Carbohydrate Recommendations for Training
Ellen Coleman, RD, MA, MPH © 2004

The relationship between muscle glycogen depletion and exhaustion is strongest at moderate training intensities (65% to 85% of VO2max). Low blood glucose, and muscle and/or liver glycogen concentrations can contribute to fatigue during other types of exercise. Building up and maintaining glycogen stores during training require an adequate intake of carbohydrate and energy. When adequate carbohydrate and energy are not consumed on a daily basis between training sessions, the pre-exercise muscle glycogen content gradually declines and training or competitive performance may be impaired. Daily restoration of the body's carbohydrate reserves should be a priority for athletes involved in intense training.

Costill et al evaluated glycogen synthesis on a 45% carbohydrate diet during 3 successive days of running 16.1 km at 80% of VO2max (1). Pre-exercise muscle glycogen levels started at 110 mmol/kg and fell to 88 mmol/kg on Day 2 and 66 mmol/kg on Day 3. Another study out of Costill's lab found that a diet providing 525 to 648 g of carbohydrate promoted glycogen synthesis of 70 to 80 mmol/kg and provided near maximal repletion of muscle glycogen within 24 hours (2).

Fallowfield and Williams (3) also evaluated the importance of a high-carbohydrate intake on recovery from prolonged exercise. Their subjects ran at 70% of VO2max for 90 minutes or until volitional fatigue, whichever came first. During the next 22.5 hours, the runners consumed isocaloric diets containing either 5.8 or 8.8 g of carbohydrate/kg. After the rest period the runners ran at the same intensity to assess endurance capacity. Those who consumed 8.8 g of carbohydrate/kg were able to match their running time from the first race. Even though the two diets were isocaloric, the running time of those who consumed only 5.8 g of carbohydrate/kg decreased by more than 15 minutes.

For many athletes the energy and carbohydrate needs of training are greater than the requirements of competition. Some athletes involuntarily fail to increase caloric intake to meet the energy demands of increased training. Costill et al (4) studied the effects of 10 days of increased training volume at a high intensity on muscle glycogen and swimming performance. Six swimmers self-selected a diet containing 4,700 calories per day and 8.2 g of carbohydrate/kg per day, while four swimmers self-selected a diet containing only 3,700 calories per day and 5.3 g of carbohydrate/kg per day. These four swimmers could not tolerate the heavier training demands and swam at significantly slower speeds, presumably due to a 20% decline in muscle glycogen.

Athletes who train exhaustively on successive days should consume adequate carbohydrate and energy to decrease the threat of fatigue caused by the cumulative depletion of muscle glycogen. This includes athletes in sports which require repeated, near-maximal bursts of effort (such as football, basketball, and soccer) as well as endurance sports.

Jacobs and Sherman (5) conducted a literature review on the effectiveness of carbohydrate supplementation and chronic high-carbohydrate diets for improving endurance performance. They conclude that overwhelming evidence indicates carbohydrate supplementation before and during exercise improves endurance performance. The use of short term dietary and training strategies to increase muscle glycogen stores (e.g. carbohydrate loading) also improve performance. Though chronic high carbohydrate diets maintain higher muscle glycogen concentration that moderate carbohydrate diets, the effect on performance is not clear. They note that research clearly demonstrates that a high carbohydrate diet is necessary for optimal training adaptations and greater improvements in endurance performance in previously untrained individuals. Until research shows otherwise, Jacobs and Sherman conclude that a high carbohydrate diet is still the best recommendation for endurance athletes.

Carbohydrate recommendations for athletes range from 6 to 10 g/kg per day (9,10). Burke and colleagues suggest an intake of 5 to 7 g/kg/day for general training needs and 7 to 10 g/kg/day for the increased needs of endurance athletes (6).

Athletes should consume sufficient calories in addition to carbohydrate. Consumption of a reduced-energy diet will impair endurance performance due to muscle and liver glycogen depletion (7). Adequate carbohydrate intake is also important for athletes in high-power activities (eg, wrestling, gymnastics, and dance) who have lost weight due to negative energy balances (7).

Desire for weight loss and consumption of low-energy diets are prevalent among athletes in high-power activities. Negative-energy balance can harm high-power performance due to impaired acid-base balance, reduced glycoloytic enzyme levels, selective atrophy of Type II muscle fibers, and abnormal sarcoplasmic reticulum function. Adequate dietary carbohydrate may ameliorate some of the damaging effects of energy restriction on the muscle (7).

Athletes participating in ultraendurance events (those lasting over 4 hours) have the highest carbohydrate requirements. Saris et al studied food intake and energy expenditure during the Tour de France (8). In this demanding 22-day, 2,400-mile race, the cyclists consumed an average of 850 g of carbohydrate per day or 12.3 g/kg per day. About 30% of the total energy consumed was provided by high-carbohydrate beverages. Brouns et al (9,10) evaluated the effect of a simulated Tour de France study on food and fluid intake, energy balance, and substrate oxidation. Although the cyclists consumed 630 g of carbohydrate (8.6 g/kg per day), they oxidized 850 g of carbohydrate per day (11.6 g/kg per day). In spite of ad libitum intake of conventional foods, the cyclists were unable to ingest sufficient carbohydrate and calories to compensate for their increased energy expenditure. When the diet was supplemented with a 20% carbohydrate beverage, carbohydrate intake increased to 16 g/kg per day and carbohydrate oxidation rose to 13 g/kg per day.

Ultraendurance athletes who require over 600 g of carbohydrate per day should consider supplementing their dietary intake with high-carbohydrate beverages if they cannot eat enough conventional foods to meet their carbohydrate and energy requirements (10).

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