Do you pass by water stations in road races or skip water breaks during practice because you are not thirsty? Have you ever felt fatigued, dizzy, or cramps while exercising in hot weather?

If you answered "yes" to either of these questions, read on to learn how important fluids are for optimal health and performance. If your answers were "no", read on to make sure you are following fluid recommendations that will optimize your health and performance.

Role in the body

Water is key to life. Humans can survive more than a month without food, but only a few days without water. Water is found inside and outside cells and circulates in the blood. Our body consists of about 60-65% of water; for a 150 pound person, this represents approximately 90 to 98 pounds of water.

Water plays many important roles in the body including:


  • Water transports glucose, oxygen, and fat to working muscles.
  • Water transports waste products, such as carbon dioxide and lactic acid away from working muscles.

Body temperature regulation

  • The body wants to keep a constant temperature of approximately 98.6ºF or 37ºC. If the body temperature increases to >106ºF or 41ºC, cells will die.
  • Working muscles generate heat. In fact, the body can generate 20 times more heat when working, than when at rest.
  • To prevent overheating, the body regulates temperature by sweating. Water absorbs heat from the working muscles and dissipates this heat to circulating blood and ultimately through the skin.
  • Sweat evaporates from the skin, which cools it down. This cools down the blood and the rest of the body. More than 80% of metabolic heat is dissipated by sweat evaporation.


  • Water transports and eliminates waste-products in the urine.
    • The darker the urine indicates a greater concentration of waste-products and less water -- this indicates dehydration.
    • Vitamin/mineral supplementation can result in dark urine. Since supplements of individual vitamins or minerals (i.e. Vitamin C, selenium or chromium) may contain more than the body can absorb, they are excreted in the urine.


  • Water is an important component of saliva and gastric juices, which help digest food.


  • Water is a good lubricator of joints, organs, and tissues.

Role in the diet

Water comes from more than just fluids, it is a major component of many foods. In fact, it is estimated that 20% of our water needs are met through food, not fluids. Meal consumption is critical to ensure full hydration on a day to day basis. Eating food promotes fluid intake and retention.


Foods with high water content add volume but minimal calories to the diet. Thus, these foods high in water are known to promote a feeling of fullness. Fruits and vegetables are two food groups that have generally high water content. Even meat, bread, and dairy products contain some water.

Example of fruits and vegetables high in water include:
Fruit: Watermelon, Citrus fruits, Grapes, Apples, Papaya, Strawberries, Apricots, Cherries
Vegetables: Carrots, Bell peppers, Lettuce, Tomato, Cucumber, Squash, Celery, Broccoli, Cauliflower, Spinach


The daily recommended fluid intake is 10-15 glasses (8oz/glass) depending on your age and sex.

  • Females and younger people are at the lower end of the range while males are at the upper end of the range.
  • Foods contribute 20% of the fluid requirements so actual fluid recommendations are 8-13 glasses (8 oz/glass) daily.
  • Make Half your Plate Fruits and Vegetables to meet the additional 2 cups of fluid through your food intake.

Athletes have greater fluid needs related to the role of water in physical activity. Thirst alone does not adequately supply these fluid needs, it will only supply 50% of fluid needs.


Athletes need to stay hydrated for optimal performance. Many athletes drink when they are thirsty and fail to hydrate before they become dehydrated. Thirst is a biological indicator of dehydration; however, dehydration has already occurred when an athlete becomes thirsty. Even a small drop in body fluids (1% of body weight, or 1.5 pounds in a 150 pound person) can impair performance.

Causes of dehydration

  • Inadequate fluid intake
  • Profuse sweating
  • Failure to replace losses after exercise
  • Exercising in hot weather, regardless of fluid consumption
  • Relying on thirst to hydrate
dehydration dehydration

Dehydration, or excessive fluid loss, causes the blood volume to drop. This increases the heart rate as it tries to compensate for the decreased blood supply to the organs. Muscle cramps, dizziness and fatigue are caused by dehydration and can increase the risk for injury. Perceived exertion also increases with dehydration.

Dehydration can lead to heat illness, which impacts physical performance. Signs of heat illness are:

  • Headache, dizziness
  • Nausea, vomiting
  • Weakness, reduced performance
  • Irritability
  • Irrational behavior
  • Inability to concentrate
  • Fatigue
  • Muscle Cramps
  • Confusion/ disorientation

Signs of dehydration and heat illness directly related to performance include reduced muscular strength and endurance.

Note in the figure below that even a 2-4% loss of body water (3-6 pound loss for a 150# person) will reduce muscular strength and endurance. Studies have shown reduced isometric and isotonic muscular contraction of 20-25% with 4% dehydration. Cognitive/mental performance is also compromised with >2% loss of body water. Further, 24-36 hours are needed to fully recover muscular strength and endurance after dehydration. This is noteworthy because some athletes participating in weight class sports use dehydration to make weight. This practice compromises their muscular strength and endurance because there is not enough time between weigh-in and competition to adequately rehydrate. 

