Ketosis
From Wikipedia, the free encyclopedia
| Ketosis | |
|---|---|
| Classification and external resources | |
 Ketone bodies | |
| ICD-9 | 276.2 |
| DiseasesDB | 29485 |
| MeSH | D007662 |
Contents[show] |
[edit] Metabolic pathways
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.[1] 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.
[edit] Ketoacidosis
Main article: Ketoacidosis
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.[2] Prolonged excess of ketone bodies can overwhelm normal compensatory mechanisms, leading to acidosis if blood pH falls below 7.35.There are two major causes of ketoacidosis:
- Most commonly, ketoacidosis is diabetic ketoacidosis (DKA), resulting from increased fat metabolism due to a shortage of insulin. It is associated primarily with type I diabetes, and may result in a diabetic coma if left untreated.[3]
- Alcoholic ketoacidosis (AKA) presents infrequently, but can occur with acute alcohol intoxication, most often following a binge in alcoholics with acute or chronic liver or pancreatic disorders. Alcoholic ketoacidosis occurs more frequently following methanol or ethylene glycol intoxication than following intoxication with uncontaminated ethanol.[4]
[edit] Diet
If the diet is changed from a highly glycemic diet to a diet that does not provide sufficient carbohydrate to replenish glycogen stores, 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.Ketosis is deliberately induced by use of a ketogenic diet as a medical intervention in cases of non-responsive epilepsy.[5] Other uses of low-carbohydrate diets remain controversial.[6][7]
[edit] Diagnosis
Whether ketosis is taking place can be checked by using special urine test strips such as Ketostix.When the body is in ketosis, subjects often smell of acetone, sometimes described as a 'fruity breath' because the breath smells like pear drops.
[edit] Controversy
Some clinicians regard ketosis as a dangerous and potentially life-threatening state that stresses the liver and causes destruction of muscle tissues.[8][9][10][dubious ] It should be remembered from the above discussion that ketogenesis does not destroy muscle tissue. Ketogenesis can occur solely from the byproduct of fat degradation: acetyl-CoA. Ketosis, which is accompanied by gluconeogenesis (the creation of glucose de novo from amino acids), is the specific state with which clinicians are concerned.The anti-ketosis conclusions have been challenged by a number of doctors and advocates of low-carbohydrate diets, who dispute assertions that the body has a preference for glucose and that there are dangers associated with ketosis.[11][12][13] It has been argued that hunter-gatherer societies lived for thousands of years on a diet that would have been ketogenic, and there are many documented cases of modern humans living in these societies for extended periods of time.[14] While it is believed by some[who?] that exercise requires carbohydrate intake in order to replace depleted glycogen stores, studies have shown that, after a period of 2–4 weeks adaptation, physical endurance is unaffected by ketosis, as long as the diet contains high amounts of fat.[14]
