Water balance

For optimal performance, the body’s water content should remain relatively constant.

Water balance at rest

Under normal resting conditions, the body’s water content is relatively constant: water intake equals water output. About 60% of our daily water intake is obtained from the fluids we drink and about 30% is from the food we consume. The remaining 10% is produced in our cells during metabolism. Metabolic water production varies from 150 to 250 ml per day, depending on the rate of energy expenditure: higher metabolic rates produce more water. The total daily water intake from all sources averages about 33ml per kilogram of body weight per day. For a 70kg(154lb) person, average intake is 2.3L per day. Water output, or water loss, occurs from four sources:

• Evaporation from the skin;
• Evaporation from the respiratory tract;
• Excretion from the kidneys;
• Excretion from the large intestine.

Human skin is permeable to water. Water diffuses to the skin’s surface, where it evaporates into the environment. In addition, the gases we breathe are constantly being humidified by water as they pass through the respiratory tract. These two types of water loss(from the skin and respiration) occur without our sensing them. Thus, they are termed insensible water losses. Under cool, resting conditions, these losses account for about 30% of daily water loss.

The majority of our daily water loss – 60% at rest – occurs from our kidneys, which excrete water and waste products as urine. Under resting conditions, the kidneys excrete about 50 to 60ml of water per hour. Another 5% of the water is lost by sweating(although this is often considered along with insensible water loss), and the remaining 5% is excreted from the large intestine in the feces. The sources of water gain and water loss are presented in the picture below.

Water balance during exercise

Water loss accelerates during exercise, as seen in the table below. The ability to lose the heat generated during exercise depends primarily on the formation and evaporation of sweat. As body temperature increases, sweating increases in an effort to prevent overheating. But at the same time, more water is produced during exercise because of increased oxidative metabolism. Unfortunately, the amount produced even during the most intense effort has only a small impact on the dehydration, or water loss, that results from heavy sweating.

In general, the amount of sweat produced during exercise is determined by:

·         Environmental temperature, radiant heat load, humidity and air velocity;

·         Body size;

·         Metabolic rate.

These factors influence the body’s heat storage and temperature. Heat is transferred from warmer areas to cooler ones, so heat loss from the body is impaired by high environmental temperatures, radiation, high humidity, and still air. Body size is important because large individuals generally expend more energy to do a given task, so they typically have higher metabolic rates and produce more heat. But they also have more surface area(skin), which allows more sweat formation and evaporation. As exercise intensity increases, so does the metabolic rate. This increases body heat production, which in turn increases sweating. To conserve water during exercise, blood flow to the kidneys decreases in an attempt to prevent dehydration; but like the increase in metabolic water production, this too may be insufficient. During high- intensity exercise under environmental heat stress, sweating and respiratory evaporation can cause losses of as much as 2 to 3L of water per hour.

Dehydration and exercise performance

Even minimal changes in the body’s water content can impair endurance performance. Without adequate fluid replacement, an athlete’s exercise tolerance shows a pronounced decrease during long-term activity because of water loss through sweating. The impact of dehydration on the cardiovascular and thermoregulatory systems is quite predictable. Fluid loss decreases plasma volume. This decreases blood pressure, which in turn reduces blood flow to the muscles and skin. In an effort to overcome this,heart rate increases. Because less blood reaches the skin, heat dissipation is hindered, and the body retains more heat. Thus, when a person is dehydrated by 2% of body weight or more, both heart rate and body temperature are elevated during exercise above values observed when normally hydrated.

As one might expect, these physiological changes will decrease exercise performance. Figure below illustrates the effects of an approximate 2% decrease in body weight attributable to dehydration from the use of a diuretic on distance runners’ performance in 1500m, 5000m, and 10,000m time trials on outdoor track. The dehydration condition resulted in plasma volume decreases between 10% and 12%. Although the average VO₂max did not differ between the normally hydrated and dehydrated trials, mean running velocity decreased by 3% in the 1500m run and by more than 6% in the 5000m and 10,000m runs. The greater the duration of the performance, the greater is the expected decline in performance for the same degree of dehydration. These trials were conducted in relatively cool weather. The higher the temperature, humidity, and radiation, the greater the expected decrement in performance for the same degree of dehydration. The decrement in performance would be progressively greater with greater levels of dehydration.

The effect of dehydration on performance in muscular strength, muscular endurance, and anaerobic types of activities is not as clear. Decrements have been seen in some studies, whereas other studies have shown no change in performance. In one of the best-controlled studies, researchers at Penn State University reported that 2% dehydration resulted in significant deterioration of basketball skills in 12- to 15-year-old boys who were skilled basketball players.

Wrestlers and other weight-category athletes commonly dehydrate to get a weight advantage during the weigh-in for a competition. Most rehydrate after the weigh-in before the competition and experience only small decrements in performance.