During exercise bouts lasting 3+ hours, burning more calories than we can consume and absorb is likely if we’re fit. Burning 500 to 800 calories per hour during a hard 8-10 hour event is not difficult to do for those that have trained well. What is the fuel source for those calories? Carbohydrates, proteins, and fats from foods we ingest and substances that have previously been stored in the body.
The maximum number of calories we can assimilate per hour while we exercise falls short of the calories we actually burn. Let’s say someone can consume AND ABSORB 350 calories per hour while doing an endurance event. (BTW, I think this is overdoing it and taking the chance of ruining your race.) Where do we get the other 150 to 450 calories per hour? Fatty acids cover the caloric gap. Even the leanest athletes have a fatty acid reservoir that won’t run out in any length endurance event. This isn’t to say that we need only fatty acids to fuel our efforts, but they can easily bridge that caloric gap between the carbohydrates we can assimilate and what we burn while exercising. In order to tap into this vast reservoir of calories, we have to train the body to access them.
Pace while training and racing plays a very important role in our ability to utilize these fatty acids. We can quickly and inefficiently breakdown the limited supply of simple sugars/carbohydrates to release the energy the working muscles need to function. This is a short-term solution for providing energy to the working muscles, but endurance events aren’t short term affairs. We can’t store enough sugars in the blood, liver and muscle to fuel long events. And to say it again, we can’t ingest and absorb enough to replace what we burn. Pacing for endurance events allows an athlete to release the energy from fatty acids, proteins and carbohydrates and meet the energy needs. Too fast of a pace burns through the limited supply of carbohydrates. Also, the product of this metabolism is lactic acid. The acid changes the pH inside the muscle. This changing pH inhibits the enzymes needed to deliver energy to the working muscle. Going too fast is self- limiting. Carbohydrates are the predominant fuel source to support the faster paces. Lactic acid is the product of quickly burning the carbs, and the price that is paid is that the athlete becomes less able to continue to deliver the needed energy. They start to shut down.
To get better at delivering energy from metabolizing fatty acid, we can’t do all of our training at too fast a pace. If the long slow distance workout for the week is truly slow, we won’t depend as much on burning sugars to maintain the pace. The enzymes needed to break down the fatty acids won’t be as inhibited. The adaptation will be an increase in the amount of enzymes needed to deliver energy form the breakdown of fatty acids. The result, a faster long slow distance pace – one that can allow for bridging the caloric gap between what you can absorb and what you actually burn.
Happy fast racing!