Metabolic Considerations For The Personal Trainer

Barbell and Kettlebell Weights

The Endocrine System: An Introduction


Inseparable Relationship

"Nutrient intake exerts a fine control over synthesis and release of hormones," according to Eisenstein, in his paper, "Interrelationship of Nutrition and Endocrinology," published in the American Journal of Clinical Nutrition. He adds, "The physiological effects of insulin and growth hormone in utilisation of carbohydrates, amino acids and lipids serve to emphasise further the inseparable relationship of nutrition”


The endocrine system is a collection of hormone producing glands and cells located in various parts of the body, such as the pancreas and the ovaries. The function of the endocrine system is to produce hormones.


Hormones are chemical substances that are secreted into the bloodstream and control and influence body functions, such as metabolism, growth and sexual reproduction. Hormones are secreted from the endocrine gland and are then transported in the blood to their target organ. Hormones are complex chemical substances produced by specialised endocrine glands, or by cells in some body organs, such as the heart or a part of the gastro-intestinal tract.


Secreted into internal body fluids, many hormones target specific tissues, controlling their function and, in some organs, stimulating the production of further hormones. A special class of hormones, the prostaglandins, produce only local effects at their site of production.

Structure & Function of the Endocrine System 

The endocrine system is closely linked to the nervous system and together they act as communication centres of the body. The nervous system communicates its messages via electrical impulses and the endocrine system communicates with chemical messages in the form of hormones. Hormones tend to be slower acting than nerves and are responsible for changes within the body, e.g. growth.


The endocrine system comprises a series of internal secretions which help to regulate body processes by providing a constant internal environment. Hormones act as catalysts in that they affect the physiological activities of other cells in the body.


A hormone is a chemical messenger secreted by an endocrine gland, which reaches its destination by the bloodstream, and has the power of influencing the activity of other organs. Some hormones have a slow action over a period of years (e.g. growth hormone from the pituitary gland) while others have a quick action (e.g. adrenaline from the adrenal glands).Hormones, therefore, regulate and coordinate various functions of the body.


The endocrine glands are ductless glands and secrete their hormones directly into the bloodstream to influence the activity of other organs or glands. The main endocrine glands are:

  • Pituitary gland

  • Thyroid gland

  • Parathyroid glands

  • Adrenal gland

  • Islets of Langerhans

  • Ovaries in women

  • Testes in men


Pituitary Gland


This is situated at the base of the brain behind the nose and consists of two lobes. The pituitary gland is also known as the master gland because of its controlling effect on the other glands. The anterior lobe produces hormones that control other endocrine glands and other body systems:

  • ACTH (adrenocorticotrophic hormone) controls the cortex of the adrenal gland.

  • TSH (thyroid stimulating hormone) controls the thyroid gland.

Gonadotrophins (FSH – follicle stimulating hormone and LH luteinising hormone) control the ovaries and testes.


GH (growth hormone) promotes growth of the skeletal and muscular systems.


PRL (prolactin) promotes growth of the ovaries, testes and mammary glands and stimulates lactation (milk production) in the breasts.


MSH (melanocyte stimulating hormone) promotes the production of melanin in the skin. The posterior lobe produces two hormones that have an effect on the kidneys and the reproductive organs in women.


ADH (antidiuretic hormone) decreases urine production by the kidneys to regulate fluid balance.


OT (oxytocin) stimulates the uterine and mammary gland contraction in preparation for childbirth and breastfeeding.


The endocrine glands in the body have a feedback mechanism which is co-ordinated by the pituitary gland. The pituitary is influenced by the hypothalamus, and will increase its output of releasing factors if other glands start to fail, or will decrease its output if the level of the hormones in the bloodstream starts to rise.


Pineal Gland


This is situated deep within the brain between the cerebral hemispheres. It is often referred to as the ‘third eye’ because of its position. It is responsible for the hormone melatonin which is known as ‘the chemical expression of darkness’ as it is produced at night in response to fading daylight. The pineal gland is thought to be associated with seasonal affective disorder (SAD). The darker winter months bring about an increased production of melatonin making a person feel tired and sad. Melatonin regulates the daily sleep/wake cycle, monthly menstrual cycle and body rhythms, i.e.

the cycles of life.


Thyroid Gland

This is situated just below the larynx and in front of the trachea in the neck. It consists of two lobes and is responsible for the secretion of three hormones in response to the production of TSH in the pituitary gland. Thyroxine and Triiodothyronine regulate metabolism. Calcitonin helps to maintain calcium and phosphorus levels by stimulating the storage of calcium and phosphorus in the bones and the release of excess in urine.

Parathyroid Glands

These are situated in two pairs on either side of the back of the lobes of the thyroid gland and are responsible for assisting the thyroid gland in the regulation of calcium and phosphorus levels with the production of parathormones.


PTH (parathormone) stimulates the re-absorption of calcium and phosphorus when levels in the body are low, from the bones and decreases the amount lost in urine.

Thymus Gland

Situated behind the sternum in between the lungs. It is made of lymphatic tissue contributing to the immune functions of the body by producing a group of hormones called thymosins. Thymosin stimulates the production of Tlymphocytes to protect the body against antigens (harmful substances).

Adrenal Glands

This is situated on top of each kidney.Each gland consists of an outer cortex and inner medulla.


Cortex – produces hormones known as steroids in response to the production of ACTH in the pituitary gland:

  • Glucocorticoids including cortisol and cortisone stimulate metabolism, development and inflammation.

  • Mineralocorticoids including aldosterone regulate mineral concentrations in the body.

  • Gonadocorticoids including androgens stimulate sexual development .

