There are two types of triathletes. Those who admit they’ve bonked in a race, and dirty dirty liars. I used to think that “hitting the wall” was a mysterious process, ungoverned by the laws of nature, which could strike an athlete at any time like an unlucky lightning bolt from an angry Zeus.
Now I know the truth – your body hits the wall whenever it runs out of stored glycogen and has to switch to using stored fat for fuel. One way to avoid this phenomenon is to take in adequate carbohydrates during exercise to replace the ones you’re burning up. Unfortunately for long-distance triathletes, it can be nearly impossible to completely replace the fuel burned by your body over a long race such as a half-Ironman or Ironman. This is where metabolic efficiency training comes into play.
Metabolic efficiency training is also known as fat adaptation, and is the process of improving your body’s ability to use stored fats for fuel. The average person can only hold 1,400-2,000 calories worth of stored carbohydrates in their body at once much less than is used up during a multi-hour workout. However, each pound of stored fat holds approximately 3,500 calories, meaning that even the leanest triathlete with a single-digit body-fat percentage holds tens of thousands of calories worth of energy in adipose tissue (body fat).
Great news, right? Not so fast my friend. Retrieving and making use of this stored fat is much more difficult than burning the carbohydrates stored in your liver and muscle tissue.
Dr. Benjamin Rapoport wrote a fascinating (if you’re kinda nerdy) paper called “Metabolic Factors Limiting Performance in Marathon Runners” in which he breaks down the biochemistry behind hitting the wall.
One of his more arresting findings is detailed in the below image:
This graph clearly shows that higher intensity exercise burns a much higher percentage of carbs for energy as opposed to stored fats. He also finds that carbohydrate oxidation produces approximately 20% more energy per unit of respired oxygen than fatty acid oxidation. In layman’s terms, this means that burning carbs is about 20% more oxygen-efficient than burning fats. As oxygen intake and usage is a obviously a limiting factor in racing triathlons, this seems to point to preferring the usage of stored glycogen over stored fats for energy.
The good news is that you can train your body to adapt and become more efficient at burning fat, and this is the crux of Metabolic Efficiency Training.
The experts can explain it much better than I, and I’d highly recommend the following books for further information: