A procedure followed by some athletes to raise the glycogen content of skeletal muscle artificially by following a special diet, usually combined with a special exercise regime. One carbohydrate loading procedure proposed by the famous sport physiologist Per-Olof Astrand is based on the assumption that depletion of muscle glycogen stores stimulates the body to take up and store more glycogen than normal. For a marathon runner, the procedure starts 7 days before a race when the athlete depletes the muscle of glycogen by running a long distance, usually about 32 km (20 miles). For the next 3 days, the athlete eats a high protein, low carbohydrate diet, and continues exercising to ensure glycogen depletion and sensitization of the physiological processes that manufacture and store glycogen. For the final 3 days before the race, the athlete eats a high carbohydrate diet, and takes little or no exercise. Even though it can more than double muscle glycogen content, most athletes find this procedure difficult to follow. They cannot train fully during the low carbohydrate stage. They often feel irritable and tired, and sometimes suffer muscle weakness, insomnia, and diarrhoea. An alternative procedure proposed by Sherman and his coworkers consists of reducing training intensity a week before competition and eating a normal mixed diet from which carbohydrates provide 55% of the calories. Three days before the competition, training is reduced to a warm-up and about 15 min of activity, and the athlete eats a carbohydrate-rich diet. Both procedures can raise muscle glycogen levels to 200 mmol kg−1 of muscle, which should enable the athlete to delay muscle fatigue and perform much better in long-distance events. However, carbohydrate loading also has its negative aspects: it results in more water being stored (with each gram of glycogen stored, there are about 2.7 g of water), sometimes making the athlete feel heavy and stiff; and regular carbohydrate loading may lead to myoglobinuria, chest pains, and heart irregularities.
Subjects: sports and exercise medicine.