The endocrine system is the functional part of the body that produces hormones, substances that act at locations remote from the glands that produce them. Many hormones play a role in physical exercise; without them, exercise would be severely limited, if not impossible. By the same token, exercise itself influences hormone activity. As a result, modulation involving the endocrine system and exercise is a two-way street.
Overview of the Endocrine System
Most hormones are secreted from the glands that produce them under the influence of stimulating hormones from the hypothalamus. These hormones in turn are activated by releasing hormones from the pituitary gland. Hormones important in physical exercise include testosterone from the testes, norepinephrine and cortisol from the adrenal glands, insulin and glucagon from the pancreas, thyroxine from the thyroid gland and human growth hormone -- also called HGH or somatotropin -- from the pituitary gland. Failure of any of these glands to function properly rapidly leads to serious health problems if the relevant hormone is not replaced.
Exercise and Testosterone Levels
The anabolic steroids that weightlifters and other athletes illegally use to improve performance are artificial versions of the sex hormone testosterone. Testosterone contributes to muscle mass and strength, and without adequate levels of it you cannot exercise to your potential. A 2009 study in "Adaptive Medicine" demonstrated that a single exercise bout results in a rapid rise in serum testosterone, and that the amount of this increase is correlated with the duration, intensity and type of exercise. Overexertion, however, was found to have the opposite effect on testosterone levels.
The Pituitary-Adrenal Axis
The adrenal glands produce numerous hormones. These include norepinephrine, commonly called adrenalin and the driving force behind the "fight-or-flight" response, and cortisol, which is released in response to stress and blunts the immune system. Plasma cortisol levels rise rapidly in response to exercise at various intensities, as do levels of adrenocorticotropic hormone or ACTH, the pituitary hormone that stimulates cortisol release. Norepinephrine is also released in greater amounts during exercise, providing your system with increased energy in the form of glucose and free fatty acids.
Insulin and Glucagon
Insulin and glucagon are both synthesized in the pancreas, a gland that sits above the bowel and below the stomach. These hormones have competing functions; insulin stimulates glucose uptake by body tissues, whereas glucagon stimulates its release into the bloodstream. During exercise, these hormones continue to keep each other's actions in check, but in a way that allows you to exercise at various levels. Glucagon levels rise and insulin levels fall so as to increase glucose release into the bloodstream. When exercise ceases, insulin rises and glucagon drops so that ingested foods are rapidly used to replenish glycogen, the body's storage form of glucose.
References
- Virtual Medical Centre: Endocrine System
- "Adaptive Medicine"; Exercise and Testosterone; Te-Chi Liu, et al.; August 31, 2009
- "Journal of Applied Physiology"; Hypothalamic-Pituitary-Adrenal Responses to Short-Duration High-Intensity Cycle Exercise; W. J. Kraemer, et al.; January 1989
- Colorado State University: Adrenal Medullary Hormones
- "Canadian Joournal of Physiology and Pharmacology"; Glucose Metabolism During Exercise in Man: The Role of Insulin and Glucagon in the Regulation of Hepatic Glucose Production and Gluconeogenesis; Carole Lavoie, et al..; January 1997
