In the movie Fletch, released the same year that ACE was founded (1985), comedian Chevy Chase plays the role of Irwin “Fletch” Fletcher, a reporter working undercover to expose drug dealing on the beaches of Los Angeles. Over the course of his investigation, Fletch assumes a variety of creative characters as he identifies the corrupt businessmen and cops involved in the drug-dealing ring. In one scene, Fletch pretends to be an airplane mechanic and tries to fib his way into a hangar by saying he was there to check the ball bearings. “It’s all ball bearings nowadays,” is one of the most memorable quotes of this iconic ‘80s movie. 

If you work as a personal trainer or group fitness instructor you have no doubt been asked how exercise can help someone achieve a specific goal. If you want a fun, but honest way to answer these questions, you can channel your inner Fletch by replying, “It’s all hormones nowadays.” 

The endocrine system regulates the production of hormones, which are chemicals that control cellular functions. Hormones can affect a number of different cells; however, they only influence the ones with specific receptor sites. Hormones control a number of physiological reactions in the body including energy metabolism, reproductive processes, tissue growth, hydration levels, synthesis and degradation of muscle protein, and mood. Hormones are responsible for both building new muscle and helping to burn fat, so it is important to have an understanding of which ones are released in relation to exercise as well as understanding the physiological functions they influence. 

There are three major classifications of hormones: steroid, peptide and amines (modified amino acid hormones). Each class of hormones has a unique chemical structure that determines how it interacts with specific receptors. Steroid hormones interact with receptors in the nucleus of a cell, peptide hormones are comprised of amino acids and work with specific receptors sites on the cell membrane, and amines contain nitrogen and influence the sympathetic nervous system. 

Hormones can either be anabolic, which means they help build new tissue, or catabolic because they play a role in breaking tissue down. The term “anabolic steroids” is often mentioned as a method of cheating used by athletes who want to improve performance; however, anabolic steroids are actually natural chemicals produced by the body that are responsible for promoting tissue growth. 

Listed below are some important exercise and hormones along with the physiological functions they control.

Insulin

A peptide hormone produced by the pancreas, insulin regulates carbohydrate and fat metabolism. When blood sugar is elevated, insulin is released to promote the storage and absorption of glycogen and glucose. Insulin helps reduce levels of glucose in the blood by promoting its absorption from the bloodstream to skeletal muscles or fat tissues. It is important to know that insulin can cause fat to be stored in adipose tissue instead of being used to fuel muscle activity. When exercise starts, the sympathetic nervous system suppresses the release of insulin; consequently, it is important to avoid foods with high levels of sugar (including sports drinks) before exercise because it can elevate insulin levels and promote glycogen storage instead of allowing it to be used to fuel physical activity. Wait until the body has started sweating before using any sports drinks or energy gels. 

Glucagon 

Released in response to low levels of blood sugar, glucagon is produced by the pancreas to stimulate the release of free fatty acids (FFAs) from adipose tissue and increase blood glucose levels, both of which are important for fueling exercise activity. As glycogen levels are depleted during exercise, glucagon releases additional glycogen stored in the liver. 

Cortisol 

Cortisol is a catabolic steroid hormone produced by the adrenal gland in response to stress, low blood sugar and exercise. It supports energy metabolism during long periods of exercise by facilitating the breakdown of triglyceride and protein to create the glucose necessary to help fuel exercise. Cortisol is released when the body experiences too much physical stress or is not sufficiently recovered from a previous workout. While cortisol helps promote fat metabolism, exercising for too long can elevate levels of cortisol to catabolize muscle protein for fuel instead of conserving it to be used to repair damaged tissues. 

Epinephrine and Norepinephrine 

These amine hormones play an important role in helping the sympathetic nervous system (SNS) produce energy and in regulating the body’s function during cardiorespiratory exercise. Classified as catecholamines, epinephrine and norepinephrine are separate but related hormones. Epinephrine, often referred to as adrenaline because it is produced by the adrenal gland, elevates cardiac output, increases blood sugar (to help fuel exercise), promotes the breakdown of glycogen for energy and supports fat metabolism. Norepinephrine performs a number of the same functions as epinephrine, while also constricting blood vessels in parts of the body not involved in exercise. 

Testosterone 

Testosterone is a steroid hormone produced by the Leydig cells of the testes in males and the ovaries of females, with small amounts produced by the adrenal glands of both genders. Testosterone is responsible for muscle protein resynthesis and the repair of muscle proteins damaged by exercise, and plays a significant role in helping grow skeletal muscle. Testosterone works with specific receptor sights and is produced in response to exercise that damages muscle proteins. 

Human Growth Hormone 

Human growth hormone (HGH) is an anabolic peptide hormone secreted by the anterior pituitary gland that stimulates cellular growth. Like all hormones, HGH works with specific receptor sites and can produce a number of responses, including increasing muscle protein synthesis responsible for muscle growth, increasing bone mineralization, supporting immune system function and promoting lipolysis, or fat metabolism. The body produces HGH during the REM cycles of sleep and is stimulated by high-intensity exercise such as heavy strength training, explosive power training or cardiorespiratory exercise at or above the onset of blood lactate (OBLA, the second ventilatory threshold). 

Insulin-like Growth Factor 

Insulin-like growth factor (IGF) has a similar molecular structure to insulin and is stimulated by the same mechanisms that produce HGH. IGF is a peptide hormone produced in the liver and supports the function of HGH to repair protein damaged during exercise, which makes it an important hormone for promoting muscle growth. 

Brain-derived Neurotrophic Factor 

Brain-derived neurotrophic factor (BDNF) is a neurotransmitter that helps stimulate the production of new cells in the brain. The production of BDNF is closely related to the production of HGH and IGF—the same exercises that elevate levels of those hormones also increase amounts of BDNF. High-intensity exercise can stimulate anabolic hormones for muscle growth while elevating levels of BDNF, which can help improve cognitive function. 

Understanding how exercise influences the hormones that control physiological functions can assist you in developing effective exercise programs for your clients. Hormones have both short- and long-term responses to exercise. In the acute phase immediately post-exercise, testosterone (T), HGH and IGF are produced to repair damaged tissue. Over the long-term, there is an increase in the receptor sites and binding proteins, which allow T, HGH and IGF to be used more effectively for tissue repair and muscle growth. For clients who want muscle growth, the levels of T, HGH and IGF are produced in response to the amount of mechanical stress created during resistance-training exercises. Moderate to heavy loads performed until momentary fatigue generate high levels of mechanical force, which creates more damage to muscle protein, which signals the production of T, HGH and IGF to repair protein, which results in muscle growth. 

While there are a myriad of hormones responsible for an almost infinite number of physiological functions, the hormones listed above are directly influenced by physical activity and play important roles in helping the body adapt to the imposed physical demands of exercise. Many fitness professionals understand that the nervous and muscular systems play important roles in determining the outcomes of an exercise program. However, the reality is that hormones influence many of the physiological adaptations to physical activity. That means that, “It’s all hormones nowadays,” is the appropriate response to many questions about how the human body responds to exercise.