Thermoregulation and Exercise

Body Temperature and Hydration Maintaining body temperature and ample hydration can be challenging for athletes and people participating in exercise Sustaining hydration is essential for normal bodily functions and for peak exercise performance

Thermoregulation Thermoregulation is the manner in which the body is able to maintain a consistent internal temperature, notwithstanding significant fluctuations in external temperatures caused by the environment

Body Temperature Regulation The human body regulates temperature by keeping a tight balance between heat gain and heat loss ▫Humans regulate heat generation and preservation to maintain internal body temperature or core temperature. ▫Normal core temperature at rest varies between 97.7 to 99.5 °Fahrenheit (°F).

Body Temperature Regulation Core temperature is regulated by the hypothalamus (in the brain), which is often called the body’s thermostat ▫The hypothalamus responds to various temperature receptors located throughout the body and makes physiological adjustments to maintain a constant core temperature ▫Example: On a hot day, temperature receptors located in the skin send signals to the hypothalamus to cool the body by increasing the sweat rate.

Body Temperature and Regulation Your temperature regulation system is more analogous to the operation of a home furnace, as opposed to the function of an air conditioner ▫It is constantly producing heat and then dispersing it through various processes

Thermoregulation and Exercise The body’s ability to thermoregulate is challenged during all types of exercise ▫Heat is produced as a bi-product of metabolism (all of the reactions that occur in the human body) ▫The human body is only 25% efficient, therefore you lose approximately 75% of energy as heat During exercise, heat is produced mainly from working muscle contractions and core temperature can go above 104 °F

How Does the Body Lose Heat? Heat can be lost through the processes of conduction, convection, radiation, and evaporation

How Does the Body Lose Heat? Conduction is the process of losing heat through physical contact with another object or body ▫For example, if you were to sit on a metal chair, the heat from your body would transfer to the cold metal chair Convection is the process of losing heat through the movement of air or water molecules across the skin ▫The use of a fan to cool off the body is one example of convection – ▫The amount of heat loss from convection is dependent upon the airflow

How Does the Body Lose Heat? Radiation is a form of heat loss through infrared rays - this involves the transfer of heat from one object to another, with no physical contact involved ▫For example, the sun transfers heat to the earth through radiation Evaporation is the process of losing heat through the conversion of water to gas ▫Evaporation of sweat

Maintaining the Body’s Core Temperature Exercise levels will naturally place a correspondingly greater impact on the body's ability to regulate temperature The impact of exercise on the thermoregulatory system will also vary subject to the presence or absence of environmental conditions such as heat or humidity

Maintaining the Body’s Core Temperature Responses to heat stress ▫Peripheral vasodilatation is the enlargement of the diameters of blood vessels to allow greater volume of blood to reach the skin and dissipate the heat ▫Evaporative heat loss by sweating is common and obvious cooling mechanism ▫Hyperthermia is a rise in core temperature, with devastating effects such as heat exhaustion and heat stroke

Maintaining the Body’s Core Temperature During a fever, hypothalamus resets the body’s thermostat to a new temporary core temperature ▫At the onset to fever, heat loss decreases and heat production increases: the person feels chilled ▫When the fever breaks, peripheral vasodilatation and sweating increases as the body attempts to reduce the core temperature to normal

How Much Water is in the Body Water makes up approximately 60% of your total body composition ▫In addition, 73% of lean body mass or muscle is composed of water Water is the essential nutrient for survival and is required for all cell functions

Water and the Body Water is also an important constituent in thermoregulation, because it is a major component of blood volume ▫It is mainly lost through sweat, respiration, and waste ▫However, when the body is dehydrated, most of the water lost is from the blood

Water and the Body Osmoregulation of the body is the control of the body's levels of water and mineral salts, particularly those of sodium and potassium Osmoregulation, thermoregulation, and the maintenance of the body's blood glucose levels, are the three main aspects of homeostasis in the body

Sweat Basics The average person has 2.6 million sweat glands ▫Sweat is made up of water and electrolytes such as sodium, chloride, and potassium When the hypothalamus senses an increase in core temperature it will act by increasing blood flow to the skin, stimulating the sweat glands ▫The result is an increase in the rate of water lost through sweating

Sweat Basics During low- to moderate-intensity exercise of less than one hour, there are minimal electrolyte losses because the body reabsorbs most of the electrolytes from the sweat During moderate- to high-intensity exercise of greater than one hour, the electrolyte loss in sweat becomes significant and the sweat rate is too fast for re-absorption of electrolytes

