Proper hydration for athletes requires more than water. During exercise electrical impulses are being carried, muscles are contracting, calories are burning, heat is being produced, and sweating occurs. To keep these systems within optimum parameters, electrolytes needs to be maintained and energy stores need to be replaced—all within a fluid medium. The endurance athlete is faced with an ever-increasing variety of products to meet these nutritional needs. We have put together a comparison of 15 currently available drinks, and some thoughts to consider when selecting an exercise drink. Click here for comparison chart.
Yes, our bodies depend on fluid to do the business of exercise, but that fluid contains more than water. It contains electrolytes primarily, along with some amino acids and vitamins. Electrolytes are dissolved mineral salts that are found in the fluid both inside and outside of the cells in our bodies. The primary minerals lost are sodium, potassium, chloride, calcium, and magnesium. Sodium, potassium, and chloride are key to the conduction of electrical impulses, and are involved in transportation of nutrients into cells and wastes out of cells. Many people think of bones when they think of calcium. That is true; bones are the largest reservoir of calcium in the body. However, soluble calcium in body fluid is also necessary for neuromuscular conduction, muscular contraction, inter- and intracellular messaging, and plays a key regulatory role in glycogen metabolism. Magnesium is important for proper transmission of nerve impulses, muscular contraction, and energy production. Nutrition during exercise should include these five critical electrolytes.
For athletes participating in longer duration and/or higher intensity exercise, the electrolytes lost through exercise can exceed what is available in many sports drinks. Some companies have recognized this and recommend supplementing with electrolyte tablets during exercise. Adequate amounts of electrolytes should be available in the sports drink.
From Maughan and Shirreffs, 1998. Fluid and electrolyte loss and replacement in exercise. In Oxford textbook of sports medicine, 2nd Edition. Edited by Harris, Williams, Stanish, and Micheli. New York: Oxford University Press, pp. 97-113.
Carbohydrates are present in sports drinks as an energy source. Most athletes have about 2 hours worth of energy stored as muscle glycogen. Carbohydrates consumed during exercise spare muscle glycogen stores and delay the onset of fatigue during exercise. Carbohydrates from multiple sources make use of multiple absorption systems, and therefore allow more energy to be absorbed by the small intestine. Higher Glycemic Index carbohydrates are absorbed faster. The following is a list of carbohydrate sources from highest to lowest glycemic index:
Maltose>Glucose or glucose polymers (maltodextrin)> Sucrose>Honey>Lactose>Galactose>Fructose
The concentration of the carbohydrate solution is also important. Too much or too little carbohydrate can delay gastric emptying and impair the absorption of both carbohydrates and fluid. The optimal absorption of carbohydrates and fluid has been shown to occur in a 6-8% carbohydrate solution.
Should an athlete use a sports drink with protein? While there is evidence to suggest that protein consumed during exercise improves time to exhaustion and decreases muscle damage, these benefits may be from the amino acids that make up the proteins consumed in these studies. These same benefits are supported by literature for amino acids consumed during exercise. The most critical amino acids for exercise are glutamine and the branched chain amino acids (BCAAs). The branched chain amino acids are leucine, isoleucine, and valine. When choosing a sports drink with whole proteins versus amino acids, consider that whole proteins are harder to digest, are absorbed slower in the intestines, do not dissolve easily in the water bottle, and may not taste good during exercise.
Finally, the taste of the sports drink (which is personal preference) can influence its effectiveness. Drinks that taste better are more readily consumed.