The relationship between exercise and the level of fluids present in the body is as important as any to effective athletic performance. The intensity and duration of the exercise are variables to be considered on the exercise side of the equation. Factors such as heat, humidity, physical illness, all substances ingested by the athlete prior to the event (including those with a diuretic effect), and the types of fluids consumed during competition will also impact on this relationship. Fluid replacement may involve a number of distinct products—water will be the common component of any exercise fluid.
Exercise by its nature requires the body to generate energy to perform the required movements. Energy is produced by the body in one of three specific systems: the aerobic, the anaerobic lactic, and the anaerobic alactic systems. Each of these processes involves the cardiovascular system, the network of blood vessels in which the flow of blood is regulated to a significant degree by the power of the heart. Blood is the transportation system within which the raw material for human energy-producing fuel is carried. The average adult person contains a blood volume of approximately 1 gal (5 l); trained athletes and larger persons may possess a greater quantity of blood. Sugar, stored in the muscles and the liver as glycogen, are reduced into its simple form, glucose, which is then used to make adenosine triphosphate (ATP), which produces energy when exposed to the oxygen carried in the red blood cells of the bloodstream. The waste byproduct of the energy production process, carbon dioxide, is then carried away by red blood cells to be ultimately exhaled through the lungs.
The blood of the cardiovascular system is essential to energy production. The fluid component of blood is plasma is 90% water by volume. The maintenance of this level in the blood plasma is therefore essential to the ability of the body to produce energy. Where the plasma loses some of its water volume, it does not move as quickly through the blood vessels, and it tends to coagulate more readily. The minerals essential to a variety of exercise functions, such as sodium (important to both fluid regulation and the transmission of nerve impulses to muscles), potassium, and calcium (bone formation and maintenance) are all dependent upon the cardiovascular system to function properly.
Exercise also produces heat, irrespective of the present external temperature surrounding the body. The body cannot function if the accumulated heat generated by its energy production is trapped below the surface or within its organs; the body compensates for the production of heat through the generation of perspiration, or sweat. Blood warmed in the production of energy process moves closer to the surface of the skin, where the capillaries, the smallest of the vessels, release fluid composed primarily of water, with minerals such as sodium and potassium also present. The external temperature and humidity will also affect the production of perspiration, as the body will produce greater volumes of perspiration to seek a balance between its internal temperature and that it is experiencing as skin temperature.
While perspiration is the primary means by which the fluid within the body is depleted during exercise, water may be released from the body by other mechanisms. During exercise, as blood is circulated through the body, it is filtered through the kidneys, where the nephron functions to remove toxins or waste materials. Water is also separated from the blood; the amounts of water released through the processes of the kidney are related in part to the balance of minerals sensed by the kidney in the bloodstream. The combination of these waste substances will be ultimately excreted from the body as urine. The amount of urine produced will usually be a secondary factor in determining how much fluid will be necessary to return the body to an optimum state, or homeostasis.
The body also releases a smaller amount of fluid through breath, especially in colder temperatures. The body will generate fluids in the throat and breathing passages to compensate for the generally lower humidity in colder weather. This fluid is then released through the breathing mechanism. The other circumstance that will cause a significant amount of fluid loss is a serious injury in the course of exercise, such as a laceration, when an amount of blood plasma will escape through the wound.
Two commonly ingested substances will impact upon the body and its fluid levels maintained through exercise. Caffeine is a central nervous system stimulant. While heightening the ability of the body to react to external circumstances, it is also a diuretic, causing a chemical reaction in the kidney that stimulates the production of urine and causing dehydration. Alcohol is a central nervous system depressant that is highly water soluble, meaning that it will be absorbed readily into the bloodstream. Alcohol also creates a pronounced diuretic effect.
The mineral most central to the balance between the ability of the body to produce energy and the maintenance of fluid levels is sodium. The kidney acts on signals sent by the hypothalamus gland to maintain the sodium level in the body; when the sodium level is too high, sodium will be retained in the kidney and a lesser amount of water excreted as urine to achieve the desired balance; when sodium levels fall below the necessary level, water will be directed out of the bloodstream and converted into urine for excretion to arise the proportion of sodium to available fluid.
No matter how rapidly, either alone or in concert, the factors contributing to dehydration may occur within the body, the warning mechanism—thirst—is a slow and uncertain indicator as to the degree of fluid loss. The average person will only experience thirst after the body has lost 500 ml of fluid, or over 10% of the amount of water in blood plasma. A significant decrease in performance results when fluid loss is equal to 1-2% of total body weight. It is for this reason that athletes implement hydration strategies that include the consumption of appropriate fluids before, during, and after training or competition.
The physical symptoms of dehydration include overheating; loss of speed, muscular power, agility, and coordination; muscle cramping (known as heat cramps); headache, dizziness, and nausea. More advanced cases of heat illness, known as heat exhaustion, can lead to heat stroke.
The strategies to combat fluid loss through exercise begin with the preparation for an event through conclusion. In chronological order, such strategies will include the following components:
It is possible to over-hydrate the body, a circumstance that may result in a condition known as hyponatremia, or hyper-hydration. Hyponatremia is caused when the sodium level of the body during exercise becomes too low, due to either a low sodium level prior to the commencement of exercise, or the depletion of sodium stores during exercise. Hyperhydration, or water intoxication, floods the body with fluid and drives the sodium level into an imbalance. This condition occurs with some degree of frequency among endurance athletes. It is a deceptive condition, in that the athlete may actually be able to hear the sound of water moving in the stomach as the competition progresses; the athlete would be forgiven for believing that this sound is consistent with good hydration. In fact, the water is not entering the small intestine and it is not being processed for use in the cardiovascular system, due to the shut down of the regular physical processes that occur when sodium is depleted. If not treated, the body will potentially shut down those systems dependent upon sodium, such as the transmission of nerve impulses, and death may result.