How Much Water is Lost During Exercise? During high-intensity exercise, a person can lose up to 2.0 liters of water per hour! ▫However, 1.0 liters is more common Sweat rate can vary depending on the environmental temperature, humidity, type of clothing worn during exercise, intensity of exercise, fitness level of the individual and acclimation of the individual to the environment

Thirst All humans, when their fluid levels are low, will crave water or other fluids The thirst mechanism is also progressive in its signal from the involuntary system to the human senses, a sensation that cannot be shut out or deactivated through any means

Thirst The human thirst mechanism does not activate until the body supply of water is depleted by approximately 15 oz ▫Although a person cannot survive for more than three or four days without water, it is ironic that a person may survive as long as 30 days without food, even though the hunger craving is a more powerful one than that of thirst

Daily Fluid Requirements Of the water consumed daily in various forms, a sedentary person will eliminate: ▫45 oz (1,500 ml) as urine ▫15 oz (500 ml) through evaporation and perspiration ▫10 oz (300 ml) through the lungs ▫6 oz (200 ml) through the digestive and other gastrointestinal processes

Daily Fluid Requirements An athlete engaged in a demanding workout or who is active in a warm weather environment may lose between 1 qt (1L) and 4 qt (4L) of fluid through perspiration in less than 90 minutes of activity ▫The fluid losses in such circumstances outpace the thirst mechanism, putting athletic performance and athlete health in jeopardy

Staying Hydrated The gap between the triggering of the thirst mechanism and fluid levels is also a factor at the beginning of the activity If an athlete relies on the thirst sensation to determine hydration, the athlete will often begin the activity in a mildly dehydrated state

Staying Hydrated The warm weather activation of the anti-diuretic hormone (ADH) is also an important factor when assessing the function of the thirst mechanism ▫ADH is released when the body senses, through the hypo-thalamus, that it is becoming dehydrated and that the blood volume has been reduced through additional perspiration ▫ADH is the signal conveyed to the kidneys to produce less urine, and to direct greater amounts of water into the blood. This process also may be triggered in advance of any thirst experienced by the athlete

Staying Hydrated Because the thirst mechanism is an unreliable indicator of the body's true thermoregulatory and osmoregulatory condition during exercise, an athlete must develop a fluid replacement/hydration strategy that permits optimal function irrespective of the thirst sensation

Staying Hydrated The consumption of water in the period prior to, during, and subsequent to performance is essential ▫When the athlete will be involved in significant exercise for periods greater than one hour, the consumption of sport drinks that will assist in maintaining the sodium level

Staying Hydrated In warm weather circumstances, the consumption of water to address dehydration, absent proper sodium levels, can lead to a state where the water will not be absorbed into the body as the involuntary systems strive to maintain a desired sodium balance with the sodium remaining ▫This condition is hyponatremia, a state of water intoxication

Dehydration Replacing fluids during and after exercise is very important for staying hydrated and preventing dehydration Signs of dehydration include: ▫Dark colored urine (urine should be the color of water with a splash of lemon) ▫Muscle cramps ▫Decreased sweat rate ▫Increased fatigue

What’s the Best Way to Stay Hydrated? According the American College of Sports Medicine (ACSM), before, during and following exercise, water or a carbohydrate/electrolyte drink is recommended to stay hydrated. The drink of choice should be cold in temperature and taste good to the individual. If it’s more palatable to the person, more will be ingested!

What’s the Best Way to Stay Hydrated? ACSM makes the following general recommendations for the amount and type of fluid that should be ingested before, during and after exercise: ▫Approximately 24 hours before exercise, an individual is recommended to consume fluids and foods to promote hydration. Fruits, vegetables, and carbohydrates are examples of foods that promote hydration. In addition, avoid too much alcohol and caffeine, as these fluids can cause water loss and promote dehydration.

What’s the Best Way to Stay Hydrated? ▫Two hours before exercise, 16 ounces (2 cups) of fluid should be ingested to promote hydration and allow time for excretion of excess water. ▫During exercise of less than an hour, it is recommended to ingest water every 15 minutes to prevent dehydration. Electrolyte loss is negligible; therefore a carbohydrate drink is not necessary. ▫During exercise of greater than an hour, it is recommended to ingest a carbohydrate and electrolyte drink every 15 minutes.

What’s the Best Way to Stay Hydrated? ▫Never restrict fluids during exercise! Quite the contrary. Encourage your students to take water breaks during the class. ▫After exercise ingest a carbohydrate and electrolyte solution. The carbohydrate will replenish glycogen stores (muscle carbohydrate stores) and the electrolytes will replenish sodium, chloride, and potassium lost in sweat. ▫Avoid carbonated drinks, as they make you feel full and decrease fluid intake.