A part of my studies in exercise physiology.
Did you know that your endocrine system plays a huge roll in your body especially when it comes to hormones and how these hormones work during exercise? Now you ask what is your endocrine system? The endocrine system is one of the two communication systems in the body. It is the chemical communication system. The other communication system is the nervous system.
The endocrine system does react slower than your nervous system, but in the long run it lasts longer. It maintains homeostasis (define) via hormones. Hormones are chemical communicate control and regulate cell/organ activity and act on target cells. It constantly monitors your internal environment. Coordinates integration of physiological systems during rest and exercise and helps maintain homeostasis during exercise by controlling substrate metabolism (aka carbohydrate metabolism and fat metabolism) while regulating fluid and electrolyte balance. There are lots of different types of hormones in the body but there are only some that help with carbohydrate and fat metabolism.
Did you know that there are 4 major endocrine glands responsible for metabolic regulation besides just your lean muscle mass? These glands release hormones that affect metabolism of carbohydrate and fat during exercise. We will be getting into fat metabolism in the next blog entry. The four major endocrine glands are: anterior pituitary gland, thyroid gland, adrenal gland, and pancreas.
So the pituitary gland is attached to the inferior hypothalamus and has three lobes: anterior, intermediate, and posterior. It secretes hormones in response to the hypothalamic hormone factors. It either brings on the release of these hormones or inhibits these hormones from releasing. Exercise increases secretion of all anterior pituitary hormones.
The first hormone is a growth hormone (GH). This is a potent anabolic hormone that builds tissues and organs, promotes muscle growth (known as hypertrophy) and also stimulates fat metabolism. The release of the growth hormone is proportional to exercise intensity.
The next hormone is called thyrotropin, also known as thyroid -stimulating hormone (TSH). It travels to the thyroid to stimulate T3 and T4. T3 and t4 lead to increase in the metabolic rate of all tissues, protein synthesis, number and size of mitochondria (part of a cell), glucose uptake by cells, rate of glycolysis and gluconeogenesis (one of the ways we produce energy for the body) and free fatty acid (FFA) mobilization.
Next gland is the adrenal medulla (one of the adrenal glands). It releases catecholamines, which are hormones responsible for your fight or flight mechanism. They are 80% epinephrine and 20% norepinephrine. When exercise increases, which communicates to the sympathetic nervous system, which also increases, to release the epinephrine and norepinephrine. When the catecholamine hormones are released, it increases your heart rate, the force at which your muscles contract and your blood pressure. It also increases the rate at which glycogenolysis and free fatty acids work, as well as the blood flow that runs to your skeletal muscles.
Next adrenal gland is the adrenal cortex. This is where the corticosteroids are released. There are three corticosteroids that are released here called glucocorticoids, mineralocorticoids, and gonadocorticoids. One of the major glucocorticoids is cortisol and we all know what that can do. Cortisol in normal amounts is responsible for inflammatory and anti-immune responses. It also is used in energy making through gluconeogensis. It increases free fatty acid mobilization and protein catabolism as well.
The final gland is the pancreas. As we know the pancreas is primary responsible for making of insulin which is one of the hormones. Insulin lowers our blood sugar and opposes the glucagon that the body makes. Glucagon is the other hormone and this raises our blood glucose levels. Insulin on the other hand is used for glucose transport to cells and increases synthesis of glycogen (muscle energy), protein, and fats. It inhibits gluconeogensis. Glucagon on the other hand increase glycogenolysis and gluconeogensis.
Now what does these have to do carbohydrate metabolism and fat metabolism? You probably heard mention things that use fat or glucose which is one of the things carbohydrates are made out of which is sugar, but how do these help regulate those metabolisms in the body using exercise?
For carbohydrate metabolism to properly work, there first must be glucose available to the tissues. Glycogenolysis is when the body uses your glycogen and transforms it back into glucose. Glycogen is what glucose is stored as. To be able to make energy for the muscles, it has to be made back into glucose. Now there is another energy making pathway called gluconeogensis, which I mentioned previously. This pathway is where those free fatty acids (FFAs) and proteins are made into glucose. This is a harder pathway and takes a lot more energy to work. This is why the body likes to use carbohydrates as its main source of energy. Also the brain only uses carbohydrates for energy. Fats and proteins can not be used by the brain.
For exercise to work properly, there needs to be an adequate amount of glucose so its released in two areas: muscles (glycogen) and liver. The body will use the stores in the muscles before it goes to use the glucose in the liver. Now certain hormones that were before mentioned help increase the circulation of glucose in the blood stream. These are glucagon, epinephrine, norepinephrine, cortisol, growth hormone, and the T3 and T4 hormones. Now remember circulating glucose can be affected by other things as well, including growth hormone which increases free fatty acid mobilization, but decreases glucose being brought into cells. The T3 and T4 hormones increase glucose catabolism and fat metabolism (which we will discuss in a moment). The amount of glucose that is released from the liver does depend on two factors: intensity and duration. Higher the intensity and longer the work the more that is released from the liver for glycogenolysis. As your glycogen levels decrease though, your glucagon levels go up. With exercise duration, when the muscle glucose uptake increases the amount of liver glucose that is being released goes up.
Now there is more to insulin than just glucose mobilization. Insulin enables glucose uptake into the muscles to be stored as glycogen. Now during exercise, insulin concentrations in the body go down, but the body becomes more sensitive to the insulin that is already there. So basically the more exercise you are doing the better insulin sensitivity you will have, meaning the more glucose that is taken into the cells the less insulin the body will use.
The carbohydrate metabolism is regulated by the central nervous system by these hormones as well as nutrients brought into the body (glucose, fatty acids, amino acids). The brain is sensitive to the glucose and helps control the insulin release. Two other hormones that are released from adipose (fat) tissue is leptin and GLP-1. These act through the central nervous system by decreasing glucose production. Now remember glucose is the only substrate (nutrient) that brain metabolism uses.
So as you can see carbohydrate are an important part of your metabolism as well as hormones. They are also a very important part of brain metabolism and brain function as they are the only type of nutrient that the brain can use.
Stay tuned for the next blog article which will be on Regulation of Fat Metabolism During Exercise.