Carbohydrate As Ergogenic Aid

Since carbohydrate is typically the limiting energy substrate in exercise (i.e., it will run out before fat or protein runs out), it is critical to begin a bout of physical activity with enough stored carbohydrate to see the athlete through the session; doing so will aid exercise endurance, regardless of the exercise modality. In high-intensity exercise, carbohydrate is the primary fuel used by the musces. In low-intensity exercise of long duration, fat may be the primary fuel, but fat requires carbohydrate for complete oxidation.⁹ In addition, the storage capacity for fat is far greater than that of carbohydrate, even in the leanest athletes. In either form of exercise, carbohydrate depletion results in a dramatically reduced exercise performance.^{10, 11} It is the intent of carbohydrate-loading techniques, therefore, to store the maximal amount of carbohydrate the tissues can hold.

Not all sports and activities are suitable for carbohydrate loading. Keep in mind that for every gram of stored glycogen, the body stores approximately 3 grams of water. Tissues that are packed full with glycogen and water are likely to cause some degree of muscle stiffness. In sports such as gymnastics and diving where flexibility is important, carbohydrate loading may cause difficulties. It also appears that carbohydrate loading may be less beneficial for women than for men. In a study comparing higher carbohydrate intakes in men and women, men showed both a glycogen and performance improvement, whereas women, because of a higher lipid and lowerprotein and carbohydrate oxidation rate than men, did not experience the same level of improvement.¹²

In sports or events where jump height is a factor, as in the high jump, athletes may benefit more from a 250-calorie carboyhydrate supplement than they will from a creatine supplement.

The type of carbohydrate does appear to make a difference. Glucose polymer products (including commercially available sports gels and polycose) and maltodextrins (which are found in numerous sports beverages) are easily digested into glucose and appear to be more effective for glycogen production than are other carbohydrates. However, starches from pasta, bread, rice, and other cereals are also effective at maximizing glycogen storage.^13,14

Different forms of carbohydrates have different rates of digestion and provide varying rates of glucose release into the blood. In a study assessing the rise of blood glucose after a high-carbohydrate meal 2.5 hours before a 90-minute bicycle ride, subjects were given a candy bar with either a high glycemic (HG) index or a low glycemic (LG) index; a no-food control was also used. Both blood glucose and insulin were higher in the LG group than in either of the other two conditions. Blood free fatty acids (FFAs) were highest in the control group, while the LG trial showed higher FFAs than did the HG trial during the ride. This study suggests that a low glycemic index food will provide more sustained energy (and thus improved endurance) during prolonged exercise.¹⁵

Ingesting a glucose polymer solution before exercise results in a smaller reduction in power than does a noncaloric placebo during 1 hour of maximal exercise. When the same amount of glucose polymer is taken in 15-minute intervals during exercise, there is no observed benefit. This suggests that consumption of a glucose polymeris performance enhancing if consumed before exercise but is without any ergogenic benefit if taken during exercise.¹⁶

How quickly an athlete can recover from exhaustive exercise by reestablishing muscle glycogen is also an important performance factor, particularly in sports where athletes compete on sequential days. Glycogen depletion may occur in 2 to 3 hours of high-intensity exercise (60 to 80 percent of VO₂max) and even faster in maximum-intensity activity. Besides reducing performance, low muscle glycogen may predispose athletes to higher injury risk. In activities such as soccer and hockey, where competition frequency and exercise intensity make glycogen depletion more likely, consumption of carbohydrate during a competition or practice is a logical strategy to avoid glycogen depl tion. Many athletes in these and related sports continue to miss an important ergogenic opportunity by consuming only water during an event.¹⁷

After exercise, it seems logical to consume a carbohydrate food (1 gram per kilogram of body weight is recommended) to reduce protein breakdown and aid in protein synthesis. Failure to consume carbohydrate after exercise results in a higher than necessary level of muscle breakdown, thereby reducing the benefit that might be derived from resistance training.¹⁸