Posted May 26, 2012 by sourced
By Peter Pfitzinger | DistanceCoach.com
You train for 5 months for the big day. You’ve done 23 long runs, worn out the inside lane on your local track, spent a small fortune on massage and new shoes. You are the fittest that you have ever been. The gun fires and you feel great. You are on personal best pace for 18 miles. Then something starts to go wrong. You feel progressively more sluggish. By 22 miles, you’ve slowed down to the old familiar death trot. Guess you need to train harder next time….
NO! Chances are that what makes you slow down in the last few miles of the marathon (or any event that takes longer than an hour and a half to complete) is dehydration or glycogen depletion. A few extra miles in training aren’t going to help much, but a well-laid plan to take in water and carbohydrates both before and during the race can make a huge difference in your performance.
Sounds great. So, what’s the plan?
Before the race you need to concentrate on: 1) carbohydrate loading to improve your muscles’ glycogen stores at the start of the race; and, 2) drinking enough water to ensure you are fully hydrated. During the race you need to: 1) take in carbohydrates to prevent glycogen depletion; and, 2) take in fluid to prevent dehydration.
Let’s start with the carbohydrates.
Your Glycogen Bank
There are only 2 fuels used for endurance exercise; carbohydrates and fat. Carbohydrates are stored in the body as glycogen. Fat is stored in the body as, well, you know. When you run, your body burns a mixture of carbohydrate and fat. The harder you run, the more carbohydrate you use, and the slower you run, the more fat you use.
As your glycogen stores become progressively more depleted during the run, your body tries to conserve what’s left by burning more fat. Because fat is 15% less efficient than carbohydrate as an energy source, when you have to burn more fat you slow down.
If you eat a normal runner’s diet with about 60% of your calories from carbohydrate, you probably store about 1,600 to 2,000 calories of glycogen in your muscles. If you “glycogen load” (see sidebar) however, your muscles have the capacity to store about 2,500 to 2,700 calories of glycogen. Each mile that you run burns about 90-120 calories, depending on your weight and metabolism. If you do a great job of loading, you will have just about enough glycogen for a marathon.
What happens if you don’t load? Say you hope to run a half-marathon in about 1 hour 40 minutes. The last 3 days before the race, you cut back your training a little bit and you just eat normal meals. Your glycogen stores will only be partially filled. You could start to run low on glycogen anytime after the one-hour mark, and end up jogging to the finish line wondering what happened.
Glycogen loading is also important before any training run lasting more than 1 1/2 hours. The last thing you want both physically and psychologically is to struggle home in your long runs. If you focus on carbohydrates for 1-2 days before your long runs (and stay hydrated) you will increase your chances for a better training effort and a positive psychological experience.
Carbohydrate loading before the event is a sufficient precaution against glycogen depletion for races up to about 2 hours in duration, but for the marathon you are still at risk. Since you can only store about 2,500 to 2,700 calories of glycogen, you don’t have much of a buffer in a 26 mile race. Say for some reason you only do a fair job of loading and store 2,200 calories worth of glycogen. Or say you did a great job of loading, but, due to your size or metabolism, you happen to burn 120 calories per mile. You would severely deplete your glycogen stores during the race despite pre-race loading. The solution is to take in additional calories during the race.
Carbing on the Run
If you consume carbohydrates during the race, you will supply additional fuel to your muscles, and delay or prevent muscle glycogen depletion. The easiest way to consume carbohydrates on the run is in a sports drink, and you will have the added benefit of taking in needed fluid at the same time. Research has shown that drinks with a high concentration of sugars take longer to empty from your stomach. Every runner is an experiment of one, so you should try out what to drink and how much during your training runs.
The trick is to find the right balance between a carbohydrate solution that is strong enough to give you needed calories, but not so strong that it is absorbed slowly from your stomach. For most people, 4-8% solutions are about right. Drinking 28 ounces per hour of a 4% solution will supply 32 grams of carbohydrate, while an 8% solution will supply 64 grams of carbohydrate per hour.
Each gram of carbohydrate contains 4.1 calories, so you will be taking in 130-260 calories per hour. If you run the marathon in 3 hours, therefore, you will take in about 400-800 calories during the race. At 100 calories per mile, that’s enough carbohydrate fuel to last an extra 4-8 miles!
Exercise physiologist and ultramarathoner Ron Johnston knows first-hand about the need to take in carbohydrates during the run. Ron was having a great day during the 1997 Maine Track Club 50 mile race. Ron recalls it this way, “I was cruising at 34 miles, feeling the best I had ever felt, and running 6:40 to 7 minute pace. All of a sudden it was like someone pulling down the shades. I felt fuzzy headed and didn’t even realize that I had slowed down to 8:20 mile pace.”
Johnston continues, “My pace changed within a few minutes, and it was clear that the culprit was glycogen depletion. The problem is, there aren’t really any warning signs until it’s too late. When you have to slow down quickly like that it’s probably glycogen depletion and not dehydration which tends to get you more slowly. I went home and calculated that I was only taking in 50-100 calories on each 4-mile loop. That won’t happen again.”
Okay. The spaghetti’s in the pot. Let’s talk dehydration.
Camels Have It Made
What’s so bad about dehydration anyway? Can’t you just gut it out?
Several sinister events occur inside your body that limit performance when you run on a warm day. High humidity only makes matters worse. First, in an effort to cool you off, your body automatically sends more blood to the skin for evaporative cooling, leaving less blood going to your leg muscles. The result is that you get less oxygen to your muscles and you slow down. (Of course, if your body didn’t send blood to the skin to cool you off, you would overheat and maybe even die, but you wouldn’t have to slow down!)
Second, on hot days you sweat more and become dehydrated. That doesn’t really surprise you, does it? Physiologically, what happens when you get dehydrated is that your blood volume decreases. This means that your body has to decide what to do with the blood that’s left. So your body says, “Let’s see, better send some blood to the brain, don’t want this crazy runner to pass out (although it may solve all my problems). I better send some to the heart, and better send some to the skin to get rid of all this body heat from running. Hmmm. There’s not alot left for the old leg muscles.” That’s why you slow down.
If you think the Olympic Marathon in Athens is going to be won in World record time, think again. In fact, studies have shown that you slow down about 2% for each 1% loss in bodyweight due to dehydration. So, a 150 pound runner would slow down 4% after losing 3 pounds (which is about the same as 3 pints).
It is not unusual to lose 3-4 pounds of water per hour when running on a warm day. After 2 hours of running, our 150 pound runner would have lost 6-8 pounds, representing a 4-5% loss in bodyweight and an 8-10% loss in performance. At eight minute mile pace, slowing down 10% represents 50 seconds per mile. If you lose more than 4-5% of your bodyweight, however, you won’t just slow down, you will find yourself in a first-aid tent.
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