Medulla – produces stress hormones in response to stimulation by the sympathetic nervous system to prepare the body for ‘fight or flight’:


Adrenalin and noradrenalin stimulate the body systems needed for the physical action, e.g. muscular and respiratory, and shutting down those not needed, e.g. digestive and urinary. Humans have two adrenal glands located near the kidneys.Each gland is really a double gland consisting of an outer cortex and an inner medulla.

Adrenalin causes a rise in blood pressure, acceleration of heartbeat, increased conversion of glycogen to glucose, rapid release of glucose into the blood, increased oxygen consumption and release of reserve red blood cells from the spleen into the circulating blood. The blood flow in skeletal and heart muscle is increased; the flow to smooth muscle in the digestive tract is decreased. Pupils of the eye dilate.

The Pancreas

The pancreas is a compound organ composed of two kinds of glandular tissue. Most of it is exocrine (meaning that it secretes digestive enzyme products through a duct). About 1 percent of the gland is composed of clusters of quite different cells that have an endocrine function.

Islets of Langerhans

These are situated in small clusters at regular intervals within the pancreas. These are responsible for the regulation of the blood sugar levels with the production

of two hormones:

  • Insulin reduces blood sugar levels by promoting the storage of sugar (glycogen) in the liver and muscles.

  • Glucagon increases blood sugar levels by promoting the release of glycogen from the liver and muscles.



These are situated within the female pelvic girdle on either side of the uterus. They are responsible for the secretion of female sex hormones:

  • Oestrogen and progesterone are responsible for the development of secondary sexual characteristics e.g. start of menstruation, development of breasts, widening of hips and the growth of pubic and axillary hair.



These are situated in the male scrotum which hangs externally from the body. They are responsible for the secretion of the male sex hormone:

  • Testosterone is responsible for the development of secondary sexual characteristics, e.g. production of sperm and semen, the change in voice, development of muscles, bones and male pattern hair growth.


Functions of the Endocrine System


The way the endocrine system works is to ensure that the changing needs of the body are met in terms of homeostasis, growth and sexual development. Homeostasis is the maintenance of a constant state, e.g. body temperature, mineral balance, blood pressure, fluid balance, etc.. Growth occurs in natural phases throughout life with rapid growth in the first year of life, slow steady growth during childhood, rapid growth during puberty with growth ceasing at the average age of 16- 17.


Sexual development occurs in stages throughout life starting with puberty in both sexes and ending with the menopause in women and andropause in men. These functions are controlled, communicated and maintained by the links between the nervous and endocrine systems. The nervous tissue in the hypothalamus in the brain forms an attachment with the pituitary gland allowing a two-way communication between the two systems. The endocrine system and nutrition intertwine to help you

sustain health. The endocrine system secretes hormones into the bloodstream to regulate your body, and eating different types of foods can affect certain hormones, which in turn increase or decrease your risk of disease.

A balanced diet of healthy foods enhances the function of your endocrine system, whereas eating an unhealthy diet increases the risk of hormonal imbalance. The connection between blood sugar levels and diabetes is one obvious example; because the pancreas of people with diabetes does not secrete adequate insulin, management of the disease involves nutritional regulation as well as insulin intake. More recently, researchers have found complex relationships between nutritional status and endocrine health in regulation of blood fats, or lipids, obesity and proper function of the gonads. There is a connection between eating disorders and bipolar illness, and the mind-body interface is evident in nutritional problems associated with bulimia

and anorexia nervosa.

Insulin & Glucagon

All cells of your body need glucose to fulfil their energy needs. For their proper functioning, the body has to maintain the levels of glucose in your blood within a normal range. While too little blood glucose can exhaust your cells, too much glucose in the blood can also damage them. The pancreas plays a primary role in maintaining normal levels of blood glucose by secreting insulin and glucagon. Together, these two hormones allow you to store energy when you eat and efficiently spend it during muscular and mental exercise. Abnormally low or high levels of glucose usually imply a health problem. 



Insulin is a hormone made in the beta cells of your pancreas. When you eat a meal rich in sugars, insulin causes the utilisation of blood glucose by your body cells and is also responsible for storing the surplus energy as glycogen in the muscles and liver. Interestingly, once your blood glucose reaches the desired levels, insulin generation is reduced by the pancreas to avoid hypoglycemia (low blood glucose). If you have diabetes, in which the body's generation or recognition of insulin is ineffective, normal blood glucose levels need continual monitoring. Your physician may prescribe injections of insulin to help you utilise the glucose in your blood.



Glucagon is a hormone made in the alpha cells of your pancreas. Its effect opposes that of insulin. While insulin decreases the levels of glucose in your blood, glucagon breaks down glycogen in the liver to increase the levels of blood glucose. Glucagon is produced in considerable amounts when you exercise and your blood glucose levels are therefore lower than what they should be. Once the glucose in your blood has increased to the desired levels, glucagon generation is reduced by the pancreas as a feedback step.

The Role of Insulin 

Insulin is a hormone secreted by the pancreas in response to elevated blood sugar (glucose). It is important that an individual's blood sugar level does not rise too high or too quickly. High blood sugar alerts the pancreas. The pancreas then detects the excess glucose and secretes the hormone messenger, insulin. Insulin then lowers the elevated blood sugar by shifting the metabolism into storage mode. Insulin converts the excess glucose into glycogen, removes it from the blood stream, and stores it in the liver and muscles. The excess blood sugar that cannot be stored as glycogen will be converted to new fat and stored in the adipose tissue (butt, hips, and back). Insulin commands the body to save food energy stored in fat cells for a time when no food is available.


Excess Insulin & Weight Gain 

Excess insulin removes fat from the blood and transports it into fat cells. Excess insulin forces the body to burn carbohydrates for energy instead of stored fat. Elevated insulin levels inhibit the release and utilisation of stored body fat for energy. Elevated insulin

converts and stores excess glucose as fat. Elevated insulin can be caused by too many carbohydrates in a meal and not enough protein, essential fats, and fibre.

