Ketosis

Ketosis

[ki-toh-sis]
Ketosis is a state in metabolism occurring when the liver excessively converts fat into fatty acids and ketone bodies which can be used by the body for energy.
Adipose tissue consists of highly specialized cells which store energy in the form of a triglyceride and release it upon hydrolysis in a process known as lipolysis, yielding three fatty acids and one glycerol molecule. These ketone bodies are a by-product of the lipid metabolic pathway after the fat is converted to energy. Ketoacidosis, by contrast, is the accumulation of excessive keto acids in the blood stream (specifically acetoacetate and beta-hydroxy butyrate).

Metabolic pathways

Most medical resources regard ketosis as a physiological 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 carbohydrates in the diet. In recent years this viewpoint, both the body's preference for glucose and the dangers associated with ketosis, has been challenged by some doctors.

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 will slowly decarboxylate into acetone. That molecule is excreted in the breath and urine. 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. It is important to note that acetyl-CoA can only enter the Krebs Cycle bound to oxaloacetate. When carbohydrate supplies are inadequate, however, the body naturally converts oxaloacetate to glucose for use by the brain and other tissues. When acetyl CoA does not bind with oxaloacetate, the liver converts it to ketones (or ketone bodies). If these ketones are not used for fuel by muscles, nerves, or the brain, they begin to accumulate and ultimately lead to a condition known as ketosis. During this process a high concentration of glucagon is present in the serum and this inactivates hexokinase and phosphofructokinase-1 (regulators of glycolysis) indirectly, 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. Glucose is a simple sugar made by the body and is necessary for proper bodily function. This glucose is used exclusively for energy by cells such as neurons and red blood cells.

Similar conditions

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 signals 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.

Diet

If the diet is changed from a highly glycemic diet to a diet that does not substantially contribute to blood glucose, the body goes through a set of stages to enter ketosis. During the initial stages of this process the adult brain does not burn ketones, however the brain makes immediate use of this important substrate for lipid synthesis in the brain. After about 48 hours of this process, the brain starts burning ketones in order to more directly utilize the energy from the fat stores that are being depended upon, and to reserve the glucose only for its absolute needs, thus avoiding the depletion of the body's protein store in the muscles.

Whether ketosis takes place can be checked by using special urine test strips such as Ketostix.

Deliberately induced ketosis through a low-carbohydrate diet has been used to treat medical conditions although most such treatments remain controversial.

See also

External links

Diabetic Ketoacidosis
Alcoholic Ketoacidosis

References

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