Brain : Metabolic Pathways

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. 1, ).
[Note: If blood glucose levels fall to <50 mg/dl (normal fasted blood glucose is 70–99 mg/dl), cerebral function is impaired .] 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 2.

Figure 1: Major metabolic pathways in the brain in the absorptive state. [Note: The numbers in circles, which appear both in the figure and in the text, indicate important pathways for carbohydrate metabolism.] CoA = coenzyme A; TCA = tricarboxylic acid; P = phosphate; GLUT = glucose transporter.

Figure 2:  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; CoA = coenzyme A; NADPH = nicotinamide adenine dinucleotide phosphate; TCA = tricarboxylic acid; VLDL = very-low-density lipoprotein.