By Mike Darnley, MS,RD
Many people do not think of water as a nutrient, possibly due to its simplicity or maybe because there is no energy provided by water. Nevertheless, water is the most vital nutrient that we consume. Besides being a medium for cellular processes that occur in the body, water is the main component of blood, which is responsible for transporting nutrients and oxygen to tissues of the body. Water is also important for dissipating heat through evaporation from the skin. In addition to the role of water in survival, athletic performance can be compromised by only slight changes in hydration status(1).
Because the primary focus of this paper is water, I will not go into detail about the need for oral rehydration solutions or carbohydrate solutions during exercise.
Normally, we lose about 2 1/2 cups of water per day simply by evaporation from the skin and by regular breathing. A greater volume of water is lost through sweat and urine, and exercise dramatically increases the requirement for water.
Water that is lost through sweat during exercise can amount to 2-6% of a person’s body weight (2). Fluid needs are dependent on temperature and humidity, activity levels, and the clothing that covers our bodies. For example, football players wearing uniforms can become dehydrated in as little as 30
minutes (3).
Water is the best source of fluid for exercise sessions lasting less than one hour (1). Drinking carbohydrate solutions, such as Gatorade or Powerade, may be necessary to replete blood glucose during longer events. The intensity of exercise, as well as the volume of fluid in the stomach are both major determinants of gastric emptying rate. Exercise intensities above 80% of maximal capacity can slow the rate at which fluid is absorbed from the stomach (4). A high volume of fluid intake (up to approximately 1 L/hr) increases gastric emptying rate (5). Regardless of whether an athlete consumes water or a carbohydrate solution, fluids containing more than 10% carbohydrate slow gastric emptying and should be avoided. Cool water is preferred by most athletes and can promote increased voluntary intake of fluids during exercise.
Dehydration can affect our performance in many ways; one is by exacerbating a condition known as cardiovascular drift. Cardiovascular drift is a state that occurs during prolonged activity (more than one hour) at a moderate intensity (at least 50% VO2max) and is characterized by a progressive increase in heart rate with a corresponding decrease in the amount of blood that is pumped from the
heart with each beat (stroke volume). During prolonged exercise blood is directed to the surface of the body for cooling to take place; this creates a drop in stroke volume and a subsequent reduction in central venous pressure (blood returning to the heart). Dehydration can lead to cardiovascular drift by causing a decrease in plasma volume (6). Although dehydration is not believed to be the cause of cardiovascular drift, the responses to prolonged exercise are negatively affected by a state of dehydration, and being adequately hydrated can help prevent these detrimental responses (7).
Practical Considerations
Water can be consumed through foods and by drinking fluids. Try carrying a water bottle with you throughout the day, and avoid becoming thirsty by drinking frequently. Thirst is not a reliable indicator of hydration status, so drink water before you feel thirsty. Alcohol and caffeine have diuretic properties, so tea, caffeinated sodas, and alcoholic beverages should not be used to replete fluids.
Early symptoms of dehydration include fatigue, loss of appetite, loss of body weight, heat intolerance, and low volumes of dark yellow urine. Severe dehydration can lead to muscle spasms, excessive body-core temperatures, and total exhaustion.
Fluid Guidelines for Exercise Lasting Less than One Hour
1. Drink adequate fluids throughout the day, and consume approximately 2 cups
of water during the 2 hours prior to exercise.
2. Every 15 minutes during activity consume 1/2-3/4 cups of water.
3. Fluids should be kept at a cool temperature (between 59° & 72° F).
4. Use a container, such as a sports bottle that allows one to drink while
exercising.
5. Consume approximately 2 cups of water for every pound lost during exercise
(acute shifts in body weight during exercise indicate fluid loss).
References
1. American College of Sports Medicine (1996) Position stand on exercise and fluid replacement. Med. Sci. Sports Exerc. 28(1):i-vii.
2. Noakes, TD (1993) Fluid replacement during exercise. Exerc. Sports Sci.Rev. 21:297-330.
3. Mathews, DK, Fox, EL, and Tanzi, D (1969) Physiological responses duringexercise and recovery in a football uniform. J. Appl. Physiol. 26:611.
4. Costill, DL, and Saltin, B (1974) Factors limiting gastric emptying during restand exercise. J. Appl. Physiol. 37:679-683.
5. Noakes, TD, Rehrer, NJ, and Maughan, RJ (1991) The importance of volume in regulating gastric emptying. Med. Sci. Sports Exerc. 23:307-313.
6. Raven, PB, and Stevens, GH (1988) Cardiovascular function and prolonged exercise. In D. Lamb, and R. Murray (Eds.), Perspectives in Exercise Science and Sports Medicine: Prolonged Exercise. Benchmark Press Inc. Indianapolis,IN.
7. Rowell, LB (1993) Human Cardiovascular Control. Oxford Univ. Press Inc. NY
