Ketone bodies as fuel

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. 

Tissues that can use ketones as "fuel" brain, muscle, kidney, intestine P-hydroxybutyrate + NAD+ —> NADH + acetoacetate acetoacetate + succinyl CoA —> acetoacetyl CoA + succinate acetoacetyl CoA + CoA —> 2 acetyl CoA Ketone Bodies as Fuel... 

A) The brain uses glucose and ketone bodies as fuel sources... 

Skeletal muscles, the heart, the liver, and many other tissues use fatty acids as their major fuel during fasting and other conditions that increase fatty acids in the blood. However, a number of other tissues (or cell types), such as the brain, use ketone bodies to a greater extent. For example, cells of the intestinal muscosa, which transport fatty acids from the intestine to the blood, use ketone bodies and amino acids during starvation, rather than fatty acids. Adipocytes, which store fatty acids in tri-acylglycerols, do not use fatty acids as a fuel during fasting but can use ketone bodies. Ketone bodies cross the placenta, and can be used by the fetus. Almost all tissues and cell types, with the exception of liver and red blood cells, are able to use ketone bodies as fuels. 

The brain nses ketone bodies as fuel. If starvation continues for more than 2 days the brain adapts to using the ketone bodies as a fuel. Remember, during fasting, the liver converts fatty acids to ketone bodies. 

The hver makes ketone bodies from fatty acids during fasting. Fortunately, after 2 days of fasting, the brain adapts to use the ketone bodies as fuel, reducing the need for glucose and therefore decreasing the need for gluconeogenesis. 

Brain Brain does not burn fat as an energy source however, after adapting to long-term starvation, brain can use ketone bodies for fuel. 

Although the acetyl CoA from fatty acids cannot he converted to glucose, it can be converted to ketone bodies as an alternative fuel for cells, induding the brain. Chronic hypoglycemia is thus often accompanied physiologically by an increase in ketone bodies. 

Figure 7.20 The major quantitative pathway for fatty acid metabolism in the liver is ketone body formation. This is another indication of the importance of ketone bodies as a fuel.

Tissues that can use ketone bodies for fuels Unlike fatty acids, ketone bodies can be used by the brain (but not by cells, such as red blood cells, that lack mitochondria) and, thus, are important fuels during a fast. The liver lacks the ability to degrade ketone bodies, and so synthesizes them specifically for the peripheral tissues. 

Unlike skeletal muscle, heart muscle functions almost exclusively aerobically, as evidenced by the density of mitochondria in heart muscle. Moreover, the heart has virtually no glycogen reserves. Fatty acids are the heart s main source of fuel, although ketone bodies as well as lactate can serve as fuel for heart muscle. In fact, heart muscle consumes acetoacetate in preference to glucose. 

Upon waking in the morning, measure your urine with the Ketostix. Measuring ketones in the morning is necessary because the sticks show only the unused level of ketones in your body and should show trace to moderate ketone levels. If you measure them in the afternoon the numbers will not be as accurate, because most of the ketones that were produced will have been used by your body as fuel. 

Some organs (e.g., heart and skeletal muscle) can use ketone bodies (/J-hydroxybutyrate and acetoacetate) as an energy source under normal conditions. During starvation the brain uses them as an important fuel source. Because liver does not have /J-oxoacid-CoA transferase, it cannot use ketone bodies as an energy source. These reactions are reversible. 

The remnants of chylomicrons, depleted of most of their triacylglycerols but still containing cholesterol and apolipoproteins, travel in the blood to the liver, where they are taken up by endocytosis, mediated by receptors for their apolipoproteins. Triacylglycerols that enter the liver by this route may be oxidized to provide energy or to provide precursors for the synthesis of ketone bodies, as described in Section 17.3. When the diet contains more fatty acids than are needed immediately for fuel or as precursors, the liver converts them to triacylglycerols, which are packaged with specific apolipoproteins into VLDLs. The VLDLs are transported in the blood to adipose tissues, where the triacylglycerols are removed and stored in lipid droplets within adipocytes. 

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