Ketone Bodies

Even when a person is not losing weight, the triglycerides in adipose tissue are continuously being broken down and resyn-thesized. New triglycerides are produced, while others are hy-drolyzed into glycerol and fatty acids. This turnover ensures that the blood will normally contain a sufficient level of fatty acids for aerobic respiration by skeletal muscles, the liver, and other organs. When the rate of lipolysis exceeds the rate of fatty acid utilization—as it may in starvation, dieting, and in diabetes mellitus—the blood concentration of fatty acids increases.

If the liver cells contain sufficient amounts of ATP so that further production of ATP is not needed, some of the acetyl CoA derived from fatty acids is channeled into an alternate pathway. This pathway involves the conversion of two molecules of acetyl CoA into four-carbon-long acidic derivatives, acetoacetic acid and P-hydroxybutyric acid. Together with acetone, which is a three-carbon-long derivative of acetoacetic acid, these products are known as ketone bodies (see chapter 2, fig. 2.19).

Ketone bodies, which can be used for energy by many organs, are found in the blood under normal conditions. q Under conditions of fasting or of diabetes mellitus, however, the increased liberation of free fatty acids from adipose tissue results in the increased production of ketone bodies by the liver. The secretion of abnormally high amounts of ketone bodies into the blood produces ketosis, which is one of the signs of fasting or an uncontrolled diabetic state. A person in this condition may also have a sweet-smelling breath due to the presence of acetone, which is volatile and leaves the blood in the exhaled air.

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