Endurance competitors (i.e., distance runners, soccer players, rowers, etc.), perhaps more than other athletes, must not overlook the distinct advantage provided by adequate carbohydrate intake. As defined by the authors, endurance activities are those activities that include prolonged, continuous, low- to moderate-intensity exercise that is principally aerobic in nature. Endurance athletes expend an inordinate amount of energy in training and competing, and, because carbohydrates are the preferred fuel source in most sports, they therefore must be the first macronutrient accounted for in any dietary regimen.
Many studies have demonstrated the undeniable relationship between high-carbohydrate diets and ample pre exercise glycogen stores, improved endurance, and optimal performance.A review of the scientific literature at the International Consensus Conference in 1991 examined the connection between food, nutrition, and sports performance, and consequently devised carbohydrate recommendations for athletes. The most significant contributor to endurance capacity was pre-exercise glycogen concentration.
Therefore, a recommendation was made that athletes should allow carbohydrates to comprise approximately 60-70% of their daily energy intake. However, it was also advised that this high level of carbohydrate intake should not be part of the athlete’s normal diet-but simply preparation for heavy exercise. For example, only athletes who train exhaustively on successive days or compete in prolonged endurance events should follow a dietary program that consists of 70% carbohydrates.
Regarding an endurance athlete’s routine diet, a common recommendation is to ingest approximately 60% of daily calories from carbohydrates. Although this guideline may be accurate, it neglects the recommended mandat for the total amount of carbohydrates required for ideal performance, replenishing glycogen stores, and assisting in recovery. Specifically, carbohydrate needs of all athletes should be expressed relative to body weight.
The importance of this is illustrated by a study by Walberg-Rankin. In their research, the authors computed the daily carbohydrate intake and dietary macronutrient ratios of a number of athletes from both aerobic and anaerobic sports. Surprisingly, they found that even in those athletes who consumed 60% of their total calories from carbohydrates, most still fell short of their projected total when carbohydrate requirements were formed on the basis of body weight.
Furthermore, as gleaned from dietary surveys, it is apparent that endurance athletes are not meeting their daily carbohydrate requirements. In many cases, daily energy and carbohydrate requirements to support strenuous training may even surpass those required for competition. Consequently, most studies support the notion that endurance athletes should consume carbohydrates (CHO) on the order of -10 g/kg body weight/day to restore and maintain glycogen levels.
This total also represents an ideal quota for optimal sports performance, as depicted in a study by Sherman et al. The authors examined runners and cyclists who completed 70 minutes of intense interval training over 7 days. They found that the subjects who consumed 10 g CHO/kg/day maintained significantly higher glycogen levels than those who ingested 5 g/kg/day. The goal of carbohydrate intake is to maximize available energy throughout exercise, therefore, inadequate glycogen stores will likely reduce training intensity and eventually lead to premature fatigue.
For athletes involved in strenuous training, it appears that maximum benefits are derived from a carbohydrate intake of 500 g daily According to Costill et al,39 an intake of 524 to 648 g CHO/day is ideal for maximal muscle glycogen replenishment following strenuous exercise. In fact, intake beyond this range does not appear to provide additional benefit. 39 Additionally, even if this ideal amount makes up less than the recommended 60% of the athlete’s total daily calories, it still appears sufficient for glycogen restoration and optimal performance. Hawley et al. 40 demonstrated that athletes who consume 550 g CHO/day or more can adequately replenish glycogen stores, although this amount may only make up 45% of total daily calories.
They also point out that there is little research to support the use of chronically high carbohydrate diets to enhance training in athletes with high-energy intake. Furthermore, because elevating carbohydrate ingestion to 70% of total energy may result in insufficient protein and fat ingestion, this type of diet is strongly discouraged for those athletes who are restricting caloric intake (figure skaters, distance runners, gymnasts). The practices of athletes who habitually reduce fat or protein intake could potentially jeopardize their health and performance (i.e., significant low-fat intakes may promote deficiencies in fat-soluble vitamins).
Carbohydrate loading is a technique used to increase muscle glycogen stores beyond the maximum attained through the conventional dietary methods as previously outlined. Traditionally, carbohydrate loading is performed in two stages: a glycogen depletion stage and a carbohydrate loading phase. During glycogen depletion (which begins about 6 days before endurance competition), the athlete performs intense, exhaustive exercise over a period of to 3 days while eating a low-carbohydrate diet. Specifically, exercise should target the muscles to be used during competition, and should consist of approximately 90 minutes of intense submaximal exercise. The athlete will also maintain a daily carbohydrate intake of only 60 to 100 g during this period.
Essentially, the depletion phase is used to create a glycogen debt in the chosen muscles. Then, at least 3 days before competition, the carbohydrate loading phase begins, and creates what is termed glycogen supercompensation. This is a phenomenon in which glycogen storage is enhanced significantly, and is likely a result of the increased concentration of glycogen-storing enzymes created by the previous glycogen debt. In addition to abundant water, vitamins, minerals, and protein, carbohydrate intake during the loading phase increases to 50 600 g/day.
Carbohydrate loading has resulted in significant increases in endurance capacity (the time taken from the start of exercise to exhaustion), although large improvements in endurance performance (the time taken to complete a predetermined workload) have not been reported. In other words, carbohydrate loading may allow athletes to maintain a chosen pace for longer periods than if they had not carbohydrate loaded. These results have increased recommendations that carbohydrate loading is only beneficial for endurance activities lasting 1 hour or more.
Possible disadvantages of carbohydrate loading typically occur during the glycogen depletion stage, and occasionally produce such undesirable side effects as weight gain, irritability, and mental and physical fatigue. Of course, all of these adverse reactions may be detrimental to performance.
Because of these side effects, the authors recommend that athletes follow a modified procedure if undergoing carbohydrate loading for the first time. This may consist of a truncated depletion phase (1 day instead of 2 or 3), or may involve a more agreeable carbohydrate intake of 150 to 250 g/day during depletion.
Nevertheless, many athletes derive success from consuming a normal mixed diet and simply tapering their training in the days leading to competition. In fact, this procedure can generate higher than normal pre-exercise muscle glycogen levels, even without carbohydrate loading+ 2 Another modified approach involves a 50% carbohydrate diet and no exhaustive exercise during the depletion phase, and a loading phase in which carbohydrate intake is increased to 70% of total calories and training is tapered.
Perhaps surprisingly, this procedure results in an accumulation of glycogen reserves to approximately the same level as attained with the classic protocol. Regardless of the method used, however, athletes should become well informed about carbohydrate loading before manipulating their dietary and exercise habits to achieve glycogen super compensation.