The Primary importance of the brain
The brain needs as much as 50% of the body’s blood sugar (serum glucose). In order to function properly, the brain must have an adequate supply of glucose at all times. An inadequate supply of blood sugar results in a condition called neuroglycopenia, which causes the brain to malfunction and which can cause permanent brain damage, coma, and even death.
The brain has two different hormone systems at its disposal to maintain blood sugar at an adequate level. When the diet provides an adequate supply of glucose, the well known system involving the pancreas and the hormones insulin and glucagon is used. But during an emergency, when the diet does not produce an adequate supply of glucose, i.e., during starvation, the less well known system involving the hypothalamic-pituitary-adrenal axis and the hormone cortisol is used under the brain's direct control.
Pancreatic regulation when the supply of glucose is adequate
All glucose is extracted from food, either directly or indirectly. When the amount of glucose from a meal is more than adequate, the surplus glucose is stored temporarily for later use. The pancreas releases insulin, which acts on the insulin receptors in the walls of muscle, fat, and liver cells to remove some of the glucose from the blood and store it in the cells in the form of glycogen.
When the amount of glucose remaining in the blood becomes inadequate, glucose is brought back out of storage into the blood. The pancreas releases the hormone glucagon, which acts on the glucagon receptors in the walls of fat and liver cells to cause the stored glycogen to be converted back into glucose and released into the blood stream.
HPA axis regulation when the supply of glucose is not adequate
An emergency occurs when the amount of stored glucose is not sufficient to maintain the glucose level needed by the brain. A state of starvation has occurred. The brain uses the hypothalamic-pituitary-adrenal axis and the hormone cortisol to create (1) a condition called insulin resistance to retain the remaining glucose in the blood by preventing it from entering the tissues of less important organs and (2) a condition called gluconeogenesis, in which the liver converts protein and amino acids into glucose and releases this newly manufactured glucose into the bloodstream.
