Blood Glucose Is Regulated Between Meals And During Exercise
How is blood glucose maintained in between meals and overnight?
The complete digestion and absorption of a meal can take several hours, depending upon its size and composition. Therefore, carbohydrate or more specifically glucose from that meal may be available for several hours as well. However, once this ends, a new blood glucose scenario begins to take shape. Cells throughout the body will continue to help themselves to glucose in the blood to help meet their energy needs. The net effect is that our blood glucose concentration will begin to decrease. When this happens the pancreas responds again. However, this time it responds by releasing the hormone glucagon into our blood. In addition, epinephrine (adrenalin) and cortisol will promote efforts in different tissue that will help maintain blood glucose levels in between meals.
How does gluagon help maintain blood glucose levels in between meals?
Glucagon works in a manner that is generally opposite to insulin. It will labor to increase blood glucose concentration, thereby returning it toward normal levels. To accomplish this, glucagon promotes the breakdown of liver glycogen to glucose, which is released into circulation.
Glucagon will also promote another activity in our liver that will generate glucose. The process is called gluconeogenesis, which literally means to create new glucose if you read its root words right to left. In this process, certain amino acids, lactate (lactic acid), and glycerol from our circulation will be taken up by our liver and used to make glucose. Like the glucose generated from glycogen breakdown, this glucose can also be released into our blood to maintain blood glucose levels.
How does epinephrine (adrenalin) help maintain blood glucose levels during fasting?
During a fasting period, a little epinephrine (adrenalin) is released into circulation from our adrenal glands. Among epinephrine’s many roles will be its influence upon the liver and skeletal muscle. It will support the effects of glucagon in the liver that were just mentioned. In skeletal muscle, the slightly elevated epinephrine will lightly promote the breakdown of glycogen to glucose. Contrary to the glucose produced from the breakdown of liver glycogen, this glucose is not released into the blood. Rather, this glucose becomes a supportive energy source for those muscle cells while fat is the major energy source. However, when this glucose is used for energy in those cells, a little bit of lactate may be produced. This lactate can enter circulation, reach the liver, and be converted to glucose. This glucose can then be released into the blood. Therefore, our skeletal muscle can modestly contribute to maintaining our blood glucose concentration during fasting.
What does cortisol do to help maintain blood glucose levels during fasting?
Cortisol is often regarded as the “stress hormone” and, as will be discussed later. It is important to realize that fasting, especially prolonged fasting is a form of stress and stress results in the release of cortisol from the adrenal glands along with adrenalin mentioned in the previous question. Cortisol also supports the breakdown of glycogen and the conversion of amino acids, lactate, and glycerol to glucose in our liver. Because cortisol also promotes the breakdown of our body protein, especially skeletal muscle protein, it ensures a supply of amino acids for conversion to glucose in our liver.
What happens to stored carbohydrate (glycogen) during exercise?
The hormone picture that develops during exercise is similar to the one discussed regarding a fasting period; however, there are relative differences. Epinephrine is released from our adrenal glands as a direct effect of exercise. Quite simply, the greater the exercise intensity, the greater the epinephrine release. Epinephrine stimulates the breakdown of muscle cell glycogen. This makes glucose available for the muscle cells hard at work. Epinephrine also promotes the breakdown of glycogen to glucose in the liver. Some of this glucose will then circulate to working muscle to provide support. Cortisol may also be released in response to moderate to intense exercise, particularly as the exercise becomes prolonged (i.e., endurance cycling and running). Cortisol will also support the breakdown of glycogen as well as gluconeogenesis in our liver.