That “moderate amount of dehydration” is usually surprisingly small. Studies have found that dehydration of 2 percent of body weight leads to about a 4 to 6 percent reduction in running performance. Ed Coyle, PhD, a former competitive runner and now professor of exercise physiology at the University of Texas at Austin, has provided evidence that even a small amount of dehydration causes a decrease in running performance. This is because any reduction in blood volume will reduce the amount of blood returning to your heart.
During high-mileage runs or runs in hot weather, it’s important to stay hydrated. Carry drinks with you or place drinks in planned positions along your route before you begin the run.
It’s not unusual to lose 3 pounds (1.4 kg) of water per hour when running © on a warm day. At this rate, during a 2-hour run you would lose about 6 pounds (2.7 kg). For a 140-pound (63 kg) runner, this represents more than a 4 percent loss in body weight and an 8 to 12 percent decrement in performance. The effect increases as the run progresses, so this runner wouldn’t be any slower the first few miles but would likely slow by up to a minute per mile by the end of the run. Staying well hydrated, then, can be the difference between training hard enough to provide a strong stimulus for your body to improve or just going through the motions in training.
For any marathoner training in warm conditions, preventing dehydration must be a high priority. How much you need to drink to stay well hydrated during your marathon preparation depends on a number of factors, including the heat and humidity, your body size, how much you’re training, and how much you sweat.
Your baseline fluid needs when you’re not training are about 4 pints (1.9 L) per day. On top of that, you need to add your fluid losses from training and other activities. Weigh yourself before and after running and calculate how much weight you lost, then drink with the objective of bringing your weight back up to normal. Becoming fully hydrated typically requires drinking one and a half times the amount of weight you lost – the extra amount is required because some of what you drink will quickly wind up as urine, necessary to rid your body of waste products. So, for example, if you lost 3 pounds (1.4 kg) during a training run, you would need to drink about 4.5 pounds (2 kg) of fluid (4.5 pints; 2.1 L) during the next several hours to be sure you’re fully rehydrated. (If your postrun beverage contains sodium, you’ll retain more of the fluid you take in.)
If you add 4.5 pints (2.1 L) to make up for the fluid lost during training to the 4 pints (1.9 L) you require as a baseline, that makes a total fluid requirement of 8.5 pints (4 L) for the day in this example. This is a large amount of fluid, and consuming this much during the day requires a strategy, particularly for those with normal jobs. Keeping a water bottle at your workstation is a must. You’ll regain fluids most effectively if you discipline yourself to drink regularly throughout the day. Try to avoid waiting until shortly before training to replace your fluids – you can’t rush the process, and so you’ll go into your workout either bloated from too much fluid ingested too quickly or dehydrated from not having enough fluid.
Interestingly, although it’s important to drink throughout the day, recent research has found that drinking more fluid less frequently, compared with drinking the same total volume spread out in smaller, more frequent intakes, speeds gastric emptying (from your stomach to your intestines). That is, drinking, say, a pint of water all at once at the beginning of the hour should result in greater gastric emptying than drinking that same pint divided among 4-ounce servings every 15 minutes. The key is not to overload your system, because that will slow gastric emptying. If you feel bloated and have intestinal distress after drinking a large amount of fluid at once, you’ve probably exceeded what your system can tolerate at once.
How much you should drink during your race is discussed under “Marathon Race-Day Nutrition and Hydration” later in this chapter.
Water and carbohydrate-replacement drinks that contain sodium are excellent for maintaining hydration during running. The advantage of replacement drinks with 4 to 8 percent carbohydrate is that they’re absorbed as quickly as water and also provide readily usable energy. The carbohydrate can help your performance during workouts lasting longer than 1 hour. The exact carbohydrate concentration that’s best for you will depend on your stomach’s tolerance and how warm it is during training and during your marathon. On a cool day, you may want to use a carbohydrate content of 6 to 8 percent, whereas on a warm day, when fluid is more critical than extra carbohydrate, you may want to stay in the 4 to 6 percent range. Sports drinks should also contain between 250 and 700 mg of sodium per liter to enhance glucose and water absorption and improve fluid retention.
Most running books just tell you to avoid alcohol and caffeine. That’s neither realistic nor helpful advice for the majority of people who regularly enjoy coffee, tea, beer, wine, or spirits. The real issue is determining how much of these beverages you can drink before they have a significant effect on your running performance.
Alcohol (ethyl alcohol) primarily affects your brain. One or two drinks temporarily lead to reduced tension and relief from stress. In the short term, they will also increase dehydration. According to the most recent position statement on hydration from the American College of Sports Medicine, over the course of 24 hours the initial dehydration is offset by less urine output, and the diuretic effect of a small amount of alcohol is negated. Where to strike the balance? The night before your marathon, reduced tension is a good thing, but as discussed earlier, any amount of dehydration is detrimental to running performance. With this in mind, it’s best to limit yourself to one or, at the most, two beers or one glass of wine the night before the marathon. Take in enough extra fluid to make up for the dehydrating effect of the alcohol. Drink an extra ounce (30 ml) of water for each ounce of beer and an extra 3 ounces (90 ml) of water for each ounce of wine that you drink. The same guideline holds for the night before a long run.
After training or racing, wait until you’re reasonably well rehydrated to enjoy a postrun potable. Imbibing while you’re still dehydrated from running will slow your recovery. And right before training or racing, well, let’s not go there.
In its 2005 consensus on hydration, the American College of Sports Medicine wrote, “Caffeine ingestion has a modest diuretic effect in some individuals but does not affect water replacement in habitual caffeine users, so caffeinated beverages (e.g., coffee, tea, soft drinks) can be ingested during the day by athletes who are not caffeine naïve.” In other words, if you’re used to it, moderate ingestion of caffeine does not increase urine output more than a similar amount of water (Armstrong et al. 2005).
If you are not used to caffeine, there may be a mild diuretic effect in the short term, but then a compensatory mechanism results in your holding on to more water over the ensuing 24 hours. This means there is no significant diuretic effect from a cup of coffee or tea, and a cup of coffee or tea on the morning of the race is fine. In fact, if you’re used to having a cup of coffee each morning, abstaining from coffee could have a detrimental effect on your performance because of the withdrawal effects of caffeine deprivation.