The Role of Glucagon

The effect of glucagon is to make the liver release the glucose it has stored in its cells into the bloodstream, with the net effect of increasing blood glucose. Glucagon also induces the liver (and some other cells such as muscle) to make glucose out of building blocks obtained from other nutrients found in the body (eg, protein).


When excess carbohydrates are consumed and the liver and muscles have stored as much glycogen as possible, the body creates another storage form, fat. Insulin tells your body not only to store new fat, but also not to release any previously stored fat. Insulin is the storage hormone.


Glucagon, on the other hand, has the opposite effect to insulin. It tells the body to increase the blood sugar. Protein stimulates the release of glucagon, which stimulates the liver to release stored carbohydrates from its glycogen stores and from fat. Glucagon works in opposition to insulin:

  • Glucagon raises low blood sugar.

  • Glucagon puts the metabolism in burning mode.

  • Glucagon converts protein and fat too glucose.

  • Glucagon causes dietary fat to be used for energy.

  • Glucagon releases fat from fat cells to be used for energy.

  • Glucagon reduces cholesterol production.

  • Glucagon causes the kidneys to release water from the body.

  • Glucagon causes artery wall cells to return to normal.

  • Glucagon stimulates the use of fat for energy.

Thyroid Hormones

Thyroid hormones are critical determinants of brain and somatic development in infants and of metabolic activity in adults; they also affect the function of virtually every organ system. Thyroid hormones must be constantly available to perform these functions. To maintain their availability, there are large stores of thyroid hormone in the thyroid gland. The thyroid hormones increase oxygen consumption and metabolic rates in almost all cells of the body. These hormones influence carbohydrate and lipid metabolism and are necessary for normal growth and development.


It has been appreciated for a very long time that there is a complex relationship between thyroid disease, body weight and metabolism. Metabolism is determined by measuring the amount of oxygen used by the body over a specific amount of time. If the measurement is made at rest, it is known as the basal metabolic rate (BMR).


Indeed, measurement of the BMR was one of the earliest tests used to assess a patient’s thyroid status. Patients whose thyroid glands were not working were found to have low BMRs, and those with overactive thyroid glands had high BMRs. A few examples of specific metabolic effects of thyroid hormones include:

  • Lipid metabolism: Increased thyroid hormone levels stimulate fat mobilisation, leading to increased concentrations of fatty acids in plasma. They also enhance oxidation of fatty acids in many tissues. Finally, plasma concentrations of cholesterol and triglycerides are inversely correlated with thyroid hormone levels - one diagnostic indication of hypothyroidism is increased blood cholesterol concentration.

  • Carbohydrate metabolism: Thyroid hormones stimulate almost all aspects of carbohydrate metabolism, including enhancement of insulin-dependent entry of glucose into cells and increased gluconeogenesis and glycogenolysis to generate free glucose.

Parathyroid Hormone

Your parathyroid glands create parathyroid hormone. When this hormone releases, it raises the levels of calcium in the blood through multiple pathways. While it increases the release of calcium from the bone, it increases the absorption of calcium from your intestine and makes your kidneys excrete less calcium. This hormone secretes when the levels of calcium in the blood become low. At the same time, increase of ionised calcium in the blood decreases the secretion of parathyroid hormone. When abnormally high amounts of this hormone are released, unnecessary calcium can release in the blood to cause bone loss.



Of the hormones produced, prolactin is the most versatile. It stimulates milk production by female mammary glands shortly after childbirth. In its absence milk production quickly ceases. It also has roles in metabolism of fats and carbohydrates.


Adrenal Gland and Carbohydrates


Adrenaline, also known by the scientific name epinephrine, is a hormone that's responsible for the "fight or flight" response that occurs under conditions of excitement or imminent danger. One of the major effects of adrenaline is to increase the power of muscular contraction, in skeletal muscle and in the heart. In the case of the heart muscle, adrenaline also increases the heart rate. For muscles to contract, they must have an energy source such as glucose.



The adrenal glands play a role in carbohydrate metabolism through secreting adrenaline into the bloodstream. This hormone plays an important role in stress response. Once the adrenal glands release adrenaline into the bloodstream, it travels throughout the body and binds to receptors on many cells. Most relevant to carbohydrate metabolism are the receptors on liver and muscle cells. Once adrenaline binds to these receptors, cells respond by producing a molecule called cyclic AMP, which signals the cells to modify their metabolism of carbohydrates.



The purpose of adrenaline playing a role in modifying carbohydrate metabolism is that under stressful conditions, human bodies need more energy available to them. Both the muscles and the liver store glycogen, which is a storage form of the sugar glucose. Adrenaline, through causing production of cyclic AMP, signals liver and muscle cells to break down glycogen into glucose. The liver releases this glucose into the bloodstream while the muscles use the glucose for their own purposes.



While other hormones can cause the liver and muscles to break down glycogen and increase available glucose, adrenaline from the adrenal glands provides an additional benefit--it activates a pathway whereby the cells can make glucose out of other molecules, including lactic acid, glycerol and various amino acids, which are the building blocks of protein. This process, called gluconeogenesis, is very important under stressful circumstances because it ensures that even if glycogen levels become depleted, cells will have adequate glucose.




The adrenal glands also help direct cells--specifically muscle cells--toward the most efficient possible mechanism for burning glucose. Adrenaline, when it binds to a muscle cell receptor, causes a muscle cell organelle called the sarcoplasmic reticulum to release calcium ions, which increase the rate of glucose metabolism. In particular calcium release signals the muscles to burn glucose aerobically, meaning using oxygen as a reactant, which increases the efficiency of glucose.

