Second, the recommendation to drink enough fluid to prevent weight-loss is based on the false assumption that all the weight lost is from body fluid evaporating as sweat. However, recent studies show that a significant amount (as much as 60 percent) is actually due to the loss of water stored with fat and carbohydrate molecules, which is released from the muscles when these stores are converted to energy. Although it contributes to sweat and weight loss during exercise, this kind of fluid loss has no dehydrating effect because it doesn't reduce blood volume.
Third, the problem with drinking to completely prevent dehydration is that it tends to dilute the concentration of sodium and other electrolytes in the blood, especially during prolonged exercise of more than two hours. Electrolytes are dissolved minerals that regulate your body's fluids, helping create the electrical impulses essential to physical activity. When you sweat, you release more sodium than any other electrolyte.
More: Hydration Basics
Since even the most electrolyte-packed sports drink has a lower sodium concentration than sweat, when you replace sweat with a sports drink you essentially water down your blood. In extreme cases, blood sodium dilution leads to hyponatremia, a potentially fatal condition where fluid balance is thrown off to the point where cells literally become waterlogged, causing the brain to swell.
Therefore, instead of drinking to completely replace the fluid you sweat out during exercise, aim for keeping thirst at bay. Respond to your thirst right away with small amounts of sports drink, but don't allow your thirst to build to the point that you're forced to guzzle down a full bottle at one time. Taking a few sips about every 10 to 12 minutes will help you stay hydrated and avoid stomach upset.
Old: Use either a sports drink or water for hydration.
New: Use a sports drink instead of water.
Prior to 2003, USA Track & Field's hydration guidelines for runners suggested that water and sports drinks were equally good choices for hydration during intense physical activity. But, based on new research concerning the risks of blood sodium dilution, the USATF revised its hydration guidelines stating, "A sports drink with sodium and other electrolytes is preferred." Athletes in other sports are now following these guidelines as well.
In short, sports drinks simply hydrate better than water does. Your body absorbs fluids through the gut and into the bloodstream faster when their osmolality, the concentration of dissolved particles in a fluid, more closely matches the osmolality of body fluids such as blood.
Because a sports drink contains dissolved minerals (key electrolytes such as sodium, calcium, magnesium, potassium, and phosphate) and carbohydrates, it's absorbed into the bloodstream more quickly than water, which has fewer or no dissolved particles.
Moreover, electrolytes and other nutrients play important roles in regulating fluid in the body. They help determine how much fluid enters muscle fibers and cells, and how much remains in the blood. That's why sports drinks do a better job than water of helping the body maintain an optimal fluid balance.
Water is fine for short (less than an hour) workouts of easy to moderate intensity in which you don't sweat a lot. But in any workout where sweat losses are substantial, and especially in warm weather, use a sports drink.