Postabsorptive state

AUostetic activator of hepatic pyruvate carboxylase in the postabsorptive state. 

Increased fatty acid oxidation The oxidation of fatty acids derived from adipose tissue is the major source of energy in hepatic tis sue in the postabsorptive state (see Figure 24.11, ). 

Infusion of too much glucose over an extended period in the treatment of hypoglycemia is dangerous (101). Glucose utilization in the postabsorptive state is 2 mg/ kg/minute and can increase, if insulin concentrations are high, to 6 mg/kg/minute (about 600 g/day in a 70 kg individual). It is better to infuse glucose in concentrations of 10 or 20% rather than 40%. 

The well-fed, or postabsorptive, state. After we consume and digest an evening meal, glucose and amino acids are transported from the intestine to the blood. The dietary lipids are packaged into chylomicrons and transported to the blood by the lymphatic system. This fed condition leads to the secretion of insulin, which is one of the two most... 

Ekberg K, Landau BR, Wajngot A, et al. Contributions by kidney and liver to glucose production in the postabsorptive state and after 60 h of fasting. Diabetes 1999 48 292-298. 

Lipoproteins complexes are soluble aggregates of lipids and proteins that transport lipids through the blood and lymph. Despite their differences in lipid and protein composition, all lipoproteins share common structural features, notably a spherical shape that can be detected by electron microscopy. The lipoproteins help maintain in solubilized form some 500 mg of total lipid per 100 mL of human blood in the postabsorptive state, after the contents of a meal have been digested and absorbed into the bloodstream. 

If we eat another meal within a few hours, we return to the fed state. However, if we continue to fast for a 12-hour period, we enter the basal state (also known as the postabsorptive state). A person is generally considered to be in the basal state after an overnight fast, when no food has been eaten since dinner the previous evening. By this time, the serum insulin level is low and glucagon is rising. Figure 3.1 illustrates the main features of the basal state. 

A. Interorgan Flux of Amino Acids in the Postabsorptive State... 

Fig. 42.3. Interorgan amino acid exchange after an overnight fast. After an overnight fast (the postabsorptive state), the utilization of amino acids for protein synthesis, for fuels, and for the synthesis of essential functional compounds continues. The free amino acid pool is supported largely by net degradation of skeletal muscle protein. Glutamine and alanine serve as amino group carriers from skeletal muscle to other tissues. Glutamine brings NH4 to the kidney for the excretion of protons and serves as a fuel for the kidney, gut, and cells of the immune system. Alanine transfers amino groups from skeletal muscle, the kidney, and the gut to the liver, where they are converted to urea for excretion. The brain continues to use amino acids for neurotransmitter synthesis.

Fig. 42.5. Hormonal regulation of hepatic amino acid metabolism in the postabsorptive state. Circled + = glucagon-mediated activation of enzymes or proteins circled t = induction of enzyme synthesis mediated by glucagon and glucocorticoids. Induction of urea cycle enzymes occurs both during fasting and after a high-protein meal. Because many individuals in the United States normally have a high-protein diet, the levels of urea cycle enzymes may not fluctuate to any great extent.

Fig. 42.13. Amino acid metabolism in the gut. The pathways of glutamine metabolism in the gut are the same whether it is supplied by the diet (postprandial state) or from the blood (postabsorptive state). Cells of the gut also metabolize aspartate, glutamate, and BCAA. Glucose is converted principally to the carbon skeleton of alanine. a-KG = a-ketoglutarate GDH = glutamate dehydrogenase TA = transaminase.

After a protein meal, dietary glutamine is a major fuel for the gut, and the products of glutamine metabolism are similar to those seen in the postabsorptive state. The gut also uses dietary aspartate and glutamate, which enter the TCA cycle. Colonocytes (the cells of the colon) also use short-chain fatty acids, derived from bacterial action in the lumen. 

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