Adrenaline & Cortisol

Our bodies response to stress is to change the homeostasis that exists in the body by increasing and secreting the hormones adrenaline and cortisol. By increasing the amount of adrenaline and cortisol the body is preparing to respond to some unusual situation. This could be a stand up row with your partner, dealing with an unhappy customer or taking on a huge project at work. This response is the fight or flight response because it gives us the energy to fight or run away from a situation and prepares the body for any injuries or potential harm.


The purpose of the fight or flight response is to respond to a finite incident that starts and ends in a short space of time. However much of the stressors that we face today, like the mountain of work, in the huge new project, do not end, and could go on for years. Thus if we respond to the stressor by constantly releasing adrenaline and cortisol we are telling the body that the normal homeostasis is wrong and that the new normal state includes an excessive amount of adrenaline and cortisol.


The analogy can be made to a car engine. If you constantly run at maximum revs and never service it, the engine eventually breaks down. The same goes for the body. In terms of the metabolism, adrenaline makes the fat cells more efficient at turning fat into energy. Adrenaline also increases the metabolism. Cortisol has the effect of increasing the amount of glucose in the blood and creating more energy. Whilst these changes in the body's metabolism are good for an incident where you have to expend a lot of physical energy, we rarely have to do this at work. We spend long hours sitting at a desk whilst adrenaline and cortisol build up and give us excess energy. This often causes chronic stress or stress that has no end in sight.


Chronic stress will cause cortisol to create excess glucose. This will cause excess energy that may not be used. This will eventually end up being stored as fat. Adrenaline will continue to be secreted but the body and the fat cells, in particular, will become less sensitive to it. This means that the fat cells won't be converted to energy and thus reduced, whilst cortisol will be increasing in quantity. This means an increase

in weight. Thus stress can improve the efficiency of the metabolism. This might be seen as good, but if the stress goes on for too long the homeostasis of the body's metabolism is changed. This can ultimately lead to eating disorders and a breakdown of some of the multitude of systems that combine to help metabolise food in the body.


Growth Hormone

Human growth hormone does not directly produce growth but induces other tissues to secrete a number of protein growth factors which have an affect skeletal and muscle growth. There is epidermal growth factor, nerve growth factor, ovarian growth factor. Intake of carbohydrates and protein affects both production of human growth hormone and insulin, according to a study published in the American Journal of Clinical Nutrition. Human growth hormone, or HGH, is a key factor in lean body mass, and its release is triggered when blood glucose levels drop precipitously. HGH acts to mobilise free fatty acids from adipose tissue deposits as part of the body's survival mechanisms.

  • Protein metabolism: In general, growth hormone stimulates protein anabolism in many tissues. This effect reflects increased amino acid uptake, increased protein synthesis and decreased oxidation of proteins.

  • Fat metabolism: Growth hormone enhances the utilisation of fat by stimulating triglyceride breakdown and oxidation in adipocytes. 


  • Carbohydrate metabolism: Growth hormone is one of a battery of hormones that serves to maintain blood glucose within a normal range. Growth hormone is often said to have anti-insulin activity, because it suppresses the abilities of insulin to stimulate uptake of glucose in peripheral tissues and enhance glucose synthesis in the liver. Somewhat paradoxically, administration of growth hormone stimulate insulin secretion, leading to hyperinsulinemia.


Human Growth Hormone

Doctors often prescribe human growth hormone, or HGH, to patients who have a growth deficiency. Injections of the hormone promote the growth of muscle mass and reduce fat, although healthy adults who take HGH to obtain the body benefits suffer an increased risk for heart disease and other harmful side effects.

  • Swelling in the legs and arms

  • Carpal tunnel syndrome

  • Joint and muscle pain.

  • Gynecomastia, or breast enlargement (Men) - this enlargement often contributes to life-threatening conditions like diabetes and heart disease.

  • Fluid retention

  • Insulin resistance (Type 2 Diabetes)

  • Severe Headaches

  • Cancer

Human growth hormone, or HGH, is a naturally occurring substance produced by the pituitary gland. Growth hormone supports growth during childhood and adolescent years. Insulin-like growth factor(IGF-1), which is a primary component of the hormone's growth-promoting effects, is produced as human growth hormone interacts with the liver and various tissues.

HGH has been touted as an anti-aging drug by boosting metabolic rate and increasing muscle mass, both critical factors in weight loss. A high muscle mass to body weight ratio can accelerate your body's metabolism which can lead to fat loss. Current research into the effect of IF (Intermittent Fasting) or ADF (Alternate day fasting) on reducing the production of IGF-1 to in turn reduce incidence of cancer, diabetes and heart disease.


A hormone produced in cells of the thyroid that participates in regulating the blood level of calcium and stimulates bone mineralisation. A synthetic preparation of the hormone is used in the treatment of certain bone disorders. Calcitonin acts to reduce the blood level of calcium and to inhibit bone resorption, whereas parathyroid hormone acts to increase blood calcium level and bone resorption.


Vitamin D also contributes to the regulation of calcium homeostasis.


Calcium is abundantly present in your bones along with phosphates. This mineral also proves necessary to the body for muscle contraction, blood clotting and proper functioning of the nervous system. Your body has to maintain calcium in the blood at

constant levels because a decrease in calcium levels, called hypocalcemia, or an increase in its levels, called hypercalcemia, can seriously impact the activities of your nerves and muscles. The parathyroid glands and the thyroid gland play significant roles in regulating the levels of calcium in the blood by releasing parathyroid hormone and calcitonin, the two hormones that regulate calcium.


Cortisol is a hormone that is manufactured and released by the adrenal glands located on the top of each kidney. The release of the hormone is controlled by another hormone, adrenocorticotropic hormone from the pituitary gland. Cortisol has several regulatory functions in the body in a normal state. It is at the highest level in the morning and at the lowest level at night. Although stress isn't the only reason for secretion, cortisol is called the stress hormone because it is secreted in higher levels during a 'fight or flight' response to stress.



