Hydration is an essential part of your race strategy. Fluid and body salts are lost through sweating and need to be replaced— start the race with your fluid levels topped up, and maintain them throughout. Staying properly hydrated during your run will reduce the strain on your body and lower your perception of exertion.
HYDRATE TO WIN
You will be pushing your body to its limits during the race, so you need all the help you can get. Staying hydrated will help your body work at maximum efficiency and is an easy and effective way to get the most out of your performance. Your water intake affects many of the physical processes that enable you to compete at your best: sweating helps stabilize body temperature, while drinking enough fluid also helps balance your blood plasma volume. Blood plasma is vital for transporting nutrients to your muscles as they work, and moving waste products such as lactic acid away from them. Both your physical performance and mental clarity will start to be detrimentally affected as soon as you begin to become dehydrated. Experiment with your fluid intake during training, so that you are familiar with your body’s needs and know how to hydrate effectively during your race.
TOO LITTLE OR TOO MUCH
If you don’t replace fluids lost through sweat, your blood thickens, reducing your heart’s efficiency, and increasing the time it takes for oxygen to be delivered to cells. This is known as hypernatremia. It raises your body temperature and increases the concentration of salts in the blood, causing dehydration. Rarely, drinking too much fluid too quickly can over-dilute your blood. This is called hyponatremia, and can lead to dizziness, confusion, and, in severe cases, seizures and respiratory failure. If you follow a good race hydration strategy, the chances of either occurring are small.
Runners’ hydration needs vary, so use this chart below as a guide. It is best to drink water before a race, but sports drinks can be better during and after a race as they also replace glucose and body salts. If you pick up a drink while running, it is best to drink it in small sips, slow to a walk for a few steps, and don’t consume more than the recommended amounts to avoid excess fluid causing discomfort in your stomach, or hyponatremia. Don’t forget that it is equally important to replace lost fluid at the end of the race.
BEFORE THE RACE
You need to be properly hydrated before you run. Runners who have sufficient fluid intake before a race will have lower heart rates and body temperatures than those who are not adequately hydrated. Drinking water around 2 hours before your race will generally ensure you are properly hydrated—see chart, opposite. If you do not feel the need to urinate within an hour of this, then you can top up with additional liquid. About 15 minutes before the start, try to consume some more water to boost levels.
DURING THE RACE
Feeling thirsty is the brain’s way of telling you that you are already dehydrated and must drink immediately. So if you wait until you feel thirsty, it will be too late. Fluid needs vary from runner to runner, and are also determined by weather conditions and the amount you sweat, but it is vital to drink enough on runs over 6 miles (10 km), or if it is very hot. Both water and sports drinks (hypotonic or isotonic) can be drunk while running. Sports (or energy) gels are a good way of maintaining blood-sugar levels, but need to be washed down with water.
AFTER THE RACE
When restoring your hydration levels after a race, it is important to replace the body’s electrolytes (salts and glucose) too, because they regulate your hydration levels. Sports drinks can help with this: isotonic drinks contain the same levels of salts and glucose as your body while hypertonic drinks have a higher concentration, so they should not be drunk during a run but are ideal for drinking afterward. If you are drinking water, then try to eat some easily digestible source of carbohydrate, such as a banana, or take a sports glucose tablet.
Taken From "The Complete Running and Marathon Book" -- OmRiyadat