Preventing dehydration through the long hot summer is essential during both training and racing. Becoming dehydrated negatively affects your running performance during today’s run, and also slows your ability to recover for the next workout. To enhance your running performance, reduce your recovery time, and ensure your health, you need a strategy to minimize fluid losses during running, and to replace lost fluids as quickly as possible after running.
What happens in your body as you become dehydrated? When you sweat, your blood volume decreases, so less blood returns to your heart. As a result, the amount of blood your heart pumps with each beat decreases, consequently less oxygen-rich blood reaches your working muscles. Your rate of aerobic energy production decreases, therefore, and you must run at a slower pace.
These effects are magnified on a hot day because one of your body’s major responses is to send more blood to your skin to help remove heat from your body which results in even less blood returning to your heart to be pumped to your working muscles. The result is a higher heart rate for a given pace, and inability to maintain the same pace as on a cool day. Looked at in another way, dehydration also reduces your body’s ability to maintain your core temperature, because less blood is available to be sent to your skin and your sweat rate decreases. Struggling to maintain a fast pace on a hot day becomes more dangerous as you become progressively more dehydrated, and can lead to heat stroke.
Studies have found that dehydration of 2 percent of bodyweight leads to about a 6 percent reduction in running performance. For years, the general wisdom was that running performance was only affected when runners lost two percent of bodyweight or more due to dehydration. Ed Coyle, PhD, a former competitive runner and now Professor of Exercise Physiology at the University of Texas at Austin, has provided evidence, however, that even a small amount of dehydration causes a decrease in running performance by reducing blood volume and the amount of blood pumped to the working muscles.
How much should you drink?
How much you need to drink to stay well-hydrated during hot summer running depends on the severity of the heat and humidity, whether you tend to sweat lightly or heavily, your body size, and how much and how intensely you are training. Your baseline fluid needs when you are not training are about three to four pints per day. On top of that, you need to add your fluid losses from training and other activities. Runners sometimes become chronically dehydrated without realizing it. If you replace a little less fluid than you lose each day, after a few days you will run poorly but the reason why may not be obvious to you.
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 its pre-run level. Because you do not retain all of the fluid that you drink, becoming fully hydrated typically requires drinking an amount of fluid equivalent to about 1.5 times the amount of weight that you lost. For example, if you lost three pounds during a training run, you would need to drink about 4.5 pounds of fluid (4 ˝ pints) over the next several hours to be sure that you are fully rehydrated.
In the example above, adding 4 ˝ pints to make up for the fluid lost during training to the three or four pints required as a baseline equals a total fluid requirement of 7 ˝ to 8 ˝ pints for the day. 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 work-station or work-site is a must. You will regain fluids most effectively if you discipline yourself to drink regularly throughout the day. Try to avoid waiting until shortly prior to training to replace your fluids as you cannot rush the process and will go into your workout either bloated from too much fluid ingested too quickly or dehydrated from not having enough fluid. From personal experience, I can verify that drinking too much too quickly prior to running can also give you an unpleasant case of “the runs.”
What should you drink?
During running, water and carbohydrate replacement drinks are both excellent for maintaining hydration. After running, many other options are available, but I favor watered-down fruit juice for the taste. The advantage of replacement drinks with 4 to 8 percent carbohydrate while you run is that they are absorbed as quickly as water, and also provide readily usable energy. The carbohydrate concentration that is most appropriate for you will depend on your stomach’s tolerance, and how warm it is during training or racing. Beverages that you drink during running should also contain between 250 mg and 700 mg sodium per liter to enhance glucose and water absorption, and improve fluid retention.
The following tips will help ensure that you train successfully and safely through the heat of the summer:
1. Start each workout fully hydrated by making rehydration a priority after the previous day’s run.
2. Drink small amounts all day to maximize absorption.
3. Place water or carbohydrate drinks strategically around your course (or identify places to stop to drink) before long runs.
4. On a hot, humid day, slow down your pace from the outset.
5. Have no more than one beer or glass of wine and no more than one cup of tea or coffee per day to prevent further dehydration.
6. Run in the morning when heat and pollution levels are at their lowest.
7. Work out indoors or take a day off when the heat or pollution level is dangerously high.