In the control and regulation of metabolism, cortisol plays a key part in glucose metabolism and blood sugar regulation. According to physicians, cortisol stimulates the production of glucose in the liver, moves specific amino acids out of the tissues and into the liver where they are used in the production of glucose to feed the body cells, stops the absorption of glucose by the cells when conservation is necessary for survival and begins the breakdown of fat cells in the production of energy.

Thryoid Functions

Cortisol works in concert with thyroid hormone at the cellular level. In essence, cortisol makes thyroid hormone work more effectively and is very important for normal thyroid function. This is why if a person has an imbalance of cortisol, he can have thyroid type symptoms but normal blood thyroid levels. Cortisol is an important hormone that is involved in many important biological processes within the human body including regulation of glucose levels. Diseases and medications that affect cortisol levels can

significantly alter blood glucose levels. Synthetic steroid medications are often designed to mimic the effects of cortisol within the body.


Diseases & Medications

Diseases of the adrenal gland and pituitary gland can cause abnormal levels of cortisol secretion. Prolonged use of synthetic steroid drugs can inhibit the production of endogenous cortisol, which can cause abnormally low levels of cortisol once the patient stops using the steroid medication. Abnormal cortisol levels can cause many problems, including impairment of normal metabolism and the body's ability to respond to stress.

Cortisol & Cushing Syndrome

Cushing syndrome demonstrates the effects of having a high amount of cortisol in the bloodstream. People with Cushing syndrome can have a round face, an obese abdomen, osteoporosis, menstrual irregularity if female, balding whether female or male, and high blood pressure.


People with symptoms of Cushing's disease are tested to see if a tumour in the pituitary gland is releasing a high amount of the adrenocorticotropic hormone, for this type of tumour causes the disease. The hormone stimulates the adrenal glands to release the hormone cortisol, which in turn stimulates the production of cholesterol. Those with Cushing's usually have high blood pressure as well, a disorder that's also associated with a high cholesterol level.


What is Cushing's Disease?

People with Cushing's disease have a benign tumour in the anterior pituitary gland of the brain. The tumour is called a pituitary adenoma and it is referred to as benign because it does not spread to other places or invade other structures.


Antidiuretic Hormone

Antidiuretic hormone, or ADH, is produced by the hypothalamus and released by the posterior pituitary gland as a result of either low blood volume or increased sodium concentration. When released, ADH acts in the kidneys to increase the reabsorption of water. In some people, ADH is secreted in excess, resulting in decrease blood sodium. Small cell lung cancer is a common cause of excess ADH secretion. Roughly 60% of the mass of the body is water, and despite wide variation in the amount of water taken in each day, body water content remains incredibly stable. Such precise control of body water and solute concentrations is a function of several hormones acting on both the kidneys and vascular system, but there is no doubt that antidiuretic hormone is a key player in this process.


Antidiuretic hormone, also known commonly as arginine vasopressin, is a nine amino acid peptide secreted from the posterior pituitary. Within hypothalamic neurons, the hormone is packaged in secretory vesicles with a carrier protein called neurophysin,

and both are released upon hormone secretion.


Physiologic Effects of Antidiuretic Hormone

Effects on the Kidney

The single most important effect of antidiuretic hormone is to conserve body water by reducing the loss of water in urine. A diuretic is an agent that increases the rate of urine

formation. Antidiuretic hormone binds to receptors on cells in the collecting ducts of the kidney and promotes reabsorption of water back into the circulation.

In the absence of antidiuretic hormone, the collecting ducts are virtually impermeable to water, and it flows out as urine. Antidiuretic hormone stimulates water reabsorption by stimulating insertion of "water channels"  into the membranes of kidney tubules. These channels transport solute-free water through tubular cells and back into blood, leading to a decrease in plasma osmolarity and an increase osmolarity of urine. In many species, high concentrations of antidiuretic hormone cause widespread constriction of arterioles, which leads to increased arterial pressure.


Control of Antidiuretic Hormone Secretion


The most important variable regulating antidiuretic hormone secretion is plasma osmolarity, or the concentration of solutes in blood. Osmolarity is sensed in the hypothalamus by neurons known as an osmoreceptors, and those neurons, in turn, stimulate secretion from the neurons that produce antidiuretic hormone. When plasma osmolarity is below a certain threshold, the osmoreceptors are not activated and secretion of antidiuretic hormone is suppressed. When osmolarity increases above the threshold, the ever alert osmoreceptors recognise this as their cue to stimulate the neurons that secrete antidiuretic hormone.

As seen the the figure below, antidiuretic hormone concentrations rise steeply and linearly with increasing plasma osmolarity. Osmotic control of antidiuretic hormone secretion makes perfect sense. Imagine walking across a desert: the sun is beating down and you begin to lose a considerable amount of body water through sweating. Loss of water results in concentration of blood solutes - plasma osmolarity increases. The osmotic threshold for antidiuretic hormone secretion is considerably lower than for thirst, as if the hypothalamus is saying "Let's not bother him by invoking

thirst unless the situation is bad enough that antidiuretic hormone cannot handle it alone." Secretion of antidiuretic hormone is also stimulated by decreases in blood pressure and volume, conditions sensed by stretch receptors in the heart and large



Changes in blood pressure and volume are not nearly as sensitive a stimulator as increased osmolarity, but are nonetheless potent in severe conditions. For example, Loss of 15 or 20% of blood volume by haemorrhage results in massive secretion of antidiuretic hormone. Another potent stimulus of antidiuretic hormone is nausea and vomiting, both of which are controlled by regions in the brain with links to the hypothalamus.

