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Ketone bodies are three water-soluble molecules that are produced by the liver from fatty acids during periods of low food intake (fasting) or carbohydrate restriction for cells of the body to use as energy instead of glucose. Two of the three are used as a source of energy in the heart and brain while the third (acetone) is a degradation breakdown product of acetoacetic acid. Radioactive tracing of acetone determines that between 2% and 30% is excreted from the body. Ketone bodies are picked up by cells and converted back into acetyl-CoA which then enters the citric acid cycle and is oxidized in the mitochondria for energy. In the brain, ketone bodies are also used to make acetyl-CoA into long chain fatty acids, which cannot pass through the blood-brain barrier. The liver additionally produces glucose from non-carbohydrate sources other than fatty acids by a process called gluconeogenesis during starvation. In the brain, ketone bodies are a vital source of energy during fasting or strenuous exercise. Although termed "bodies", they are molecules, not particles. The three endogenous ketone bodies are acetone, acetoacetic acid, and ''beta''-hydroxybutyric acid. Other ketone bodies like ''beta''-ketopentanoate and ''beta''-hydroxypentanoate may be created as a result of the metabolism of synthetic triglycerides, such as triheptanoin. ==Uses in the heart, brain and muscle (but not the liver)== 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 citric acid cycle. Ketone bodies cannot be used by the liver for energy, because the liver lacks the enzyme β-ketoacyl-CoA transferase, also called thiophorase. Acetone in low concentrations is taken up by the liver and undergoes detoxification through the methylglyoxal pathway which ends with lactate. Acetone in high concentrations due to prolonged fasting or a ketogenic diet is absorbed by cells other than those in the liver and enters a different pathway via 1,2-propanediol. Though the pathway follows a different series of steps requiring ATP, 1,2-propanediol can be turned into pyruvate.〔http://www.epa.gov/iris/toxreviews/0128tr.pdf〕 The heart preferentially utilizes fatty acids for energy under normal physiologic conditions. However, under ketotic conditions, the heart can effectively utilize ketone bodies for energy. The brain gets a portion of its energy from ketone bodies when glucose is less available (e.g., during fasting, strenuous exercise, low carbohydrate, ketogenic diet and in neonates). In the event of low blood glucose, most other tissues have additional energy sources besides ketone bodies (such as fatty acids), but the brain likely has an obligatory requirement for some glucose. After the diet has been changed to lower blood glucose for 3 days, the brain gets 25% of its energy from ketone bodies. After about 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). Furthermore, ketones produced from omega-3 fatty acids may reduce cognitive deterioration in old age. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「ketone bodies」の詳細全文を読む スポンサード リンク
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