Overview of Fasting

OVERVIEW OF FASTING

Fasting begins if no food is ingested after the absorptive period. It may result from an inability to obtain food, the desire to lose weight rapidly, or clinical situations in which an individual cannot eat (for example, because of trauma, surgery, cancer, or burns). In the absence of food, plasma levels of glucose, amino acids, and TAG fall, triggering a decline in insulin secretion and an increase in glucagon and epinephrine release. The decreased insulin/counterregulatory hormone ratio and the decreased availability of circulating substrates make the period of nutrient deprivation a catabolic period characterized by degradation of TAG, glycogen, and protein. This sets into motion an exchange of substrates among liver, adipose tissue, skeletal muscle, and brain that is guided by two priorities: 1) the need to maintain adequate plasma levels of glucose to sustain energy metabolism in the brain, red blood cells, and other glucose-requiring tissues and 2) the need to mobilize fatty acids from adipose tissue and the synthesis and release of ketone bodies from the liver to supply energy to other tissues. [Note: Maintaining glucose requires that the substrates for gluconeogenesis (such as pyruvate, alanine, and glycerol) be available.]

A. Fuel stores

The metabolic fuels available in a normal 70-kg man at the beginning of a fast are shown in Figure 24.10. Note the enormous caloric stores available in the form of TAG compared with those contained in glycogen. [Note: Although protein is listed as an energy source, each protein also has a function (for example, as a structural component of the body, an enzyme, and so forth). Therefore, only about one third of the body’s protein can be used for energy production without fatally compromising vital functions.]

B. Enzymic changes in fasting

In fasting (as in the fed state), the flow of intermediates through the pathways of energy metabolism is controlled by four mechanisms: 1) the availability of substrates, 2) allosteric regulation of enzymes, 3) covalent modification of enzymes, and 4) induction–repression of enzyme synthesis. The metabolic changes observed in fasting are generally opposite to those described for the absorptive state (see Figure 24.9). For example, although most of the enzymes regulated by covalent modification are dephosphorylated and active in the fed state, they are phosphorylated and inactive in the fasted state. Three exceptions are glycogen phosphorylase, glycogen phosphorylase kinase, and HSL of adipose tissue, which are active in their phosphorylated states. In fasting, substrates are not provided by the diet but are available from the breakdown of stores and/or tissues, such as glycogenolysis with release of glucose from liver, lipolysis with release of FAs and glycerol from TAG in adipose tissue, and proteolysis with release of amino acids from muscle. Recognition that the changes in fasting are the reciprocal of those in the fed state is helpful in understanding the ebb and flow of metabolism.

Figure 24.10 Metabolic fuels present in a 70-kg man at the beginning of a fast. The fat stores are sufficient to meet energy needs for about 80 days.

Figure 24.9 Intertissue relationships in the absorptive state and the hormonal signals that promote them. [Note: Small circles on the perimeter of muscle and the adipocyte indicate insulin-dependent glucose transporters.] P = phosphate; PPP = pentose phosphate pathway; CoA = coenzyme A; NADPH = nicotinamide adenine dinucleotide phosphate; TCA = tricarboxylic acid; VLDL = very-low-density lipoprotein.