By Jeff Rocco MD
Proper hydration for athletes requires more than water. Last month we established the critical role electrolytes play for exercising athletes. (See A Tale of Five Electrolytes) 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 need 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 an updated comparison of currently available drinks, and some thoughts to consider when selecting an exercise drink.
The volume of fluid consumed during exercise should nearly match the volume of fluid lost by sweat during the exercise period. Fluid losses exceeding as little as 2% of body weight can lead to significant decreases in performance. You can calculate your sweat rate by weighing yourself before and after a one-hour exercise session. This number less the amount of fluid consumed during the session is your sweat rate.
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 through sweat during exercise are sodium, potassium, chloride, calcium, and magnesium. Nutrition during exercise should include these five critical electrolytes. See A Tale of Five Electrolytes for a discussion of why electrolytes are important.
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. In the chart below notice the concentration of electrolyte losses versus the concentrations available in a typical sports drink. You might want to then refer to the label on your favorite sports drink or look at the comparison chart we have put together.
Besides the quantity of electrolytes, the source may also be important. Absorption rates of chelated Calcium and Magnesium sources in the form of DiCalcium and DiMagnesium Malate have been shown to be 20% to 100% higher than Calcium Carbonate and Magnesium Oxide. Chelated Calcium and Magnesium have the added benefit of buffering stomach acids, which may decrease gastric distress in a stressed athlete performing “on the rivet.”
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 (maltodexterin)→Sucrose→Honey→Lactose→Galactose→Fructose
Higher glycemic index carbohydrates are recommended even during longer exercise bouts. Slow-burning fuel for extended aerobic exercise comes from an athlete’s fat stores. Even a very thin athlete will have 30 hours or more worth of fat stores. Slowly absorbed carbohydrates may simply add to gastrointestinal stress during already stressful exercise.
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.
Aerobic and anaerobic metabolism depend on the Krebs cycle. The Krebs cycle consumes malic acid during the production of useful energy. Replacing malic acid during exercise will keep the Krebs cycle turning and keep the energy flowing.
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. BCAAs cannot be manufactured by the body, and must come from dietary sources and supplements. Glutamine and the BCAAs are depleted through exercise. Athletes experience fatigue as glutamine and BCAA levels drop. Chronic glutamine and BCAA depletion has negative effects on the immune system and may be a major component of overtraining syndrome. 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.
Exercise produces oxidative stress in the form of free radicals that damage cell membranes and DNA. Athletes produce more free radicals than sedentary individuals. Vitamin C is a water-soluble antioxidant that can protect the body from the ravages of free radicals. Vitamin C is also works through a variety of mechanisms to support the immune system and health in general. Vitamin C is a good addition to an exercise drink.
Bioavailability is the bottom line for all of the components of an exercise drink. When selecting an exercise drink, select one that is easily digested. Other considerations may be artificial colors, flavors, and non-nutritive sweeteners.
After you’re done exercising, we’ll have a look at recovery drinks in our next article.