In biology, ketosis is a state of the organism characterised by elevated levels of ketone bodies in the blood, by the processes of lipolysis and beta-oxidation.
Ketone bodies are formed from excess amount of fat break down. Some of these ketone bodies such as acetoacetate and ß-hydroxybutyrate can also be 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 in
which fatty acid chains are cleaved by coenzyme A (CoA) to form
acetyl-CoA, which can then be fed into the Krebs cycle.
Acetyl-CoA can only enter the Krebs cycle bound to
oxaloacetate. When carbohydrate supplies are inadequate to maintain
blood glucose levels, the liver naturally converts oxaloacetate in the
liver to glucose via gluconeogenesis for use by the brain and other
tissues.
Excess acetyl-CoA in the liver is used to produce ketone
bodies, leading to a state of ketosis. During this process, a high
concentration of glucagon is present in the serum, which inactivates
hexokinase and phosphofructokinase-1 (regulators of glycolysis)
indirectly, causing most cells in the body to use fatty acids as their
primary energy source.
The brain cannot use fatty acids for energy because the
fatty acids cannot 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.
Excess ketone bodies will slowly decarboxylate into acetone.
Acetone is excreted in the breath and urine.
Ketosis should not be confused with ketoacidosis (diabetic ketoacidosis
or the less common alcoholic ketoacidosis), which is severe ketosis
causing the pH of the blood to drop below 7.2.
Ketoacidosis is a medical
condition usually caused by diabetes and accompanied by dehydration,
hyperglycemia, ketonuria, and increased levels of glucagon. The
high-glucagon, low-insulin serum levels signal the body to produce more
glucose via gluconeogenesis and glycogenolysis, and ketone bodies via
ketogenesis. High levels of glucose causes the failure of tubular
reabsorption in the kidneys, causing water to leak into the tubules in a
process called osmotic diuresis, causing dehydration and further
exacerbating the acidosis.
Further Reading
This article is licensed under the Creative Commons Attribution-ShareAlike License.
It uses material from the Wikipedia article on
"Ketosis"
All material adapted used from Wikipedia is available under the terms of the
Creative Commons Attribution-ShareAlike License.
Wikipedia® itself is a registered trademark of the Wikimedia Foundation, Inc.