Brain

BRAIN

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 approximately 140 g/day to CO₂ and H₂O. The brain contains no significant stores of glycogen and is, therefore, completely dependent on the availability of blood glucose (Figure 24.8, 1). [Note: If blood glucose levels fall below 40 mg/100 ml (normal fasted blood glucose is 70–99 mg/100 ml), cerebral function is impaired.] The brain also lacks significant stores of TAG, and the FAs circulating in the blood make little contribution to energy production because FAs bound to albumin do not efficiently cross the BBB. The intertissue exchanges characteristic of the absorptive period are summarized in Figure 24.9.

Figure 24.8 Major metabolic pathways in 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 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.