For the most part, more research is needed to determine what types of carbohydrate sources are best for various specific occasions. Sports drinks are typically designed to provide fluid, carbohydrate, and electrolyte replacement. As of 1999, at least commercial sports drinks were available for purchase in the United States. Below et al. demonstrated that the carbohydrate content of a beverage confers a performance advantage for endurance exercise that is separate from and additive to its hydrating effect. Optimal sports drinks will promote consumption of fluid and nutrients through palatability and will provide appropriate ingredients to meet the athletes’ needs (e.g., hydration, energy, electrolytes, etc.) for a particular event with no ingredients included that can limit intake or performance or unnecessarily add to the cost of the beverage. The formulation of the beverage will impact each of these factors.
To maximize hydration, a fluid should be palatable, which will promote its consumption. Beverages sweet in taste, flavored, and providing sodium may most enhance consumption. The fluid should also maximize water absorption by limiting the osmolality (a measure of the concentration of solute particles in a solution). Fluids too high in osmolality can produce a lower rate of water absorption; therefore, fluids should be isotonic or hypotonic, depending on the other characteristics required of the sports drink. Some evidence suggests that including sodium in a sports drink may enhance intake by providing a physiological thirst response by increasing vascular sodium concentration.58 One factor that may limit hydration by decreasing intake is excess carbonation, which may produce a sense of fullness; however, light carbonation does not appear to decrease fluid intake and can contribute to palatability for some individuals.59 During training or events in which an important function of the sports drink is to provide energy, which usually includes intense activities lasting about 1 hour or longer, the formulation should provide an optimal amount of readily available energy with the least risk of malabsorption. Carbohydrate is likely the optimal macronutrient to provide the bulk of energy in a sports drink. Beverages with lower concentrations of carbohydrate are typically best absorbed.
When the carbohydrate concentration surpasses 6–7%, water absorption can be limited and gastrointestinal distress can occur. Some research has suggested that carbohydrate in the form of glucose polymers may provide a benefit for fluid absorption due to providing a lower osmolality of the solution.60 Other studies indicate that glucose, sucrose, glucose polymers (maltodextrins), or combinations of these carbohydrates with or without fructose provide relatively equal performance-related benefits. When fructose is fed as the sole carbohydrate source, it may promote gastrointestinal distress since it is absorbed by a saturable facilitated diffusion process; therefore, fructose is recommended only when in combination with other carbohydrates. In fact, recent research has suggested that the addition of fructose to a beverage containing glucose in a ratio of 2:1 (glucose:fructose) enhances exogenous carbohydrate oxidation and endurance performance relative to glucose only. Furthermore, combinations of various carbohydrate sources appear to promote absorption of fluids and enhance the rate of carbohydrate availability and utilization.61 Electrolytes (sodium, potassium, and chloride in particular) can be important ingredients in sports drinks. Popular commercial sports drinks typically provide 55–110 mg of sodium and 30–55 mg of potassium in an 8-ounce (240 mL) serving. Although the concentrations of these electrolytes in the sweat of some athletes may exceed the concentration in these beverages, reports of hyponatremia or hypokalemia in athletes using commercial sports drinks to meet 100% of sweat losses are rare. The loss of electrolytes does not typically pose a problem during competition or training unless the exercise is of a prolonged nature or is completed in conditions of high ambient temperatures or humidity; however, as stated earlier, sodium may also be beneficial within a sports drink by virtue of its tendency to promote thirst and fluid intake.
Excess sodium, on the other hand, appears to limit fluid intake either by decreasing fluid palatability or promoting increased vascular volume. Some commercial sports drinks provide ingredients aside from carbohydrate, water, and electrolytes that may or may not impact performance. Some additional ingredients commonly included are vitamins, amino acids, glycerol, caffeine, herbals, and more. Sports bars are often convenient sources of nutrients required by athletes before, during, and after training or competition. Many types of sports bars that provide varying amounts of macronutrients and micronutrients are commercially available. Whether sports bars can impart ergogenic benefits depends upon many factors including their timing of consumption as well as their nutrient content. Few studies are available to determine if sports bars are truly effective in enhancing performance. One study actually indicated that a commercial bar containing 19 grams of carbohydrate, 14 grams of protein, and 7 grams of fat impaired performance when compared with a feeding providing an equal amount of energy from a glucose polymer. Because most sports bars provide a combination of carbohydrate and protein and, typically, fat as well, their use may best be geared toward recovery. Sports gels are carbohydrate-rich semisolids used to replenish glucose utilized during a variety of exercise activities. Some sports gels contain vitamins, amino acids, protein, glycerol, caffeine, herbals, or other constituents in addition to the carbohydrate. Commercial sports gels vary in their total carbohydrate contents and the types of carbohydrates used. The carbohydrate source is typically a form of maltodextrin, but other sources of carbohydrate are used as well.
