Ketone bodies fasting

In the advanced stages of fasting, ketone bodies cease to provide substantial metabolic fuel for skeletal and heart muscle and kidneys. They are used almost exclusively by the brain, which also continues to use glucose, albeit in drastically lowered amounts. Muscle, heart, and kidneys depend exclusively on fatty acids for their subsistence. Glucose continues to be used exclusively by the... 

Muscle Fast twitch Slow twitch Rapid movement Sustained movement Glycolysis Aerobic pathways, eg, p-oxidation and citric acid cycle Glucose Ketone bodies, triacylglycerol in VLDL and chylomicrons, free fatty acids Lactate Lipoprotein lipase. Respiratory chain well developed. 

There is adequate proof that ketone bodies originate in the liver81 1H 192-199 and that the muscles can utilize the ketone bodies in fasting or diabetic ketosis200-202 as well as normally.208-208 Shaw206 has... 

With increasing metabolism of fat through p oxidation, much of the mitochondrial CoA pool may become tied up as acyl- or acetyl-CoA. In such cases, the supply of free CoA can be diminished, and this may limit the rate of p oxidation. Upon prolonged fasting and heavy reliance on fat for energy, the liver induces the enzymes required for the formation of ketone bodies and brain induces enzymes required for their metabolism. 

During periods of fasting, muscles may also derive energy from the metabolism of ketone bodies (3-hydroxybutyrate and acetoacetate). These intermediates are... 

The cyde is central to the oxidation of any fuel that yields acetyl CoA, induding glucose, fritty acids, ketone bodies, ketogenic amino acids, and alcohol There is no hormonal control of the cyde, as activity is necessary irrespective of the fed or fasting state. Control is exerted by the energy status of the cell. 

The plasma ketone body level increases twice as fast over live days starvation in lean snbjects compared with obese this may be an attempt by the body to restrict protein degradation in the lean by providing an alternative fnel to glucose for the brain as qnickly as possible. 

A. If fasting extends past 1-2 days, which is considered to be a long-term fast or starvation, further changes in fuel synthesis and use by several organs can occur, principally a conversion from a glucose economy to one dominated by ketone bodies as fuel (Figure 5—6). 

E. Within a few days of fasting, the brain adapts to be able to utilize ketone bodies as fuel and becomes less dependent on, but never completely independent of, glucose. 

Fig. 1.4.4 Ketone body levels according to fasting time and age...

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