According to the International Olympic Committee, dehydration impairs an athlete's performance in most events: endurance sports, team sports, power and sprint sports, winter sports and sports with weight classes. Athletes need to be well-hydrated before and during exercise and competition.
Dehydration and the Body
Your body needs adequate water for all activities and cannot adapt to dehydration. Internally, you're about 65% water. Without water, you can only survive a few days. Dehydration makes your blood thicker, increasing your heart rate and decreasing the amount of blood your heart can pump with one beat and causing your blood pressure to fall. Dehydration makes it harder for fat to get into your muscles to be used for fuel, so your muscles burn the limited sugars (glycogen) already there. Since your brain is about 85% water, even mild dehydration can bring on changes in your mood and a decline in your concentration and alertness.
From a sports perspective, losing as little as 2% of your body weight in fluids – for example, 2.8 pounds (representing about 44 ounces of water) in a 140-pound marathoner – can cause measurable decreases in performance. Dehydration of more than 3% of your body weight is serious, increasing the possibility of heat exhaustion and heat stroke in warm and/or humid conditions. Since athletes can sweat out 6% to 10% of their body weight during competition, you can see the importance of rehydrating.
And thirst is not a reliable indicator of dehydration. If you wait to drink until you are thirsty and stop drinking when your thirst is satisfied, you'll remain 25% to 50% dehydrated.
Dehydration and Performance
For any athlete, minimizing your fluid loss to not more than 2% of your body weight is a good rule. At that 2% body loss, you'll start to see increased fatigue, reduced endurance, the beginnings of heat illness and declining motivation. The good news is that rehydrating will reverse all these problems.
The longer your workout or competition, the more dehydration will hurt your performance. A review of scientific studies showed that endurance athletes like triathletes and marathoners had a performance drop of 7% to 60% when dehydrated. Athletes requiring muscle strength, like bodybuilders and football linemen, saw their power reduced when their sweat loss was as low as 3% of their body weight.
Dehydration and Preparation
Before a workout or competition, properly hydrate by drinking 1 to 2 cups of fluid an hour before starting, one cup about 15 to 30 minute prior and then 5 to 10 ounces of fluid every 15 to 20 minutes of activity. Adjust these amounts to the weather and how much you sweat.
Don't know your sweat rate? It's pretty easy to figure out. Before you exercise, weigh yourself nude. Then exercise, but for the simplest calculation, don't drink or urinate. After your workout, remove you clothes, dry off, then weigh yourself nude again. You'll need to drink 16 ounces of fluid for every pound you've lost, just to get back to your starting point.
Before and during exercise, you need to monitor your hydration level and drink more if necessary. One quick method is to look at the color of your urine – a dark urine indicates dehydration. Light yellow or no color usually means you're hydrated, but drinking caffeine drinks or alcohol will dehydrate you without the dark urine color.
Dehydration and Recovery
After exercise, you need to rehydrate (2 cups, 16 ounces for every pound lost). It's better to use a sports drink containing electrolytes rather than water alone – after all, we don't sweat plain water. Let's say our 140-pound marathoner did a workout and found she lost 1 3/4 pounds. That's 1.75 pounds * 16 ounces = 28 ounces, or 3 1/2 cups of a sports drink she needs post-workout to rehydrate.
For your own recovery needs, you may want to rehydrate to 150% of your fluid loss – 24 ounces for every pound you're down. After all, are you sure that you started exercising fully hydrated?