When to Consume Carbs
This doesn't mean you should avoid taking in carbs during all workouts. There are two good reasons to take carbs along for some of your more challenging runs. First, you will perform better in these sessions and thus get a little extra benefit from them. Second, you need to take in carbs to maximize your performance in longer races, and if you're going to do that, then you need to practice your fueling plan during race-specific workouts to ensure you get it right on race day.
Consume carbs during roughly half of your steady-pace runs that last between one and two hours. You will get a bigger boost in fat-burning capacity from the runs where you withhold carbohydrate and you will feel and perform better in those where you ingest carbs. By doing a mix of both you will get the best of both worlds.
Avoid the Crutch Effect
When you run longer than two hours, consume carbs every time. This will minimize the muscle damage that occurs in very long runs when the muscles run low on glycogen (their stored form of carbohydrate) and start to use their own proteins as an emergency fuel source.
You should also consume carbs during all high-intensity runs lasting longer than 45 minutes. You will run 2 to 4 percent faster in these workouts and you don't have to worry about a crutch effect because the purpose of high-intensity training is not to enhance fat-burning capacity anyway.
I've said a lot about carbohydrate and energy, but what about fluid and hydration? There is no crutch effect associated with drinking while running, so you should feel free to carry fluid as often as you like in training.
On the other hand, water is not performance-enhancing to the degree that carbohydrate is, so there's no need to carry fluid on any runs except those where severe thirst would be an issue otherwise. I drink during all runs lasting longer than 2 hours and between intervals every time I hit the track.
You can get water and carbs either from the same source (a sports drink) or separate sources (water and energy gels). It makes sense to use a sports drink in all runs where you plan to drink, so you don't have to carry gels separately. You can use gels without water to minimize your carrying burden and boost your performance in challenging workouts that don't last long enough to produce serious thirst, but gels work best when washed down with water.
Getting the optimal amount of fluid and carbs to maximize race performance usually requires using some combination of a sports drink and energy gels washed down with water. If you intend to use this strategy in races, you need to rehearse it in at least a couple of race simulation workouts ahead of race day.
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