Staying hydrated during exercise
The goal of drinking fluids during exercise is to prevent dehydration (i.e., more than two percent body weight loss from water deficit) and maintain performance. Fluids, preferably with carbohydrates and electrolytes, should be consumed in small amounts every 15 to 20 minutes during exercise.
Water and electrolytes (sodium/Na+, potassium/K+, chloride/Cl–, and others) serve very important roles in the functioning of the body, and sweating can lead to excessive losses of both critical nutrients if not properly replaced. Dehydration and electrolyte imbalances can adversely impact health and exercise performance. The magnitude of fluid and sweat losses during exercise depends on the intensity of the exercise, environmental conditions, and the type of clothing worn during the exercise. To avoid excessive fluid and electrolyte losses, a person should begin exercising in a well-hydrated state. About two hours prior to strenuous exercise, drink approximately 20 fl oz (500ml) of liquid to ensure proper hydration at the onset of exercise.
Consuming sufficient fluids during exercise will influence cardiovascular, thermoregulatory, and muscle function, as well as hydration status. To avoid dehydration, 13–32 fl oz (400–1,000 ml) of fluid should be consumed every hour by drinking small amounts frequently: 3–8 fl oz (100–250 ml) every 15 minutes or 8–11 fl oz (250 – 330 ml) every 20 minutes. Water is fine if the exercise is of short duration, but if the exercise is longer than one hour, the fluid should contain carbohydrates (from sugars) and electrolytes (from salts). The addition of carbohydrates to a fluid replacement drink can enhance intestinal absorption of water and help maintain blood glucose concentration during exercise, which may preserve muscle glycogen (sugar storage) and thereby delay fatigue.
Myths and Claims
Some common misconceptions about hydration are:
Drinking fluids during exercise slows you down. This is not the case, and in fact, fluid intake during exercise should be adjusted to accommodate sweat rate. Ingesting fluids during exercise may improve endurance.
The most effective way to replace electrolytes is by consuming sport drinks. This is not necessarily true. Fruits, fruits juices, and other foods, if combined with water, will replace electrolytes and fluids.
Even minimal dehydration (one percent body weight) can increase cardiovascular strain and limit the ability of the body to transfer heat from contracting muscles to the skin surface for dissipation to the environment. Thus, body water deficits increase the probability for compromising exercise performance and developing heat-related injury.
With dehydration, the overall amount of water in the body declines, which reduces blood volume and skin blood flow. Consuming fluids at regular intervals will prevent dehydration.
Thirst is an excellent indicator of dehydration: if you are thirsty, it is most likely you are dehydrated.
Multiple guidelines regarding fluid replacement have been published. In 2003, the U.S. Military published revised fluid replacement guidelines and in 2007 the American College of Sports Medicine published a revised Position Stand on Exercise and Fluid Replacement. In 2000 the National Athletic Trainers’ Association published their Position Statement on Fluid Replacement for Athletes. Each of these documents provides recommendations for optimal fluid replacement and maintaining hydration during exercise in all types of conditions.
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Sustained exercise, especially in the heat, can result in high sweat rates and substantial water and electrolyte loss. If sweat water and electrolyte losses are not replaced, performance can be severely compromised. Dehydration and electrolyte deficits have been associated with muscle cramps. Conversely, excessive fluid intake can result in hyponatremia (low salt levels in the blood), which is life threatening. Fluid intake should be limited to one quart (one liter) per hour to avoid hyponatremia. Women may be at greater risk than men of developing exercise-associated hyponatremia.
Summary for Military Relevance
Personnel under heat and physical stress need to pay special attention to fluid, carbohydrate, and salt intake. Maintaining hydration status is critical for the military because it affects performance. The U.S. Military has fluid replacement guidelines to address the needs of Warfighters.
* Source HRPC