After consuming a meal containing fats, most of the fatty acids (FA) that are added to the TAG stores in adipocytes come from the breakdown of exogenous (dietary) TAG in chylomicrons released by the intestine and endogenous TAG in VLDL released by the liver. Lipoprotein lipase (LPL), an extracellular enzyme attached to the endothelial cells of capillary walls in various tissues including adipose and muscle, releases the FA from the lipoproteins. In adipose tissue, insulin upregulates LPL, favoring the storage of TAG. This is facilitated by elevated levels of glucose and insulin in the fed state, where all the carbons needed for TAG are supplied by glucose.

Skeletal muscle, which accounts for about 40% of the body mass in individuals of healthy weight, can utilize glucose, amino acids, fatty acids, and ketone bodies as sources of fuel. In the well-fed state, muscle takes up glucose through GLUT-4 for energy production and glycogen synthesis, as well as amino acids for energy and protein synthesis. Unlike the liver, there is no covalent regulation of PFK-2 in skeletal muscle. However, in the cardiac isozyme, the kinase domain is activated through epinephrine-mediated phosphorylation.

Skeletal muscle has the unique ability to respond to significant changes in ATP demand during contraction. At rest, muscle consumes approximately 25% of the body's oxygen (O2), which can increase up to 90% during intense exercise. This highlights the fact that skeletal muscle is primarily an oxidative tissue, despite its capacity for temporary periods of anaerobic glycolysis.

A. Carbohydrate metabolism:
1. Enhanced glucose transport: Following a carbohydrate-rich meal, the temporary rise in plasma glucose and insulin levels stimulates increased glucose transport into muscle cells (myocytes) through GLUT-4. This process helps reduce blood glucose levels. Glucose is then phosphorylated to glucose 6-phosphate by hexokinase and metabolized to meet the energy requirements of myocytes.
2. Increased glycogenesis: The elevated insulin/glucagon ratio and the presence of glucose 6-phosphate promote glycogen synthesis, especially if glycogen stores have been depleted due to exercise.

B. Fat metabolism:
Fatty acids are released from chylomicrons and VLDL with the assistance of LPL. However, during the well-fed state, FA serve as a secondary source of fuel for resting muscle, with glucose being the primary energy source.

C. Amino acid metabolism:
1. Augmented protein synthesis: Following the ingestion of a protein-rich meal, there is an increase in amino acid uptake and protein synthesis during the absorptive period. This synthesis helps replace the proteins that have been degraded since the previous meal.
2. Increased uptake of branched-chain amino acids (BCAA): Muscle is the primary site for the degradation of BCAA due to the presence of the necessary transaminase. Dietary BCAA bypass metabolism in the liver and are taken up by muscle, where they are utilized for protein synthesis and as energy sources.

VI. BRAIN
Most of the FA added to the TAG stores of adipocytes after consumption of a lipid-containing meal are provided by the degradation of exogenous (dietary) TAG in chylomicrons sent out by the intestine and endogenous TAG in VLDL sent out by the liver (see Fig. 24.6, ). The FA are released from the lipoproteins by lipoprotein lipase (LPL), an extracellular enzyme attached to the endothelial cells of capillary walls in many tissues, particularly adipose and muscle (see p. 228). In adipose tissue, LPL is upregulated by insulin. Thus, in the fed state, elevated levels of glucose and insulin favor storage of TAG (see Fig. 24.6, ), all the carbons of which are supplied by glucose. [Note: Elevated insulin favors the dephosphorylated (inactive) form of HSL (see p. 189), thereby inhibiting lipolysis in the well-fed state.]
V. RESTING SKELETAL MUSCLE
Skeletal muscle accounts for ~40% of the body mass in individuals of healthy weight, and it can use glucose, amino acids, FA, and ketone bodies as fuel. In the well-fed state, muscle takes up glucose via GLUT-4 (for energy and glycogen synthesis) and amino acids (for energy and protein synthesis). In contrast to liver, there is no covalent regulation of PFK-2 in skeletal muscle. However, in the cardiac isozyme, the kinase domain is activated by epinephrine-mediated phosphorylation (see p. 100).
Skeletal muscle is unique in being able to respond to substantial changes in the demand for ATP that accompanies contraction. At rest, muscle accounts for ~25% of the oxygen (O2) consumption of the body, whereas during vigorous exercise, it is responsible for up to 90%. This underscores the fact that skeletal muscle, despite its potential for transient periods of anaerobic glycolysis, is an oxidative tissue.
A. Carbohydrate metabolism 1. Increased glucose transport: The transient increase in plasma glucose and insulin after a carbohydrate-rich meal leads to an increase in glucose transport into muscle cells (myocytes) by GLUT-4 (see p. 97 and Fig. 24.7, ), thereby reducing blood glucose. Glucose is phosphorylated to glucose 6-phosphate by hexokinase and metabolized to meet the energy needs of myocytes.
2. Increased glycogenesis: The increased insulin/glucagon ratio and the availability of glucose 6-phosphate favor glycogen synthesis, particularly if glycogen stores have been depleted as a result of exercise (see p. 126 and Fig. 24.7, ).
B. Fat metabolism
FA are released from chylomicrons and VLDL by the action of LPL (see pp. 228 and 231). However, FA are of secondary importance as a fuel for resting muscle during the well-fed state, in which glucose is the primary source of energy.
C. Amino acid metabolism 1.
Increased protein synthesis: An increase in amino acid uptake and protein synthesis occurs in the absorptive period after ingestion of a meal containing protein (see Fig. 24.7, and ). This synthesis replaces protein degraded since the previous meal.
2.
Increased branched-chain amino acid uptake: Muscle is the principal site for degradation of the BCAA because it contains the required transaminase (see p. 266). The dietary BCAA escape metabolism by the liver and are taken up by muscle, where they are used for protein synthesis (see Fig. 24.7, ) and as energy sources.
VI. BRAIN