The body is composed of 50-75% of water, depending on age and body fatness. Water is essential for living, significantly more so than food. Death will usually occur after one week without any liquids (2 to 3 days in the heat) but humans have been known to survive some months without food.
The common rate of sweat loss is about 275ml in the heat. Prolonged exercise performance can be impaired by depletion of the body’s energy stores and by disturbances of water and electrolyte balance. Exercise can exceed two liters per hour in highly fit and well-acclimatized athletes.
Even more modest and common rates of sweat loss (e.g., 1 – 1.5 l/h) sustained for 30 to 60 minutes can result in dehydration that is of sufficient magnitude to negatively affect cardiovascular, thermoregulatory, and performance responses. There is, however, a limit to the body’s ability to maintain physiological homeostasis during exercise in a warm environment when the body is no longer capable of coping with the demands placed upon it.
The Relationship of Water and Exercise
In Exercise Physiology, Water plays an important role in two aspects: “Heat Dissipation” and “Physiological Homeostasis”.
Heat Dissipation: Regulates body temperature through perspiration. Physiological Homeostasis: Water helps to maintain the physical equilibrium, to regulate its internal environment to maintain a stable, constant condition for adapting the physical activity.
Normal body water turnover in an adult is from 1.7 to 2.3 L/day Respiration: Vigorous exercises can dissipate1-5ml water per minute Sweat: about 500ml per day (depending upon temperature and humidity) Urine excretion: about 1000-1500ml per day Stool excretion: about 100-200mg per day
Effects of Dehydration on Physiology and Performance
Inadequate drinking during strenuous activity can result in dehydration and impaired athletic performance. During intense physical activity, athletes often do not drink enough fluids to replace what they lose in exercise, resulting in what’s called “voluntary dehydration”.
Fluid losses of as little as 1% of body weight (about 600ml), cause heart rate to be elevated, cardiac output to be declined, and core temperature to be raised. Fluid losses of 2% of body weight (less than 4 pounds in a 200-pound athlete), due to the dysfunction of homeostatic, it can impair performance by raising the body temperature and increasing fatigue who fails to replace body fluids.
Fluid losses of 4-6% of body weight (about 2400-3600ml) will decrease muscle strength and muscle endurance. And heat spasm often develops. The performance ability may lower by 20-30% and also affected the anaerobic capacity. Besides, there are progressive increases in the concentrations of sodium and other dissolved substances in the blood plasma, a lower blood flow to the skin, and a higher core temperature which will increase the chance of heat stroke.
Fluid losses over 6% of body weight (greater than 3600ml). Dehydration compromises symptoms of impaired cardiovascular function often develop (e.g., syncope, heat exhaustion, heat spasm), and, in severe cases, neurological failure and severe hyperthermia ensue (e.g., heat stroke).