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Staying Cool When Your Body is Hot
Chantal A. Vella, M.S and Len Kravitz, Ph.D.

Maintaining ample hydration can be challenging for participants in outdoor and indoor aquatic exercise classes. Sustaining hydration is essential for normal bodily functions and for peak exercise performance. Many of your students do not associate aquatic exercise with any potential risk of dehydration, however, this is a fundamental issue in aquatic exercise. This article will overview, discuss and explain some of the key physiological concepts of body temperature regulation and hydration for aquatic exercise professionals. In addition, specific recommendations for optimal hydration during exercise and aquatic exercise are provided.

How does the body regulate body temperature?
The human body regulates temperature by keeping a tight balance between heat gain and heat loss. Your temperature regulation system is more analogous to the operation of a home furnace, as opposed to the function of an air conditioner. Humans regulate heat generation and preservation to maintain internal body temperature or core temperature. Normal core temperature at rest varies between 36.5 and 37.5 °Celsius (°C), which is 97.7 to 99.5 °Fahrenheit (°F). 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. For 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.

During all types of exercise the body’s ability to thermoregulate is challenged. Heat is produced as a bi-product of metabolism (metabolism is defined as all of the reactions that occur in the human body). However, 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 40 °C (104 °F).

How does the body lose heat?
As previously discussed, the body regulates temperature like a furnace. It is constantly producing heat and then dispersing it through various processes. Heat can be lost through the processes of conduction, convection, radiation, and evaporation. 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 or in aquatic exercise, the water flow over the skin. 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. The last process of heat loss is evaporation. Evaporation is the process of losing heat through the conversion of water to gas (evaporation of sweat). The primary heat loss process for aqua enthusiasts is convection, however, in an outdoor pool on hot day evaporation will also play a primary role in heat loss.

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. It is the essential nutrient for survival and is required for all cell functions. 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.

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.

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. However, 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 liter of water per hour 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. 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, and increased fatigue.

What is 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!
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.
*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.
*Never restrict fluids during exercise! Quite the contrary. Encourage your students to take water breaks during the class. Many aquatic exercise professionals actually plan the hydration breaks into the structure of 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. In addition, avoid carbonated drinks, as they make you feel full and decrease fluid intake.

Specific Suggestions for the Aqua Instructor
As aqua instructors it is essential to promote fluid intake before, during and after exercise. Many individuals do not associate dehydration with exercise in the pool. However, in a hot humid environment, dehydration is a potential risk to your aqua exercise students. During a 60 to 90 minute class, encourage participants to take a water break every 15 minutes. In hot, humid weather a carbohydrate and electrolyte drink is preferable to water. However, in cool to moderate weather, water is sufficient to maintain hydration during exercise. Following each class, remind students to continue to re-hydrate throughout the day. Advise your students to drink 1-2 glasses of water at least one hour before each exercise class begins. For the health, safety and enjoyment of your students, aquatic exercise professionals are encouraged to develop teaching strategies that educate students about correct and appropriate hydration before, during and after exercise. In the long run, this will help your students realize their fitness goals.
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