For endurance athletes, both fats and carbohydrates are a critical energy source. During high-intensity exercise, which can last up to 90-minutes in a well-trained athlete, carbohydrate metabolism is the body's main source of energy. At this beginning phase of the exercise, our body utilizes the glycogen which it has stored in muscles. Glycogen is a form of carbohydrate that our body transforms the carbohydrates we eat into to store for later. As glycogen is depleted, the intensity of exercise decreases,
but our bodies are not ready to quit yet. At this point, our bodies start utilizing fat stores, which do not give us the intensity we get from carbohydrates but will keep us going for endurance.
There has been a lot of research in the sports nutrition world to fine-tune this transition from carbohydrate metabolism to fat metabolism for energy. Some studies have investigated the benefits of maximizing carbohydrate stores, such as “carb loading” to prolong high intensity. Other studies look at using a ketogenic diet*, so athletes utilize fat as the main energy source at the beginning to prevent athletes from hitting a “wall” as their bodies transition energy sources. So far, research on the ketogenic diet has shown an ability to avoid that tough “wall” that every endurance athlete has experienced during a race, but most studies do not show overall performance benefits. The best way to improve performance through nutrition has been shown to be through timing and the number of carbohydrates.
The amount of carbohydrates you need depends on the length of time you are exercising. Recommendations range from five-grams of carbohydrates per kilogram body weight per day (g/kg/d) for one-hour of exercise to 12g/kg/d for greater than four hours of exercise. Carbohydrate intake in days leading up to performance is important to maximize glycogen stores. The meal prior to performance can help maintain blood glucose levels and improve performance. Research suggests that this pre-performance meal should be two to four hours prior to the performance.
It is also important to be aware that even when our bodies start using fat as the main energy source it still needs a small number of carbohydrates for the metabolic process of turning fat into energy. For this need, you should aim for an intake of 60-90g/kg of carbohydrates per hour during exercise; with needs closer to 90g when exercise is greater than three hours.
*A ketogenic diet is very low in carbohydrates and forces the body to utilize fat as its main energy source, and you should always speak to a health professional prior to switching to such a diet.
Karpinski, C. Rosenbloom, C. Sports Nutrition A handbook for professionals. 6th ed. Academy of Nutrition and Dietetics. 2017: 230.