Recommendations for carbohydrate intake during exercise

Building muscle glycogen stores and maintaining them during training sessions requires a carbohydrate-rich diet. If adequate carbohydrates are not consumed daily between training periods, pre-workout muscle glycogen levels gradually decline and exercise performance during training or competition is impaired. Daily replenishment of the body's carbohydrate reserves should be a top priority for hard-training athletes.

Costill et al. assessed glycogen synthesis following a 45% carbohydrate diet during three consecutive days of 16.1 km running at 80% V02max. Muscle glycogen levels were 110 mmol kg 2 at baseline and decreased to 88 mmol kg 2 on day 2 and 66 mmol kg 2 on day 3. Another study found that a diet providing 525–648 g carbohydrate resulted in glycogen synthesis of 70–80 mmol kg 2 and provided near-maximal muscle glycogen replenishment within 24 h.

Fallowfield and Williams also assessed the role of carbohydrate intake in recovery from prolonged exercise. Their subjects ran at 70% V02max for 90 min or until fatigue. For the next 22.5 h, the runners consumed an isocaloric diet containing either 5.8 or 8.8 g carbohydrate kg. After rest, they ran at the same intensity to determine endurance, with those receiving the 8.8 g carbohydrate kg running for the same time as in the first run. Although the two diets were isocaloric, the runners receiving the 5.8 g carbohydrate kg ran for 15 min less.

For many athletes, energy and carbohydrate needs are greater during training sessions than during competition. Some athletes are unable to (unintentionally) increase caloric intake to meet energy demands during intense training sessions. Costill et al. studied the effects of 10 days of increased volume and intensity training on muscle glycogen and swimming performance. Six swimmers self-selected a diet containing 4700 kcal/day and 8.2 g carbohydrate kg/day, and four swimmers self-selected a diet containing only 3700 kcal and 5.3 g carbohydrate kg/day. These four swimmers were unable to cope with the increased demands of the training sessions and swam significantly slower, presumably as a result of a 20% decrease in muscle glycogen levels.

The feeling of sluggishness associated with muscle glycogen depletion is often referred to as fatigue, which is caused by overtraining. Athletes who train hard for several days in a row should consume sufficient carbohydrates to reduce the risk of fatigue due to the cumulative depletion of muscle glycogen.

Training-related glycogen depletion can occur during activities that require repeated near-maximal explosive efforts (soccer, basketball) and endurance training. A sign of glycogen depletion is the inability of the athlete to maintain normal exercise intensity. Glycogen depletion can be accompanied by a sudden loss of several pounds of body weight (caused by the loss of glycogen and water).

A review of the literature by Sherman and Wimer challenges the assumption that a high-carbohydrate diet optimizes training adaptations and athletic performance. They suggest that the relationship between muscle glycogen depletion and fatigue is strongest during moderate exercise (65-88% V02max). However, they also note the established fact that low blood glucose and muscle and/or liver glycogen concentrations can cause fatigue during other types of exercise. Because dietary carbohydrate is involved in maintaining carbohydrate stores in the body, Sherman and Wimer recommend that athletes continue to consume high-carbohydrate foods and monitor for signs of fatigue during exercise and take note of athletes whose eating habits make them more susceptible to glycogen depletion.

Athletes training vigorously should consume 7-10 g/kg of carbohydrate per day. The typical American diet recommends 4-5 g/kg of carbohydrate per day. Consuming 6-7 g/kg of carbohydrate per day is sufficient for an athlete training vigorously (about 70% of V02max) for about one hour per day. Consuming 8-10 g/kg of carbohydrate per day is recommended for athletes training vigorously for several hours per day.

Some athletes should reduce fat intake to 30% of total calories to get 8-10 g/kg of carbohydrates per day. Sugar may be increased to meet the increased carbohydrate needs, but most of the carbohydrates should be complex carbohydrates. These are more nutrient dense and, compared to sugary foods, contain more B vitamins needed for energy metabolism, as well as more fiber and iron. Many foods high in sugar are also high in fat.

In addition to carbohydrates, athletes must consume sufficient calories. Consuming a diet that reduces energy production will impair endurance performance by depleting muscle and liver glycogen. Adequate carbohydrate intake is also important for athletes who have lost body weight due to negative energy balance due to intense physical activity (e.g., wrestling, gymnastics, dance).

Those wishing to reduce body weight and consume low-energy foods are prevalent among athletes exposed to high loads. Negative energy balance can reduce their performance due to deterioration of acid-base balance, decreased levels of glycolytic enzymes, selective atrophy of type II muscle fibers, and abnormal function of the sarcoplasmic reticulum. Adequate intake of dietary carbohydrates can reduce some of the damaging effects resulting from limited energy supply to muscles.

Athletes participating in ultra-endurance events (lasting greater than 4 h) have very high carbohydrate requirements. Saris et al. studied food intake and energy expenditure during the Tour de France cycling race. In this 22-day, 2400-mile race, cyclists consumed an average of 850 g carbohydrate per day, or 12.3 g-kg per day. About 30% of the total energy intake was provided by high-carbon drinks. Brounc et al. assessed the effects of a simulated Tour de France study on food and fluid intake, energy balance, and substrate oxidation. Although cyclists consumed 630 g carbohydrate (8.6 g-kg per day), 850 g carbohydrate per day (11.6 g-kg per day) was oxidized. Despite ad libitum intake of normal foods, the cyclists did not consume enough carbohydrate and calories to offset their increased energy expenditure. When 20% carbohydrate drinks were added to the diet, carbohydrate intake increased to 16 g-kg/day and oxidized carbohydrate intake increased to 13 g-kg/day.

Ultra-endurance athletes who require an additional 600 g of carbohydrate per day to meet their carbohydrate and energy needs should supplement their diet with high-carbohydrate drinks if their regular food intake is insufficient. Saris and Brauns recommend that such athletes consume 12-13 g of carbohydrate per kg of body weight per day during training and competition. They also believe that this amount provides the maximum contribution of carbohydrate to energy supply during extreme endurance activities.

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