F.U.E.L. Testing and Metabolism, part 1

April 18th, 2012 10:26am - Posted By: Adam St.Pierre, MS

For a few years I’ve been dabbling in ultra-running and run a handful or 50 and 100 mile races in that time. My biggest struggle has always been fueling for these long races. In my first 50, I ran 30 miles without eating, because I could not hold any calories down at the pace I wanted to maintain. Needless to say I ended up walking 15 miles in a hypoglycemic haze then stopped and ate at the last aid station. I felt miraculously wonderful after eating, and ran the last 5 miles well, probably the best I felt the entire race. In later ultras I experimented with different fuel sources: drinks, gels, solids, real food, more salt, less salt, etc. I learned how to calm an unsettled stomach with Tums, ginger, Coke, etc. but the reality of the situation was, I can  not consume enough calories while running the pace I want to run to sustain that pace. The only solution then, was to run slower, and I still dealt with some severe GI issues on my first 100 miler (first bathroom break at mile 13, hourly thereafter for the rest of the race!). Something needed to change if I want to complete more ultras, and compete better during them. I need to somehow change my metabolism to burn more fat at faster paces.

 Let me go back and explain a little about energy production. Your body has 3 energy systems: ATP/CP which is used for short sprints (10 sec or less), anaerobic glycolysis (the incomplete breakdown of carbohydrate into some ATP and lactate), and aerobic metabolism (fat or carbohydrate (or lactate) are used to produce energy in an efficient manner). Aerobic metabolism happens in the mitochondria located within your cells and is the primary method of energy production for endurance exercise. Your body is undergoing all 3 types of energy production throughout the day. As exercise intensity increases, the amount of energy required increases (it takes more energy per minute to run 8 min/mi than 10 min/mi). As exercise intensity increases, you need to produce energy faster to meet this increased demand. Aerobic metabolism is a long, multi-step, biochemical process and at higher workloads it often can’t produce energy fast enough so your body will utilize greater amounts of anaerobic glycolysis to continue. Eventually carbohydrate stores are depleted and you “bonk.”

Your body can only store 1200-2500 calories of carbohydrate as muscle or liver glycogen. The amount stored depends mostly on body size, bigger people can store more. Contrast this with the 80000+ calories of fat even the leanest of athletes has stored. To prevent bonking you need to preserve stored carbohydrate as long as possible. So you can try to consume calories from carbohydrates while exercising, or you can slow down and use less carbohydrates. At higher intensities it can be difficult to hold down calories making slowing down the more common option. Base training increases the size and number of mitochondria in your muscle cells, and also increases the activity level of the enzymes that allow for fat to be broken down from its stored form, mobilized into the blood stream, and used for energy by cells. For 2.5 years I did tons of base training hoping to improve my fat burning with little success. I still burned too many carbohydrates to compete in ultramarathons. Needless to say, I was frustrated.

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