In biology, ketosis (pronounced /kɨˈtoʊsɨs/) is a state of the organism characterised by elevated levels of ketone bodies in the blood, by the process of ketogenesis. Ketone bodies are formed when the liver glycogen stores are depleted. The ketone bodies acetoacetate and ß-hydroxybutyrate are used for energy. 
When glycogen stores are not available in the cells, 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 called beta-oxidation. One of the products of beta-oxidation is acetyl-CoA, which can be further used in the Krebs cycle. During prolonged fasting or starvation, acetyl-CoA in the liver is used to produce ketone bodies instead, leading to a state of ketosis.
During starvation or a long physical training session, the body starts utilizing fatty acids instead of glucose. The brain cannot use long-chain fatty acids for energy because only medium-chain fatty acids (which are scarce in most foods) can cross the blood-brain barrier. However, the ketone bodies produced in the liver can cross the blood-brain barrier. In the brain, these ketone bodies are then incorporated into acetyl-CoA and used in the Krebs cycle.
The ketone body acetoacetate will slowly decarboxylate into acetone, a volatile compound that is both metabolized as an energy source and lost in the breath and urine.
Ketone bodies are acidic, but acid-base homeostasis in the blood is normally maintained through bicarbonate buffering, respiratory compensation to vary the amount of CO2 in the bloodstream, hydrogen ion absorption by tissue proteins and bone, and renal compensation through increased excretion of dihydrogen phosphate and ammonium ions. Prolonged excess of ketone bodies can overwhelm normal compensatory mechanisms, leading to acidosis if blood pH falls below 7.35.
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