Final answer:
An athlete in heavy endurance training should focus on a daily carbohydrate intake of approximately 50-70% of their total energy intake to maintain optimal performance levels, focusing on complex carbohydrates for sustained energy.
Step-by-step explanation:
An athlete in heavy endurance training should maintain a daily carbohydrate intake that typically ranges from 50-70% of their total energy intake. This high percentage is crucial as carbohydrates are a pivotal fuel source for the brain and muscles, particularly during prolonged physical activity. It is vital to modify carbohydrate intake based on the intensity of exercise, with an increased amount necessary for those engaging in high-intensity or long-duration endurance events. Beyond simply consuming carbohydrates, it's recommended that athletes focus on consuming complex carbohydrates, as these are metabolized more slowly, providing sustained energy which is advantageous during endurance activities. For overall daily caloric intake, carbohydrates should constitute the largest percentage compared to fats and proteins, which should account for 20-35% and 10-15% respectively, according to general nutritional guidelines. Given the additional energy requirements of athletes, especially those involved in endurance sports, maintaining a high carbohydrate intake can support rigorous training demands and enhance performance.