Fluid Intake During Exercise

Athletes consuming fluids during exercise derive clear benefits, including better maintenance of exercise performance and a slowing of the exercise-induced rise in heart rate and body temperature. In addition, blood flow to the skin is improved or maintained. The degree to which the cardiovascular and heat-maintenance capacity is maintained is directly related to the degree to which dehydration can be avoided. It is clear that a failure to consume sufficient fluids during exercise is a major risk factor in the onset of heat exhaustion.⁵⁸ The best strategy for athletes to follow to avoid heat exhaustion and maintain athletic performance is to drink fluids during exercise. ^59-61

Most studies evaluating the interaction between hydration adequacy and athletic performance have used either plain water or sports beverages that contain, in differing degrees, carbohydrate and electrolytes. The results of these studies are similar in confirming the importance of fluid consumption during exercise. However, the inclusion of carbohydrate and electrolytes in the fluids affords an athlete certain advantages over plain water. Recent studies suggest that inclusion of carbohydrate in the rehydration solution improves the athlete's ability to maintain or increase work output during exercise and increases the time to exhaustion.^62-65

Consumed carbohydrate helps athletes avoid depletion of muscle glycogen and provides fuel for muscles when muscle glycogen is low. Carbohydrate also helps maintain mental function, which is critical for maintaining endurance performance. Mental fatigue leads to muscle fatigue, even if muscles have plenty of glycogen and fluids.

Different activities result in different rates of carbohydrate utilization, but consuming carbohydrate-containing fluid consistently helps maintain athletic performance, regardless of the sport. For instance, in strenuous cycling the rate of muscle glycogen use is not affected when a carbohydrate solution is ingested;⁶⁶ in long-distance running, the rate of muscle glycogen usage is reduced when a carbohydrate-containing fluid is consumed;⁶⁷ and in stop- go intermittent exercise, a reduction in muscle glycogen usage is seen with the consumption of a carbohydrate-containing fluid.^68,69 In each of these scenarios, carbohydrate depletion is generally considered to be the cause of performance degradation. However, there is good evidence that consuming a carbohydrate-containing beverage may also be important for improving athletic performance in high-intensity activities where carbohydrate is not expected to be depleted because of the relatively short duration of the activity.^70-72

These data all suggest that athletes should adjust to consuming a carbohydrate-containing fluid during exercise. However, the concentration of carbohydrate and the type of carbohydrate are important considerations. Although there are no major differences between the effects of glucose, sucrose, maltodextrins, and starch on exercise performance,^73-75 beverages containing mainly fructose may cause intestinal distress.^76,77 Maltodextrins are less sweet than sucrose and fructose, so they may be used to add carbohydrate energy to solutions without making them unpalatably sweet tasting.⁷⁸ With these exceptions aside, carbohydrate energy, regardless of whether in liquid or solid form and almost regardless of the type of carbohydrate, will aid athletic performance.⁷⁹ However, since providing carbohydrate in liquid form enables athletes to address two issues at once (energy and fluid), carbohydrate liquids are preferred.

The volume of carbohydrate provided during exercise is an important consideration as well; providing too much too fast may induce gastrointestinal distress and, at least temporarily, draw needed fluids away from muscle and skin to dilute this excessively concentrated solution in the gut. By contrast, providing a fluid that contains just a scant amount of carbohydrate may induce no performance benefit. Athletes should try to consume approximately 1 gram of carbohydrate per minute of exercise. This intake level can be achieved by drinking solutions that contain between 6 and 8 percent carbohydrate⁸⁰ at a volume of .6 to 1.2 liters per hour.^{81, 82} Some sports beverages have carbohydrate levels precisely within this range, while others have concentrations outside this range. Concentrations above 8 percent may cause a delay in gastric emptying and do not necessarily lead to a faster or better carbohydrate metabolism during exercise.⁸³ Another real advantage of consuming a 6 to 8 percent carbohydrate solution is that it has a faster rate of intestinal absorption than does water alone. This means that fluid status can be more efficiently maintained, and the delivery of carbohydrate to the blood and muscles is enhanced. Doctors commonly prescribe Pedialyte (sugar water) for babies with diarrhea because it induces faster water absorption than does water alone and can more quickly rehydrate the baby.