Over the past couple of posts I've shared information on the two main energy sources for aerobic metabolism—glycogen (from carbs
) and fat. Both are important for endurance runners. Glycogen is the main source of energy, especially during the first 30 minutes of a run. After that, fat comes into play and becomes a major source of energy. These two energy sources work simultaneously. One source may be used more than another depending on how intense or how long you're running, but it's never an either-or thing. You actually have to have some glycogen metabolism going on in order for fat metabolism to easily occur. There's a saying "Fat burns in a carbohydrate flame."
Even though it's not an either-or fuel option, the type of fuel most used at any given time depends on intensity and duration of the event as well as the training of the runner. The more readily-available fuel (glycogen) will be used during more intense exercise and the energy from fat tends to kick in more as the intensity decreases. As the duration of an event continues, a higher and higher percentage of fat can be utilized as fuel. As much as 30-60% of marathon energy can come from fat. But, like I said earlier, you have to have continuous metabolism of glycogen (carbs) in order for the fat to be used efficiently as fuel.
Here lies the problem. You only have a limited amount of storage space for glycogen (about 2000 calories). This will usually last a runner about 2hrs of hard running for an advanced runner or about 20 miles for the average runner. The 20-mile mark is usually where many runners hit the wall or "bonk." That's usually because they've run out of fuel—literally. They've depleted their glycogen stores and since the brain needs glycogen to operate too, it will send signals to fatigue the muscles causing you to slow down as well as the fact that for the fat to be utilized as fuel you still need to be metabolizing some glycogen. Kind of like having an oil well in your backyard, but no drill to get to it.
The upside is that there is almost an unlimited supply of fat. The key is to make sure you don't deplete your glycogen stores so that your fat can continue to be used as energy. The following tips will help maximize your fuel sources and help prevent depletion of your glycogen stores.
Tips to Maximize your Fuel Sources:
1. Don't start out too fast. Remember, glycogen will be used first especially when quick energy is needed. So, don't burn up all your glycogen in the first 10 minutes of the race.
2. Implement VO2Max training. This will help train your body to make better use of your fat stores (see the previous post for more info).
3. Replenish glycogen stores during the race with sports gels and/or sports drink containing carbs. Be sure to use them starting about 45 minutes into the race. Don't wait until you feel like you need them. That will be too late for them to do any good.
4. Carb-load beginning three days before the race. This will maximize your glycogen stores so you'll be fueled-up on race day.
There are several different theories on carb-loading. The thinking used to be based on a 6-day plan where for the first three days a runner depleted his carb stores so that when he/she packed in the carbs during the last three days before the race, there would be better absorption of the carbs. Current research shows that a depletion stage really isn't needed.
The recommended daily allowance of carbohydrates for the average person is 45-65% of the total daily calories. Because of the amount of running during training, a runner should probably be eating more like 60-70% of his daily calories from carbs. So to carb-load, a runner should up that amount of carbs to 70-80% of his daily calories, beginning three days prior to race day (race day isn't one of the three days). During these three days, the runner should also decrease his/her exercise. You don't want to burn off the carbs you're packing in.
Carb-Loading: Day 1
On the first day of carb-loading, try to pack in as many complex carbs as you can. Complex carbs tend to come with more fiber. Packing in the complex carbs early-on during the carb-loading phase, gives it plenty of time to be processed by your body and voided, helping to decrease any stomach issues during the race.
Carb-Loading: Days 2 and 3
Over two remaining days, begin to transition from complex carbs to more simple carbs. (Note: Beer probably doesn't make a good carb-loading food.) Be careful though; don't start packing in lots of simple carbs that you've never tried. This could cause some stomach issues. It's a good idea to do a "practice carb-load" early-on in your training for two reasons. First, it will simulate what your body is going to feel like during the carb-loading phase. You'll usually gain a few pounds due to water retention. If this happens, don't panic. This is good because it will help provide water for sweat during the race. Second, the simulation will help you determine which foods work best for you during the carb-loading phase.
Good hydration is important before the race, but make sure you're not depleting your sodium and potassium levels in the process. Including some salty foods like pretzels and eating some bananas during your carb-loading phase should help avoid this potential problem.
Last Meal Before the Run:
Make sure your last big meal is about 12-15 hours before the race. This will give the food plenty of time to pass through your body before the race. You don't want to be looking for a port-a-john at mile 3 of the race. If you plan on eating before the race (which is probably a good idea), do so 2-4hrs before race time. Make sure that you've tested out (several times) your race-morning foods. It's definitely not the time to try out something new. This meal should consist of about 300 calories coming from some carb-rich foods like bagels or toast. Keep the protein and fats to a minimum, since they take longer to process and can weigh and/or slow you down during your run.
The Carb-loading phase should not be an afterthought. Take some time early on in your training to plan out this phase and begin thinking about what foods will work best for you. Then look at your training calendar and plan a mock carb-loading phase about halfway through your training just before one of your longer runs. Remember, a simulation of the "real deal" can be a very useful tool.