Ketosis
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Ketosis (
IPA pronunciation: [ki'tosɪs]) is a stage in
metabolism occurring when the
liver has been depleted of stored
glycogen and switches to a chronic fasting mode during long periods of starvation.
During the chronic stage of starvation (after glycogen has run out), fat (triglycerol) 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 switching the primary energy source of most cells from using glucose to fatty acids. At the same time, new glucose is synthesized in the liver from
lactic acid, glucogenic
amino acids, and glycerol, in a process called
gluconeogenesis. This glucose is used exclusively by cells such as
neurons and
red blood cells.
Ketone bodies, from the breakdown of fatty acids to acetyl groups, are also produced during this fasting state, and are burned throughout the body. During the initial stages of
starvation the brain does not burn ketones, since they are an important substrate for lipid synthesis in the
brain. But after several days of starvation, the brain transitions to 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 slowing the depletion of the body's protein store in the muscles.
The brain retains 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.
Ketosis, which is a physiological condition in response to chronic starvation, should not be confused with
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,
glycolysis and
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.
The breath of people in a ketagenic state commonly contains
acetone, detectable as a sweet smell that may be mistaken for
ethyl alcohol.
Deliberately induced ketosis through a
low-carbohydrate diet has been used to treat medical conditions. The
ketogenic diet is an approach to treating
epilepsy, and the
Atkins Nutritional Approach is marketed for treating
obesity. The Lighter Life diet is recommended as a safer more controllable means of reaching ketosis for the purpose of fat burning within a professionally controlled environment. __________________