Secretion of antidiuretic hormone is also stimulated by decreases in blood pressure and volume, conditions sensed by stretch receptors in the heart and large arteries. Changes in blood pressure and volume are not nearly as sensitive a stimulator as increased osmolarity, but are nonetheless potent in severe conditions. For example, Loss of 15 or 20% of blood volume by haemorrhage results in massive secretion of antidiuretic hormone. Another potent stimulus of antidiuretic hormone is nausea and vomiting, both of which are controlled by regions in the brain with links to the hypothalamus.


How Does the Endocrine System Affect Metabolism?

Your metabolism is the process of using cellular energy to conduct all of the chemical and physical processes in your body that keep you alive. These processes include the proper function of your brain and nervous system, muscle contraction, body temperature, food digestion, blood circulation and breathing. Metabolic processes of all types are heavily influenced by your endocrine system, which is a system of glands located throughout your body.


Endocrine Glands

There are many types of endocrine glands, including the pituitary and hypothalamus in your brain, the thyroid gland in your neck and reproductive glands, including your gonads. There are also major organs that act as endocrine glands, including the stomach, liver and pancreas. Endocrine glands produce signalling molecules, including hormones and small proteins, which are secreted into the bloodstream and act as communication messengers to another part of the body.


Hormones, including testosterone and estrogen, are small chemical messengers that affect many cellular processes, including your sexual function, growth and development, mood and your metabolism. The hypothalamus in your brain connects the nervous system to your endocrine system. The hypothalamus makes hormones that tell the pituitary gland what hormones too secrete. The pituitary gland is sometimes referred to as the master gland because it secretes hormones that instruct the rest of the endocrine glands what to do.

Secretion Effects

The endocrine glands secrete hormones that bind to cells that have receptors specific for a particular hormone. When they bind to their target cells, it sends a signal that instructs those cells in a particular tissue or organ what to do. For example, the pancreas secretes the protein insulin, which then binds to target cells and signals those cells to let in blood sugar, or glucose, so that it can be metabolised.

Metabolic Effects

The thyroid gland produces special hormones that control the metabolic rate at which cells use energy. Individuals who have an under-active thyroid gland are said to have hypothyroidism, which can lead to obesity, arthritis and heart disease.

Hunger, Leptin & Ghrelin

Leptin and ghrelin are hormones that decrease and increase your hunger, respectively.Hunger leads to increased food consumption, which in turn increases your body weight and fat. Leptin is a hormone made and stored in your fat cells and secreted into your blood, where it travels to your hypothalamus and

communicates that the body has enough fat and no longer needs to eat.


However, people who are obese may have leptin resistance, a condition characterised by an inability of your hypothalamus to respond to leptin, which causes you to overeat. Your stomach produces and secretes ghrelin when it is empty. The hormone travels to the hypothalamus and tells it your body is hungry. After you complete your meal, ghrelin levels go down.

Ways to Maintain a Healthy Endocrine System

The human endocrine system is a network of glands secreting hormones responsible for regulation of numerous functions such as digestion, sexual reproduction, growth and internal homeostasis. Glands constituting the endocrine system include the kidneys, thyroid, pancreas, pituitary, adrenal, ovaries/testes and parathyroid glands. An imbalance of hormones caused by an unhealthy endocrine system can produce symptoms ranging from panic disorder, to metabolic deregulation, to diabetes. Problems with the parathyroid gland may allow excessive calcium levels to build in the body, creating a condition called hypercalcemia, while pituitary adenomas, or tumors, emerge when the pituitary releases too much growth hormone.


Omega 3, 6 and 9 Fatty Acid Oils

Omega 3, 6 and 9 fatty acid oils are obtained by eating fish or taking fish oil supplements. They enhance the operation of the endocrine system by facilitating the transportation of hormones throughout the body. Certain cells require hormones to maintain their efficiency in performing targeted tasks within the body, such as reproductive cells regulating menstrual and testosterone difficulties. Omega fatty acids also promote proper fluid balance and kidney functioning, which prevents water retention and possible resulting hypertension. Blood circulation benefits from fish oil as well, which assists in carrying hormones to their destinations.

Fruits and Vegetables

Eating a good balance of fruit and vegetables keeps the body healthy and relatively free of disease. As a result, the endocrine system benefits from these foods as well, since glandular hormone release is not disrupted by disorders that may be exacerbated by too much fat, sugar or salt. Kidneys are affected by the amount of toxins they are forced to filter and excrete from the body. Occasionally, they are unable to regulate accumulations of purine caused by a diet of high purine foods. Excessive purine is directly associated with the development of uric acid crystals and gouty arthritis. In addition, eating plenty of fruits and vegetables prevents the thyroid from becoming sluggish, which causes weight gain and chronic fatigue.

Garlic and Herbs

Garlic is a natural immune system enhancer and an important endocrine system nutrient. Chewing one or two garlic cloves each day may help maintain normal levels of blood sugar which assists the pancreas in generating correct amounts of insulin. It also may act as a blood thinner and cholesterol reducer as well. Certain herbs such as ginkgo and ginseng may also keep the endocrine system healthy by regulating release of hormones throughout the body. Garlic, ginseng and ginkgo are all available as supplements.

Endocrine Disorders

Stress and the Endocrine System

When people suffer from chronic stress, the HPA axis [hypothalamus-pituitary-adrenal] is constantly activated, producing a stress hormone called cortisol. Maintaining excessive levels of cortisol can have extremely detrimental effects on the body. Heart disease, hypertension, high cholesterol, kidney failure and diabetes are all potential disorders as a result of a heightened stress response that remains unalleviated. Too much cortisol also contributes to premature osteoporosis, overly rapid inflammatory responses and increases in body fat.


Reducing stress in your life is just as important as eating healthy in order to have a functional endocrine system. The network of organs and systems in the body designed to produce, store and secrete hormones is called the endocrine system. Reproduction, growth and energy levels are controlled by the endocrine system. Growth and thyroid disorders, diabetes, osteoporosis and polycystic ovary syndrome can result from disruptions to this vital system.

