Here’s a little bit from six-time Ironman world champion Mark Allen’s article “To Hawi and Back: Can you still win Kona on the Bike?” It appears in the September 2014 issue of Lava magazine.
We have enough fat in our bodies (yes, even ultra-lean triathletes) to run about 500 miles. However, we only have about 20 miles worth of carbohydrates stored in our liver and muscles. That’s about 2,000 calories. An Ironman takes at minimum about 6,000 calories to get you from start to finish.
Here’s the catch: Humans can only absorb about 300 calories per hour of carbohydrates. However, during an Ironman an athlete is burning between 700 and 800 calories per hour. What does that mean? Let me engage your math brain for a moment. If an athlete is going through 700 to 800 calories per hour to fuel their pace, but is only able to absorb 300, you can do the calculations. They are burning about 400 calories per hour more than they can take in. If the pace is relatively slow, or if the athlete has developed their fat burning engine to the point where they can go about 80 percent of their threshold pace without activating their anaerobic metabolism, they can in theory get about half their energy needs from stored fat and half from stored carbohydrates. In other words, they will be getting about 350 calories per hour from stored stored fat and about 350 per hour from carbs. . . .
In an Ironman, well-trained athletes can do the entire bike at roughly this point, which is equivalent to racing at 80 to 85 percent of your max heart rate if you’re well-trained aerobically or 70 to 75 percent of max otherwise. However, if the pace is fast, or if there are a lot of surges where your heart rate shoots up suddenly, then carbohydrates become the dominant fuel. This kicks in your adrenal system (your fight or flight reflex), causing your fat burning to be slowed way down for hours, even if you lower your heart rate again.
So that’s pacing and carbs. Now I just have to figure out insulin. Stay tuned.