The best endurance athletes utilize higher percentages of fat for energy at higher sustainable workloads. Each person has roughly 1,500-2,000 calories of carbohydrates (glucose) stored within the body (muscles and liver) to utilize during a single athletic event. This is known as your “Glycogen Stores.” The higher the sustained exercise intensity the more glucose the body utilizes to create energy for motion. When somebody does what’s called “hitting the wall” during, say, a marathon race (usually between miles 22-24), they have used up the glycogen stores within the body and now have to shift metabolism to break down proteins (muscle) to create glucose. The brain only runs on glucose so for self preservation the body starts breaking down the muscle to feed the brain and other organs. This is the point where runners should be pulled from the race for their own good.
At lower intensities (usually around 60% of VO2max), to save glycogen stores, the body uses a very high ratio of fat to glucose (approx. 65%/35%) to create energy. As the intensity increases, the body shifts metabolism to a more rapid use of glucose for fuel and less fat (at 80% of VO2max, approx. 20%fat/80%glucose) until the intensity increases to 100% glucose and the body starts to recruit fast twitch muscles which create and accumulate lactic acid. This begins to shut down cellular metabolism and leaves you gasping for air on the side of the road (a blog post for another day).
So, to sum up the formula for being a better endurance athlete, “The best endurance athletes burn fat as the majority of their fuel intake at higher and higher sustainable exercise intensities so they can conserve their precious glycogen stores throughout the race. This is a trainable adaptation of the human body for all endurance athletes and can be improved upon. True fuel utilization assessments can only be performed in a Metabolic Lab with medical-grade metabolic equipment that detects precise O2 uptake and expired CO2 ratios. This test will soon be available from Power to the Pedals. Stay tuned!