Cushing's Syndrome

When the body produces excessive amounts of the hormone cortisol for long periods of time, you may develop Cushing's syndrome. One of the main effects of the disease is weight gain. Cortisol is important for weight loss because it's the hormone that converts fat into energy. When too much is present, it does not work efficiently, meaning that the endocrine system disrupts this vital function and interferes with your weight loss efforts. Usually Cushing's syndrome is a temporary condition that disappears when you stop taking cortisol like medications to treat other illnesses. Tumours on the pituitary or adrenal glands also can trigger the condition, which dissipates when the tumour is removed.


The thyroid gland produces hormones that regulate how your body breaks down food and whether it stores it or uses it for energy. Thyroid hormones are the main arbiter of your metabolism. Hypothyroidism, or underactive thyroid, is most common in women over the age of 50 and can lead to obesity, heart disease and joint pain. The condition occurs when your thyroid ceases to produce sufficient hormones. Symptoms are barely noticeable in the early stages of the condition and gradually strengthen as you age. You may feel fatigued and sluggish, be sensitive to cold and experience weakness and pain in your joints. Drugs used to control hypothyroidism typically result in weight loss while also lowering your cholesterol levels and giving you more energy.

Addison's Disease

You may experience unwanted weight loss when you develop Addison's disease. According to the Hormone Foundation, this condition is rare and affects less than 150 people in every million. The disease attacks the adrenal glands and causes them to produce an insufficient amount of steroid hormones in your body. The steroid hormones control blood sugar levels, sexual drive and the ability to fight off the effects of stress and infection. You may feel weak and fatigued and have no appetite as a result of the disease. A lifetime regimen of corticosteroids usually is required to treat Addison's disease and you should be able to get back to a healthy weight.



Type 1 Diabetes

Type 1 diabetes is an endocrine disorder that usually results in weight loss. Type 1 diabetes, referred to as juvenile diabetes in the past, occurs when the pancreas does not produce sufficient insulin. In addition to unintentional weight loss, you may experience intense thirst and hunger and feel the need to urinate often. The immune disorder commonly first appears in young children and teens. You need regular injections of insulin to manage the disease and maintain a healthy weight. In addition to the weight loss, unmanaged diabetes can lead to blindness, nerve damage, stroke and kidney disease.

Endocrine & The Effect Of Exercise & Training

The endocrine system refers to that functional part of the body responsible for the production of hormones substances that have effects on target organs some distance away from where the hormones are made. Physical exercise would be impossible without the contribution of hormones. On the other hand, exercise can have profound effects on the endocrine system. Therefore, the interplay between the endocrine system and physical training is complex.

Adrenal Axis

The adrenal gland produces "adrenalin" (norepinephrine) as well as cortisol and the hormone that regulates it via the pituitary gland, ATCH. Norepinephrine lies at the centre of the "fight-or-flight" response, whereas cortisol is released in response to stress and suppresses the immune system. A 2002 study published  "Aviation, Space, and Environmental Medicine" found that seven days of very intense exercise significantly elevated levels of cortisol and ACTH, suggesting that the study subjects were highly stressed and perhaps "overworked."


Some weightlifters and professional athletes use anabolic steroids to boost strength and improve performance. These substances are the artificial counterparts of the sex hormone testosterone. Testosterone is a vital hormone in various modes of exercise and contributes to muscle mass and strength. A study by Te-Chi Liu et al. (2009) suggested that a single session of exercise results in a rapid rise in serum levels of testosterone, and that the degree of increase correlates with the type, duration and intensity of exercise.

Thyroid Hormones

Thyroid hormone, produced in the thyroid gland below the neck, plays important roles in a host of metabolic functions, influencing growth, tissue differentiation, fatty acid use and temperature regulation. A shortage of the hormone often results in weight gain and general lassitude. Research suggests  that moderate to very intense aerobic exercise produced a significant rise in circulating thyroid hormone levels, and that the degree of this increase was correlated with intensity level.

Pancreatic Hormones

The pancreas produces insulin, a hormone essential for keeping blood glucose levels in a normal range by promoting the uptake of glucose into body tissues. It also produces glucagon, the actions of which oppose those of insulin and lead to release of glucose from body tissues. A 1986 study by Wolfe et al. (1986) demonstrated that during light exercise, both a rise in glucagon level and a drop in insulin level are necessary to maintain blood glucose in a normal range.

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.


Endocrine Systems Response To Resistance Training

The endocrine environment has an essential influence on acute and long-term adaptational responses to RT (Häkkinen, 1989; Kraemer and Ratamess, 2004) (Figure 2). The human endocrine system adapts to the repeated stimulus of RT by increasing or decreasing hormone secretion depending upon the acute resistance training variables (ACSM, 2009). This is achieved by increasing or decreasing the number of circulating proteins that bind the hormone and protect it from degradation while rendering it biologically inactive and changing the number of cellular hormone receptors.

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Figure 2. Physiological and biochemical responses to resistance training in relation to muscular strength and hypertrophy.

The performance of RT stimulates anabolic hormonal responses, including Testosterone (T), Growth Hormone (GH) and Insulin Growth Factor one (IGF-1) but also a catabolic hormone (C) (ACSM, 2009). Stone et al., (1988) indicated that alteration to acute RT program variables dictates the extent of this hormonal response. Training protocols (high set volume, moderate to high resistance load and short inter-set recovery) that recruit large muscle groups (i.e. squats, deadlifts, bench press) appear to produce the greatest endocrine response (Stone et al., 1988). Additionally, Staron et al., (1994) reported that changes in fibre type are due to hormonal adaptations in previously untrained men. Researchers have also observed that changes in muscular strength and power in competitive Olympic weightlifters have been correlated to chronic hormonal adaptations (Häkkinen et al., 1987; Fry et al., 2000).



