A Fluid Formula for Athletes

Most adults can maintain a healthy hydration status quite easily, thanks to the body’s thirst mechanism, plenty of access to water in foods and beverages, and the kidneys’ ability to produce more or less urine. In nonactive adults, normal water turnover is about 1 to 3 liters per day.

But, for individuals who exercise strenuously, it’s a different story. Their daily fluid needs can often exceed 3 to 4 liters per day, sometimes pushing 10 liters per day, depending on conditions. For athletes or others who engage in vigorous activity for work or play achieving an optimal water balance can be an uphill battle.

The dehydration factor 

During strenuous activity, body water loss is primarily caused by sweat loss, although urine and respiratory fluid loss contribute as well. Even a 2% loss of body weight through sweat can be a red flag for dehydration. Water loss can be significant when an individual has been sweating for more than two hours, so long or multiple workouts during a single day can further increase this risk. Exercising in hot weather, in a hot gym or in insulated gear makes dehydration even more likely.

Failure to replace lost fluid and electrolytes can lead to impaired heat dissipation, which can increase the body’s core temperature and strain the cardiovascular system. Dehydration can also negatively affect energy, performance, short-term memory, motor skills, arithmetic ability and mood. Children and older adults suffer a greater risk for dehydration.

Quenching the thirst 

Some confusion has surfaced regarding the recommendations for optimal hydration for athletes. When the Institute of Medicine (IOM) released its recommendations for water intake in 2004, “Dietary Reference Intakes: Water, Potassium, Sodium, Chloride, and Sulfate” (2004), which highlighted the principle that thirst should be a guide for hydration for the vast majority of healthy adults, the guidelines were targeted at sedentary Americans, not athletes.

In athletes, fluid needs can be much greater than the thirst mechanism can trigger. The American College of Sports Medicine (ACSM), Indianapolis, and the National Athletic Trainers’ Association (NATA), Dallas, issued statements after the IOM report cautioning physically active people against letting their thirst guide them. In reality, active adults should be proactive about drinking fluids before, during and after activity.

ACSM and NATA recommend that water losses due to sweating during exercise be replaced at a rate close or equal to the sweat rate, which can be determined by weighing an individual before and after exercise. Weight loss after exercise means that adequate fluids were not replaced during exercise, while weight gain indicates that the volume of fluid replacement was too high. Excessive fluid consumption can result in low blood sodium concentration (hyponatremia), which, although rare, can be a dangerous condition.

Balancing Na and K 

Sodium losses through sweat can be high and variable for athletes. Some athletes are “saltier” sweaters, and sodium loss can depend on heat acclimation and other factors. Unless sodium restriction is required for a specific health reason, athletes are advised to consume a liberal amount of salt (sodium chloride) and sodium in their diet, especially when exercising in hot conditions. The requirement for the electrolyte, chloride, can be met through an adequate amount of salt (3.8 grams per day for healthy adults) in the diet.

Sweat potassium represents only a small percentage of the available potassium in the body. But sweating does increase the dietary requirement for potassium, which works in the body to lower blood pressure, blunt the adverse blood-pressure effects of salt intake, reduce the risk of kidney stones and reduce bone loss. Potassium is readily found in fruit and vegetable sources.

The requirement for sulfate is easily met through sulfur-containing amino acids. Since sweat contains little sulfate, it can be replaced through an adequate intake of protein foods, water and beverages. Because the typical American diet contains adequate-to-excessive amounts of protein, no RDA is set for sulfate.

A sports drink fix 

In 2004, sales of sports drinks soared to $2.6 billion. But do these drinks actually work?

While sports drinks may provide unnecessary calories and electrolytes for non-exercisers, studies support their hydration and performance benefits for athletes. According to “American College of Sports Medicine Roundtable on Hydration and Physical Activity: Consensus Statements” (Current Sports Medicine Reports, 2005; 3(4):115-127), drinking carbohydrate- electrolyte beverages enhances performance during prolonged (45 to 50 minutes) exercise or in high-intensity, intermittent exercise, compared with drinking the same volume of water.

Optimal hydration for an athlete includes both fluid and electrolyte (sodium and potassium) replacement. A sports drink with at least 100 mg sodium per 8 oz. promotes continued drinking by the athlete, unlike water, which quenches the thirst sensation before body fluids are sufficiently replaced. Light flavor and sweetness can also encourage better hydration. An appropriate amount and type of electrolytes in a sports drink can help maintain fluid balance, physiological functions and, in some cases, prevent muscle cramping. Recommended amounts of electrolytes in a sports drink include 100 mg to 110 mg sodium, and 30 mg potassium per 8-oz. serving.

Another benefit of sports drinks is the small amount of appropriate carbohydrates they contain, which can help fuel the body’s energy needs and aid in fluid absorption, without promoting stomach upset. Drinking a carbohydrate-containing beverage to provide 30 to 60 grams of carbohydrate per hour during vigorous exercise has also been associated with delayed mental fatigue and improved cognitive function, mood, motor-skill performance and perceived exertion, compared with drinking the same amount of water. High fructose levels can lead to gastrointestinal distress, while multiple carbohydrate sources help fluid absorption.

According to researchers from the Human Performance Laboratory, Department of Health and Human Performance, Texas A&M University, College Station, endurance athletes should replace sweat loss through intake of 500 ml of fluid solution containing about 4% to 8% carbohydrate solution and electrolytes during training and competition (Nutrition, 2004; 20(7-8):651-656). The scientists also suggest that, for intense, prolonged exercise lasting more than one hour, athletes should consume between 30 and 60 grams per hour and drink between 600 and 1,200 ml per hour of a solution containing carbohydrate and sodium (0.5 to 0.7 grams per liter of fluid).

Hydration in athletes is an important issue, and it looks like help can come from well-formulated sports drinks. 

Sharon Palmer is a registered dietitian with 1 years of experience in health-care and foodservice management. She writes on food and nutrition for newspapers, magazines, websites and books. Palmer makes her home in Southern California and can be reached at [email protected].