Ketone bodies
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robot Removing: [[la:Ketonum]], [[no:Keton]]
[[Image:Ketone bodies.png|thumb|120px|right|Chemical structures of the three ketone bodies: [[acetone]] (top), [[acetoacetic acid]] (middle), and [[beta-hydroxybutyric acid]] (bottom).]]
'''Ketone bodies''' are three water-soluble compounds that are produced as by-products when [[fatty acids]] are broken down for [[Energy (biology)|energy]] in the [[liver]] and [[kidney]]. They are used as a source of energy in the [[heart]] and [[brain]]. In the brain, they are a vital source of energy during [[fasting]].
The three ketone bodies are [[acetone]], [[acetoacetic acid]], and [[beta-hydroxybutyric acid]], although [[beta-hydroxybutyric acid]] is not technically a [[ketone]] but a [[carboxylic acid]].
==Uses in the heart and brain==
Ketone bodies can be used for energy. Ketone bodies are transported from the [[liver]] to other tissues, where [[acetoacetate]] and [[beta-hydroxybutyrate]] can be reconverted to [[acetyl-CoA]] to produce energy, via the [[Krebs cycle]].
The [[heart]] gets little energy from ketone bodies, it uses mainly fatty acids.
The [[brain]] gets its energy from ketone bodies when insufficient [[glucose]] is available (e.g., when [[fasting]]). In the event of low [[blood sugar|blood glucose]], most other tissues have additional energy sources besides ketone bodies (such as fatty acids), but the brain does not. After the diet has been changed to lower blood glucose for 3 days, the brain gets 30% of its energy from ketone bodies. After 4 days, this goes up to 70% (during the initial stages the brain does not burn ketones, since they are an important substrate for [[lipid]] synthesis in the brain). The brain retains some need for glucose, because ketone bodies can be broken down for energy only in the [[mitochondria]], and brain cells' long thin [[axon]]s are too far from mitochondria.{{Fact|date=October 2007}}
==Production==
[[Image:Acetyl-CoA-2D.png|thumb|[[Acetyl-CoA]]]]
Ketone bodies are produced from [[acetyl-CoA]] (see [[ketogenesis]]) mainly in the [[mitochondrial]] matrix of [[liver]] cells when [[carbohydrate]]s are so scarce that energy must be obtained from [[fatty acid metabolism|breaking down fatty acid]]s.
[[Acetone]] is formed from spontaneous [[decarboxylation]] of acetoacetate. In a corresponding manner, the levels of acetone are much lower than those of the other two types of ketone bodies. And, unlike the other two, acetone cannot be converted back to acetyl-CoA, so it is excreted in the [[urine]] and exhaled (it can be exhaled readily because it has a high [[vapor pressure]] and thus evaporates easily). The exhalation of [[acetone]] is responsible for the characteristic "fruity" odour of the breath of persons in ketotic states.
==Ketosis and ketoacidosis==
Any production of these compounds is called [[ketogenesis]], and this is necessary in small amounts.
But, when excess ketone bodies accumulate, this abnormal (but not necessarily harmful) state is called [[ketosis]]. Ketosis can be quantified by sampling the patient's exhaled air, and testing for acetone by [[gas chromatography]].<ref>{{cite journal | author = K. Musa-Veloso, S. S. Likhodii and S. C. Cunnane | title = Breath acetone is a reliable indicator of ketosis in adults consuming ketogenic meals | year = 2002 | journal = [[Am J Clin Nutr]] | volume = 76 | issue = 1 | pages = 65–70 | doi = | url = http://www.ajcn.org/cgi/content/abstract/76/1/65 | pmid = 12081817 }}</ref> Many diabetics self test for the presence of Ketones using blood or urine testing kits.
When even larger amounts of ketone bodies accumulate such that the body's pH is lowered to dangerously acidic levels, this state is called [[ketoacidosis]].
==Impact upon pH==
Both acetoacetate and beta-hydroxybutyrate are [[acid]]ic, and, if levels of these ketone bodies are too high, the [[pH]] of the blood drops, resulting in '''ketoacidosis'''.
This happens in untreated Type I [[diabetes]] (see [[diabetic ketoacidosis]]), and also in [[alcoholic]]s after binge drinking, subsequent starvation, and the alcohol-induced impairment of the liver's ability to generate glucose (gluconeogenesis)(see [[alcoholic ketoacidosis]]).
== See also ==
* [[Fatty acid metabolism]]
* [[Ketogenesis]]
* [[Ketosis]]
* [[Ketone|Ketones]] (General article. Not limited to the three ketones occuring in the body).
== References ==
<references/>
==External links==
* Atkins Nutritionals, Inc. [http://atkins.com/helpatkins/newfaq/categories/LipolysisKetosis.html Lipolysis and Ketosis]
* {{eMedicine|emerg|135}} - Diabetic Ketoacidosis
* [http://www.unisanet.unisa.edu.au/08366/h&p2fat.htm Fat metabolism at unisanet.unisa.edu.au]
* NHS Direct Online Health Encyclopaedia, [http://www.nhsdirect.nhs.uk/en.asp?TopicID=274 Ketosis]
* BioCarta Pathways [http://www.biocarta.com/pathfiles/ketonebodiesPathway.asp Formation of Ketone Bodies]
* {{MeshName|Ketone+Bodies}}
[[Category:Metabolism]]
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[[he:גופי קטון]]
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