Fluid and electrolyte replenishment during exercise

Research has shown that cardiovascular and thermoregulatory responses and performance are optimized when sweat losses are replaced during exercise. These results are reflected in the following recommendations: During exercise, athletes should begin fluid intake immediately and then continue to do so at regular intervals to consume fluid at a rate sufficient to replace all sweat losses or to drink as much fluid as the body can tolerate. These recommendations indicate that the purpose of fluid intake during exercise is to prevent any dehydration, but recognize that fluid intake may be difficult in some circumstances. In most cases, ad libitum fluid intake alone is insufficient to completely replace sweat losses during exercise, as sweat losses vary from person to person. For example, light exercise in a cold, dry environment may result in sweat losses of only 250 ml per hour, whereas exercise in a hot, humid environment may result in losses exceeding 2 L per hour (some athletes lose more than 3 L per hour). It is therefore important that athletes and workers consume fluids according to a prescribed regimen that regulates the frequency and volume of fluid intake. Under ideal conditions, this means knowing the individual sweat rate (easily assessed by recording body weight before and after exercise and adjusting fluid intake and urine loss accordingly) and developing specific individual recommendations for fluid intake during exercise.

It is recommended that the fluid consumed be cooler than the ambient temperature (between 15 and 22 °C (59 and 72 °F)), flavoured to enhance taste and stimulate replacement. Fluids should be readily available and served in containers so that sufficient quantities can be consumed with minimal interruption to exercise. It is not surprising that people prefer flavoured and sweetened drinks. This is important to prevent dehydration, as any step towards increasing free fluid intake will help reduce the risk of health problems associated with dehydration and heat stroke.

In addition to providing athletes with tasty drinks, there are a number of other measures that should be taken. These include the following:

• Educate coaches, caregivers, parents and athletes about the benefits of proper hydration. Periodic lectures, posters, leaflets and brochures can be part of this effort.
• Providing conditions for obtaining liquid at any time. If possible, water sources should always be nearby, and there should be no restrictions on the frequency of its intake.

Practical tips and recommendations for fluid consumption before, during and after exercise:

• Bring hydration. Carry a bottle or bag of liquids on your belt and/or carry a cooler full of drinks (keep your drink bottles frozen overnight to keep them cold longer).
• Learn the signs of dehydration (unusual fatigue, dizziness, headache, dark urine, dry mouth).
• Know where to find liquids (drinking fountains, stores, etc.) and always have money on you to buy drinks.
• Drink on a schedule - not when you feel thirsty.
• Drink enough fluids before exercise to produce clear urine.
• Plan your beverage intake during competition. Practice drinking during physical training.
• Start your workout in a state of satiety.
• Know how much you sweat by monitoring your body weight before and after exercise.
• Drink 24 ounces for every pound of body weight lost after exercise (one average sip of fluid equals about one ounce).
• Fully replace lost fluid and sodium to achieve complete rehydration.
• Drink more water than you pour on your head. Pouring water on your head does not lower your body temperature.

Adding appropriate amounts of carbohydrates and/or electrolytes to the fluid replacement solution is recommended for exercise lasting longer than 1 hour, as it does not impair hydration and may improve performance.

Carbohydrates are an important component of beverages because they enhance the taste of beverages, provide fuel for active muscles, and stimulate fluid absorption from the intestines. The performance benefits of carbohydrate feeding during exercise are discussed in more detail in other chapters. Although carbohydrate feeding improves performance, high amounts of carbohydrate in beverages are not always necessary. Drinks containing more than 14 g of carbohydrate per 8 ounce serving have been shown to decrease the rate of gastric emptying and fluid absorption.

Inclusion of sodium (0.5-0.7 g L-1 water) in rehydration solution consumed during exercise lasting longer than 1 hour is recommended as it may enhance palatability, promote fluid retention, and possibly prevent hyponatremia in those who consume fluid in excess.

Sweat contains more sodium and chloride than other minerals, and although electrolytes in sweat are usually significantly lower than in plasma (plasma - 138-142 mmol-L-1; sweat - 25-100 mmol-L-1), physical activity for more than 2 hours per day can cause significant salt loss. Sodium deficiency is not usually observed among athletes and military personnel, since a normal diet often provides more than enough salt to replenish its loss in sweat. However, sodium loss can be problematic. Thus, a case is described that occurred with a tennis player suffering from frequent heat cramps. High sweating rate (2.5 L per hour) combined with higher than normal sodium concentration in sweat (90 mmol-h-1) caused muscle cramps in the player. When he increased his daily dietary sodium chloride intake from 5-10 to 15-20 g per day and increased his fluid intake to ensure adequate hydration, the cramps stopped.

It is also important to know that sodium chloride ingestion in beverages during exercise not only helps ensure adequate fluid intake, but also stimulates more complete rehydration following exercise. Both of these responses reflect the role of sodium in maintaining the drive to drink fluid and providing osmotic pressure to retain fluid in the extracellular space.

According to the ACSM guidelines, the sodium content of a fluid replacement drink does not directly affect the rate of fluid absorption. This is because the volume of sodium that can be included in a drink is small compared to the volume of sodium available in the bloodstream. Whenever fluid is ingested, plasma sodium diffuses into the intestine under the influence of an osmotic gradient that favors sodium influx. Sodium chloride is an important component of a sports drink because it improves palatability, helps maintain the incentive to drink, reduces the amount of sodium that the blood must release into the intestine before fluid is absorbed, helps maintain plasma volume during exercise, and serves as the major osmotic impetus for restoring extracellular fluid volume after exercise.

Wilk and Bar-Or provided an example of the effect of beverage composition on free fluid intake. Boys aged 9 to 12 years exercised for 3 hours in hot conditions with breaks. During this exercise, they drank one of three drinks to choose from. The drinks included water, a sports drink, and a flavored, artificially sweetened sports drink (placebo). The boys drank twice as much of the sports drink as they drank water; their placebo intake was in between. Flavor and sweetness increased free fluid intake (placebo compared with water), and the presence of sodium chloride in the sports drink further increased intake (i.e., they consumed more of the sports drink than of the placebo).

These results are consistent with the physiology of the thirst mechanism. In humans, thirst is caused by a change in plasma sodium concentration as a result of decreased blood volume. Plain water rapidly eliminates the osmotic thirst drive (dilutes the blood sodium concentration) and reduces the blood volume-dependent drive (partially restores blood volume), thereby quenching thirst. Unfortunately, this reduction in fluid intake occurs largely before adequate fluid intake has been achieved. The osmotic drive can be maintained by the presence of low levels of sodium chloride in the beverage.

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