The brain, although accounting for only 2% of the adult weight, consumes 20% of the body's basal oxygen at rest. Due to its crucial role in the functioning of all body organs, the brain's fuel needs are given top priority. In order to provide energy, substances must be able to pass through the endothelial cells that line the blood vessels in the brain, known as the blood-brain barrier (BBB). During the fed state, the brain relies exclusively on glucose as its fuel, completely oxidizing around 140 grams per day to produce carbon dioxide and water. Since the brain does not store significant amounts of glycogen, it is entirely dependent on the availability of blood glucose. If blood glucose levels drop below 50 mg/dl (normal fasting blood glucose is 70-99 mg/dl), cerebral function is impaired. The brain does not store substantial amounts of triglycerides (TAG) either, and the fatty acids circulating in the blood have little contribution to energy production, for reasons that are unclear. The intertissue exchanges that occur during the absorptive period are summarized in Figure 24.9.

Moving on to the fasted state, fasting begins when no food is consumed after the absorptive period. This can occur due to the inability to obtain food, a desire for rapid weight loss, or clinical situations where an individual is unable to eat, such as trauma, surgery, cancer, or burns. In the absence of food, the levels of glucose, amino acids, and TAG in the plasma decrease, leading to a decline in insulin secretion and an increase in glucagon, epinephrine, and cortisol secretion. This change in hormone levels, along with the decreased availability of circulating substrates, initiates a catabolic period during nutrient deprivation. During this period, TAG, glycogen, and protein are degraded. This triggers an exchange of substrates among the liver, adipose tissue, skeletal muscle, and brain, guided by two priorities: 1) maintaining adequate plasma glucose levels to sustain energy metabolism in the brain, red blood cells, and other glucose-requiring tissues, and 2) mobilizing fatty acids from adipose tissue for the synthesis and release of ketone bodies by the liver, which supply energy to other tissues and preserve body protein. As a result, blood glucose levels are maintained within a narrow range during fasting, while fatty acid and ketone body levels increase. It is important to note that maintaining glucose requires the availability of substrates for gluconeogenesis, such as pyruvate, alanine, and glycerol.

Now let's discuss the fuel stores available in a normal 70-kg man at the start of a fast, as depicted in Figure 24.10. The considerable caloric stores present in the form of TAG far exceed those found in glycogen. Additionally, it should be noted that although protein is listed as an energy source, each protein also serves a function unrelated to energy metabolism, such as acting as a structural component or an enzyme. Therefore, only approximately one third of the body's protein can be utilized for energy production without severely compromising vital functions.

Moving on to enzymic changes...
Although contributing only 2% of the adult weight, the brain accounts for a consistent 20% of the basal O2 consumption of the body at rest. Because the brain is vital to the proper functioning of all organs of the body, special priority is given to its fuel needs. To provide energy, substrates must be able to cross the endothelial cells that line the blood vessels in the brain (the blood–brain barrier [BBB]). In the fed state, the brain exclusively uses glucose as a fuel (GLUT-1 of the BBB is insulin independent), completely oxidizing ~140 g/day to carbon dioxide and water. Because the brain contains no significant stores of glycogen, it is completely dependent on the availability of blood glucose (Fig. 24.8, ). [Note: If blood glucose levels fall to <50 mg/dl (normal fasted blood glucose is 70–99 mg/dl), cerebral function is impaired (see p. 315).] The brain also lacks significant stores of TAG, and the FA circulating in the blood make little contribution to energy production for reasons that are unclear. The intertissue exchanges characteristic of the absorptive period are summarized in Figure 24.9.
VII. OVERVIEW OF THE FASTED STATE
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, epinephrine, and cortisol secretion. The decreased insulin/counterregulatory hormone ratio and the decreased availability of circulating substrates make the postabsorptive period of nutrient deprivation a catabolic period characterized by degradation of TAG, glycogen, and protein. This sets into motion an exchange of substrates among the 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 FA from TAG in WAT for the synthesis and release of ketone bodies by the liver to supply energy to other tissues and spare body protein. As a result, blood glucose levels are maintained within a narrow range in fasting, while FA and ketone body levels increase. [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. Observe 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 unrelated to energy metabolism (for example, as a structural component of the body or as an enzyme). Therefore, only about one third of the body’s protein can be used for energy production without fatally compromising vital functions.]
B. Enzymic changes