There are two types of nutrition that have the potential to significantly enhance performance when consumed during running: water and carbohydrate. It is no accident that these are the two main ingredients in almost all sports drinks intended for use during exercise. Most sports drinks also contain sodium and other electrolyte minerals to replace some of the same minerals that are lost in sweat, but there is no evidence of any performance-enhancing effect associated with these nutrients. A couple of other nutrients—caffeine and protein—are beneficial in special circumstances. We will look more closely at caffeine and protein in later chapters. But whereas many successful runners never use caffeine or protein for performance enhancement in training, every serious runner takes advantage of water and carbohydrate.
The means by which water and carbohydrate enhance running performance are completely distinct. Water consumption actually does not so much enhance running performance as it prevents the decline in performance that results from dehydration. While sweating has the positive effect of keeping the body cool, it also has the negative effect of reducing blood volume. As blood volume decreases, the heart has to work harder to deliver oxygen to the working muscles and to clear carbon dioxide away from the muscles. This loss of “cardiac efficiency” causes perceived effort to increase, which in turn causes the runner to slow down. Water consumption during running limits the decline in blood volume and maintains cardiac efficiency and performance. A study by English researchers found that men and women were able to run for one hour and forty-three minutes at a moderate intensity when supplied with water compared to only one hour and seventeen minutes when denied water.
Carbohydrate intake enhances running performance in a couple of ways. One of these ways is neurological. When carbohydrate touches the tongue, it activates special receptors that in turn activate a “reward center” in the brain. This triggers a reduction in perceived effort (running starts to feel easier) that allows a runner to run faster without exceeding his or her maximum pain tolerance. Carbohydrate consumption also provides an extra fuel source to the working muscles and thereby increases the maximum speed a runner can sustain over a finite distance without hitting the wall. In one study, runners were able to continue for 20 percent longer in a high-intensity interval workout with carbohydrate supplementation than without.
Because water and carbohydrate enhance running performance through separate mechanisms, their effects are additive. In other words, if you consume both you will perform better than you would if you took in either water or carbohydrate alone. A 1995 study found that runners were able to complete a marathon four minutes faster when they consumed water and carbohydrate together (in a sports drink) than when they drank plain water.
Given the beneficial effects of water intake on running performance, you might think it would be a good idea to drink enough water to completely offset dehydration during every training run you do in preparation for a marathon or half marathon. Not so. Water intake only makes a difference in runs lasting longer than sixty to ninety minutes. Shorter runs don’t cause enough dehydration for drinking to matter. What’s more, drinking too much may be worse than drinking too little because it causes gastrointestinal discomfort. Also, research has demonstrated that drinking by thirst (which almost never suffices to completely offset dehydration) boosts running performance just as much as drinking to minimize dehydration and with less risk of gastrointestinal distress.
Overdrinking is more common in races than it is during training runs because fluids are abundantly available and many runners believe that drinking as much as possible in races will help them perform better. Indeed, runners have been known to bring the wall on themselves by trying to avoid hitting the wall in this way. Shannon Hays did so in the 2011 Carrollton Festival of Races Marathon. Held in Michigan in late July, the CFR Marathon is always warm and humid. Fearful of becoming dehydrated, Shannon, who worked as a high school Spanish teacher in Suwannee, Georgia, drank much more than normal—too much, it turned out. She developed stomach bloating and nausea and struggled to a 3:56:27 finish—about half an hour off her personal best.
It’s the same with carbs. Given the beneficial effects of carbohydrate intake on running performance, you might think it would be a good idea to consume the maximal effective amount of carbohydrate during every training run you do in preparation for a marathon or half marathon. Again, not so. Research has shown that while consuming carbs during exercise does enhance performance in individual workouts, it also interferes with some of the physiological adaptations to training that improve fitness. In other words, carb intake during workouts functions as a kind of metabolic crutch that reduces the benefits of workouts. A runner who relies on carbs too heavily in marathon or half-marathon training therefore risks getting less out of it.
The best approach to fueling your workouts in marathon and half-marathon training is a balanced and measured one. Maximum hydration and heavy reliance on carbohydrate are clearly not the way to go. Yet it is far more common for runners to rely too little than too much on fluid and carb intake during the training process. It is a bit of a hassle, after all, to carry nutrition on a long run. While there is something to be said for unencumbered running, runners pay a price for not lugging the nutrition they need for when they really need it.
There’s a “sweet spot” between the all-or-nothing extremes that you’ll want to aim for in your workout fueling practices. Locating the sweet spot will not only help you get the most out of your training but will also give you the practice you need to fuel your body optimally in races, where it really counts.