Running burns energy... a lot of energy. Most people burn somewhere around 100-130 calories per mile traveled. Over the course of an ultra, that adds up to at least tens of thousands of calories. As I prepare for an ultra, I like to get an idea of how much food I will need to consume. I like to simplify this process as much as possible.
Before starting the process, let's look at a few principles. Our body can use two primary fuel sources- fat and carbohydrates. Carbs burn quickly and efficiently. Fat burns slowly. If you're running fast, our body will burn carbs. The slower we run, the more fat our body uses as fuel. The “crashing” feeling we get is the result of making the switch from carb-burning to fat-burning.
Our bodies have a limited supply of carbs available at any given time, but we have a HUGE supply of fat. If we want to burn carbs, we need to constantly replace them by consuming calories during the run. This will allow us to run faster. We could get by without eating anything and rely on our fat stores, but we would have to keep intensity to a minimum. It's like the difference between throwing a piece of newspaper on a fire versus a giant oak log- the paper burns quickly with a huge flame while the log burns longer with a much less intense flame.
Through training, I know I can run about 18 miles before my carbohydrate supply is exhausted. At that point, my body switches to fat-burning mode and I slow down considerably and experience a crash. I can calculate a ballpark estimate of the carbs I have to consume during the run to avoid that crash by subtracting 18 from the total miles of the race, then multiplying that number by 100.
50 miles - 18 miles = 32
32 * 100 = 3200 calories needed during the run to avoid the crash
In a 50 miler, I know I have to consume approximately 3200 calories. This is where it gets a little tricky. Most people can only digest about 200-300 calories per hour. Let's assume we can process 250 calories per hour. If you're running at a 12 minute/mile pace (for a 10 hour finish), you could consume 2500 calories during that 10 hour race. Since you need 3200, you won't be able to consume enough to avoid a crash.
The problem becomes more pronounced with longer races. How about a 100 miler?
10 miles – 18miles = 82
82 * 100 = 8,200 calories needed during the run to avoid the crash
This is what I do to remedy the situation:
1. Train to eat. I've managed to get to the point where I can comfortably eat up to 500 calories per hour when running, which allows me to keep fueling throughout most races.
2. Train to burn fat. This is the idea behind the Maffetone method discussed earlier. This is also the reason I occasionally do long runs after fasting for 24 hours.
3. Start consuming calories from the beginning of a race. The longer you wait, the less opportunity you have to stay ahead of the carb game.
4. Find foods that are palatable even after running long distances. I have at least four “backup foods' in case the aid station foods aren't cutting it.
5. Know what the 'crash' feels like. When it starts to hit, slow down and consume something sweet.