which can then be fed into the Krebs Cycle. During this process a high concentration of glucagon is present in the serum and this inactivates glucose kinase causing most cells in the body to use fatty acids as their primary energy source. At the same time, glucose is synthesized in the liver from lactic acid, glucogenic amino acids, and glycerol, in a process called gluconeogenesis. This glucose is used exclusively for energy by cells such as neurons and red blood cells.
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