Additional source of NADPH: The OAA is reduced to malate, which is oxidatively decarboxylated to pyruvate by malic enzyme as NADPH is formed (see Fig. 16.11 on p. 187).
2. Increased triacylglycerol synthesis: TAG synthesis is favored because fatty acyl CoA are available both from de novo synthesis and from hydrolysis of the TAG component of chylomicron remnants removed from the blood by hepatocytes (see p. 178). Glycerol 3-phosphate, the backbone for TAG synthesis, is provided by glycolysis (see p. 189). The liver packages these endogenous TAG into very-low-density lipoprotein (VLDL) particles that are secreted into the blood for use by extrahepatic tissues, particularly adipose and muscle tissues (see Fig. 24.3, ).
C. Amino acid metabolism 1.
Increased amino acid degradation: In the absorptive period, more amino acids are present than the liver can use in the synthesis of proteins and other nitrogen-containing molecules. The surplus amino acids are not stored but are either released into the blood for other tissues to use in protein synthesis or deaminated, with the resulting carbon skeletons being degraded by the liver to pyruvate, acetyl CoA, or TCA cycle intermediates. These metabolites can be oxidized for energy or used in FA synthesis (see Fig. 24.3, ). The liver has limited capacity to initiate degradation of the branched-chain amino acids (BCAA) leucine, isoleucine, and valine. They pass through the liver essentially unchanged and are metabolized in muscle (see p. 266).
2.
Increased protein synthesis: The body does not store protein for energy in the same way that it maintains glycogen or TAG reserves. However, a transient increase in the synthesis of hepatic proteins does occur in the absorptive state, resulting in replacement of any proteins that may have been degraded during the previous period of fasting (see Fig. 24.3, ).
IV. ADIPOSE TISSUE: ENERGY STORAGE DEPOT
Adipose tissue is second only to the liver in its ability to distribute fuel molecules. In a 70-kg man, white adipose tissue (WAT) weighs ~14 kg, or about half as much as the total muscle mass. Nearly the entire volume of each adipocyte in WAT can be occupied by a droplet of anhydrous, calorically dense TAG (Fig. 24.5).
A. Carbohydrate metabolism 1. Increased glucose transport: Circulating insulin levels are elevated in the absorptive state, resulting in an influx of glucose into adipocytes via insulin-sensitive GLUT-4 recruited to the cell surface from intracellular vesicles (Fig. 24.6, ). The glucose is phosphorylated by hexokinase.
2.
Increased glycolysis: The increased intracellular availability of glucose results in an enhanced rate of glycolysis (see Fig. 24.6, ). In adipose tissue, glycolysis serves a synthetic function by supplying glycerol 3phosphate for TAG synthesis (see p. 188). [Note: Adipose tissue lacks glycerol kinase.] 3.
Increased pentose phosphate pathway activity: Adipose tissue can metabolize glucose by means of the pentose phosphate pathway, thereby producing NADPH, which is essential for FA synthesis (see p. 186 and Fig. 24.6, ). However, in humans, de novo synthesis is not a major source of FA in adipose tissue, except when refeeding a previously fasted individual (see Fig. 24.6, ).
B. Fat metabolism