These differences can elicit varying physiological responses, as many of the carbohydrates used vary in glycemic index. As discussed, recent research has evaluated the potential efficacy of raisins and honey versus commercial sports gels and found no major differences in effectiveness. Sports gels likely provide no advantage over many whole foods or commercial sports foods. In a field study of marathon runners, Burke et al. demonstrated that carbohydrate provided in the form of a sports gel may even limit endurance performance of some individuals by promoting gastrointestinal discomfort, although no difference in overall performance was detected between a trial in which gel was fed at the rate of 1.1 g of carbohydrate per kilogram body weight versus one in which a flavored placebo beverage was consumed. A recent study indicated that performance was similar regardless of whether carbohydrate was provided as a gel, a sports drink, or as sports jelly beans, and that performance was greater for each carbohydrate trial than a water-only trial. Other researchers evaluated the effects of a gel containing both carbohydrate and protein on performance and postexercise muscle damage. That study suggested that the addition of protein to a gel improved endurance and decreased markers of muscle damage when consumed during and immediately after exercise. Food form or carbohydrate source in general may influence the capacity for carbohydrate to alter performance based on the foods’ glycemic index (GI). Glycemic index is a measure of the effect that a food has on glycemic response.
It is typically determined by comparing the 2-hour blood glucose response of 50 g of available carbohydrate from a test food with 50 g of carbohydrate from a standard food (preferably glucose but sometimes white bread). Glycemic index has been demonstrated to have potential implications in optimizing performance of endurance under specific circumstances. Burke et al. summarized the theories regarding the use of GI for exercise suggesting that lower GI foods may be of greatest value when consumed prior to exercise and that higher GI foods may work best during exercise and for resynthesis of glycogen during recovery. Relatively few studies have actually assessed the role of GI of foods consumed during exercise on physical performance. The basis that higher glycemic index foods are the best carbohydrate sources during exercise is currently founded on theory rather than empirical data. Studies of a variety of types of carbohydrate sources, typically moderate or high in GI, have demonstrated that carbohydrate consumption during exercise can improve endurance performance. In one study, feeding of liquid versus solid meals resulted in differing effects of blood glucose and insulin at rest; however, during exercise, these differences were not detected and the feedings affected exercise performance in a similar fashion.
With that in mind, since peak oxidation of carbohydrate typically occurs approximately 1 hour after feeding, foods promoting faster appearance of glucose in the blood are potentially of greatest benefit. However, many factors, including individual variation in GI response and preference of food choice, should not be discounted in the absence of strong evidence against feeding more moderate glycemic foods during exercise. Unpublished research from my laboratory comparing a high GI sports gel with raisins, which have a more moderate glycemic index, suggested no difference in cycling performance when the foods were fed during exercise. Until more well-controlled comparisons of feedings of foods of various glycemic index foods are available, recommendations regarding carbohydrate ingestion during exercise based on glycemic index are predominately unfounded. The influence of glycemic index performance when foods are provided during pre-exercise feedings has been an area of much more comprehensive investigation. Although all research is not in agreement, likely due to differences in performance testing protocols or timing, quantity, and type of feeding, some research suggests that consumption of a lower GI food prior to exercise is more effective in enhancing performance than consumption of higher GI foods. The explanation for the improvement in performance has been suggested to be a reduction in pre-exercise hyperglycemia resulting in less hyperinsulinemia, as well as decreases in pre-exercise blood lactate concentration and maintenance of higher blood glucose and free fatty acid concentrations after commencing exercise.
However, a number of studies have yielded no performance enhancement when comparing feedings of lower versus higher GI foods. It is noteworthy to mention that research by Burke et al. indicates that any potential effect of pre-exercise feedings of foods of various glycemic indexes is abolished when carbohydrate is also fed during exercise. From a practical perspective, it is important to consider that most athletes participating in prolonged events consume energy both before and during exercise. Overall, glycemic index may ultimately prove to be a useful tool for pre-event feedings used to enhance endurance performance. Its usefulness as a tool to determine which carbohydrate sources should be fed during exercise appears to be more questionable.