Effects of dehydration on physical performance

Urine volume and color are good indicators of hydration. A light colored urine indicates adequate hydration. In addition, a larger volume of urine indicates better hydration. Since vitamin supplements can make the urine darker, volume may be the better indicator for those taking vitamin supplements.


It is important to adequately hydrate before, during and after exercising to prevent dehydration. A number of factors will influence your fluid needs:

  • Altitude - Fluid requirements are greater at high altitudes.
  • Climate - If the temperature is hot or humid you will need to consume more water.
  • Sweat Rate - Sweat is the primary source of water loss during exercise. The difference in sweat rates between individuals makes it difficult to provide a ‘one size fits all’ recommendation. Individuals who sweat easily and in large quantities are at greater risk for dehydration. Women tend to have lower sweating rates than men (smaller body size and lower metabolic rate).

    Body weight changes can reflect sweat losses during exercise and can be used to estimate individual fluid replacement needs for specific exercise and environmental conditions. It is recommended that individuals monitor body weight changes during training/competition sessions to estimate sweat loss during specific practice tasks and competitions with respect to varying weather conditions. Estimate your sweat rate per hour by weighing yourself before and after exercise.

    • A 1 pound weight loss suggests 2 cups of fluid are needed to replace sweat losses.
    • To make fluid replacement easier, measure 1 cup of water and count how many gulps it takes to drink. Then use the number of gulps as a guideline for 1 cup of fluid replacement.

To get more information on hydration and fluids visit the Gatorade® Sports Sciences Institute (look in the upper right hand corner of the page for hydration).

  • Type and length of activity - The intensity and length of duration of an activity will determine the need for fluids. Endurance athletes who exercise for hours will lose more water and may be more sensitive to dehydration for optimal performance than a sprinter. Swimmers may not feel the heat and the extent of sweating from exercise due to the cooling effect of the water, thus are at increased risk for dehydration. Athletes participating in weight-class or weight-sensitive sports and dehydrate themselves before weigh-ins will impair their performance and place themselves at risk for health problems related to dehydration. Extra care should be taken to determine fluid replacement rates in prolonged exercise lasting > 3 hours.
Fluid Recommendations Before, During and After Exercise

To enhance fluid ingestion, consider the palatability of the fluid. Cool beverages (40-50 degrees Fahrenheit) with moderate amounts of sodium and/or sugar, in desirable flavors will promote fluid consumption. For more information visit the Eat To Compete information on fluid replacement beverages. (link to training diet page and if possible anchor to section at end about differentiating different types of drinks)

Hydrating prior to exercise, an individual should slowly drink beverages approximately 1 ounce per 10 pounds of body weight at least 4 hours before the exercise task. Hydrating several hours prior to exercise allows sufficient time for urine output to return towards normal before starting an event. If this fluid consumption does not produce urine, or urine is dark, slowly drink additional beverage 1 ounce per 15 pounds of body weight about 2 hours before the event.

The goal of drinking during exercise is to prevent excessive dehydration (>2% body weight loss) and excessive changes in electrolyte balance. During practice/competition 4 to 8 ounces of fluid should be consumed approximately every 15 minutes.  While the primary intent is to prevent dehydration, consuming beverages with carbohydrate can be beneficial to sustain high-intensity exercise lasting 1 hour or more. Consuming carbohydrate during exercise at a rate of 30-60 gms has been shown to maintain blood glucose levels and sustain exercise performance. Drinking beverages with caffeine may also help sustain exercise of high-intensity. Previous concerns regarding a dehydrating effect of caffeine have not been observed with a moderate dose of caffeine (about 12 oz of coffee).

After exercise the goal is to fully replace any fluid and electrolyte deficit. How aggressively an individual rehydrates depends on the degree of dehydration and/or the length of time until the next bout of exercise. If the degree of dehydration is not severe and time permits, consumption of normal meals and snacks with sufficient volume of plain water will restore hydration status. For rapid and complete recovery from dehydration an individual should drink approximately 3 cups of fluid for every pound of body weight loss. Consuming sodium (in beverages or snack items) during the recovery period will help retain ingested fluids and stimulate thirst.

Overhydration (hyponatremia)

Although rare, overhydration, also known as hyponatremia, can occur in athletes. Athletes with a high concentration of sodium in their sweat, consume large/excessive quantities of water, or exercise long periods of time (4-6 hours) are at greatest risk for hyponatremia. Hyponatremia is a serious condition, which in worst cases can be fatal. Hyponatremia occurs when plasma sodium concentrations reach <135 mmol/L (normal levels are 138-142 mmol/L).

Hyponatremia can cause gastrointestinal problems (bloating, nausea etc.), wheezy breathing, swollen hands and feet, unusual fatigue, and incoordination. Life-threatening problems include swelling of the brain, which can result in throbbing headache, confusion, seizures, coma, and death.

Determine if you are at risk...

  • Do you have high sweat volumes and high sweat salt concentrations?
    • If you sweat large amounts and notice white salt on your skin, or if your sweat stings your eyes, the sodium concentration is high.
  • Do you exercise for hours?
  • Do you eat a low sodium diet and consume excessive amounts of water?
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