T is a male anabolic sex hormone that is secreted from the Leydig cells of the testes under hypothalamic and pituitary-gonadal axis control (Viru and Viru, 2005). The primary role of T in RT is to promote protein synthesis of contractile proteins involved in muscle tissue, resulting in increased muscle mass and strength (Kraemer et al., 1993). Exercise can acutely increase or decrease circulating T and is conditional on the mode and intensity of exercise (Schmid et al., 1982). Increases in circulating T increases during relatively short, high-intensity activities (i.e. resistance exercise), while the decline in T is associated with increased duration including marathons and other ultra-running events (Kuoppasalmi et al., 1980; Schurmeyer et al., 1984). 


Ahtiainen et al., (2003) reported that T plays an important factor in strength development with a relationship between changes in isometric strength and T. Intriguingly, Ahtiainen and colleagues also indicated that subjects who exhibited acute increases in T after training had a CSA increase of muscle more than those with lowered T. Spiering et al., (2008) reported that T causes an upregulation of androgen receptors with elevated muscle protein synthetic response for approximately 48-hours, although this period is reduced with training. The hormone response to RT has been frequently studied with the concept of endogenous T and androgen receptors interacting during the recovery period that stimulates protein synthesis, muscle hypertrophy and strength (Wilkinson et al., 2006; Crewther et al., 2009). This is further reinforced by Sinha-Hikim et al., (2002) who reported that when exogenous T (by supplementation) is combined with RT, significantly increases muscle mass and strength.

Growth Hormone

GH has been associated with the promotion of anabolism in both muscle and connective tissue. The anterior pituitary gland secretes GH polypeptides molecules from the acidophilic cells and enhances cellular amino acid uptake and protein synthesis in muscle, resulting in hypertrophy (Crist et al., 1991). The most frequently studied GH is the 22-kDa isoform molecule that consists of 191 amino acids with other isoforms functioning similarly in promoting tissue anabolism (Kimball, Farrell and Jefferson, 2002). Kraemer et al., (2017) suggest that a superfamily of different GH mediate biological actions during recovery to exercise stress and not merely the 22-kDa monomer. The GH receptor is abundant in various cells and tissues, and the understanding of recovery response patterns to RT is not fully understood (Kraemer et al., 2017). The release of GH in the blood during recovery involves a multitude of GH that aggregate during recovery (Hymer et al., 2001).

Florini, Ewton and Coolican (1996) suggested that concentrations of GH in the blood depend on the specific exercise stimuli. RT has been shown to acutely elevate many GH variants and promote muscle anabolism (Spiering et al., 2008). Kraemer et al., (1993) indicated that post-exercise GH levels are elevated 30-minutes’ post-RT. However, the magnitude of GH level depends on the amount of muscle mass recruited, volume, intensity and rest periods (Kraemer et al., 1991; Gotshalk et al., 1997; Smilios et al., 2003). GH appears to be highly influenced by the volume of the RT protocol. A further study by Kraemer et al., (2010) suggested that short inter-set recovery periods and RT stimulate higher concentrations of immunoreactive GH in both men and women. However, alterations to the recovery period and increased resistance loading resulted in significantly lower levels of immunoreactive GH (Kramer et al., 2010).

Insulin-Like Growth Factor-1

IGF-1 (originally termed somatomedin C) is produced in many tissues, primarily the liver and is secreted in response to GH-stimulated somatic growth, and as a mediator of GH-independent anabolic responses in many cells and tissues (Fry, Kraemer and Ramset., 1998). RT has been shown to increase concentrations of circulating and muscle IGF-1, although several studies have reported no change (Chandler et al., 1994; Bamman et al., 2001). Studies suggest that RT changes the concentration of IGF binding proteins that affect the biological activity of IGH-1 (Bamman et al., 2001; Nindle et al., 2000). In response to mechanical overload initiated by RT, mechanogrowth factor stimulates satellite cell activation with IGF-1 increasing the proliferation and differentiation of satellite cells that aid in muscle hypertrophy (Hather et al., (1991).

Resistance Training Variables & The Endocrine System

RT has been recognised as an effective method for strengthening muscles, connective tissue, and enhancing physical functioning associated with sporting performance. It is also acknowledged that these adaptive responses result from careful manipulation of acute training variables including load, volume, frequency, inter-set recovery, contraction velocity of movement, the range of motion, selection and order of exercise (ACSM, 2009). By altering the resistance volume and load affects several acute responses, including neural, hormonal, metabolic, and cardio-vascular responses (Häkkinen, Alen and Komi, 1985; Ratamess et al., 2007).

Manipulation of acute RT program variables stimulates anabolic hormonal responses including T, GH, and IGF-1, but also C (ACSM 2009). Typical strength and hypertrophy protocols (high volume, moderate to high resistance loading and short inter-set recovery periods) have been reported to produce the greatest endocrine response (Stone et al., 1998). Schoenfeld (2010) has suggested that increased RT volume stimulates extensive metabolic stress and mechanical tension that produces greater metabolite accumulation, substrate depletion and muscle damage (Figure 2). These factors activate anabolic responses during the recovery phase that leads to adaptational responses (Helms et al., 2015).


Subsequently, by controlling the RT volume and loading (altering the number of sets

per exercise, the number of repetitions per set and the number of exercises per session) can affect metabolic and hormonal responses to RT (Borst et al., 2001; ACSM, 2009). For example, Willoughby et al., (1991) reported significantly greater GH and T responses with multiple-sets compared to one-set programs. Borst et al., (2001) observed that long term RT that uses multiple-sets per exercise is superior compared to single sets for strength development.