Many years ago the Gatorade Sports Science Institute published a study showing that 46 percent of recreational exercisers are dehydrated (Applied Physiology, Nutrition and Metabolism, June 2006). However, the study did not say that the exercisers were harmed by their dehydration, with good reason. There is no data anywhere to show that mild dehydration affects health or athletic performance. Fit humans can tolerate significant fluid loss before their performance suffers, and most cases of muscle cramps are not caused by dehydration or salt loss. They are caused by muscle or nerve damage and can be controlled by stopping exercise and stretching the cramped muscle.
A healthy and fit exerciser can usually keep on exercising comfortably at a relaxed pace until he has lost 3-4 percent of his body weight in fluid. For example, an average fit 150-pound man can lose at least 4.5 pints of water (equal to 4.5 pounds of water loss which is three percent of his body weight) before he will weaken from dehydration. The major concern about dehydration is for people who are prone to develop kidney stones. For them, any dehydration can form more stones.
You Can Drink Too Much
When you exercise for more than an hour, particularly in hot weather, you may need to take fluid, but not too much. Taking in massive amounts of fluid can cause a potentially fatal condition called hyponatremia. Normally, the amount of salt and other minerals in your bloodstream should equal the same total mineral content in every tissue in your body. If the mineral concentrations are not equal, fluid moves from the area of lower mineral content to that of the higher concentration. If you take in so much fluid that it lowers the mineral level in your blood, levels of minerals in your brain are higher than those in your bloodstream. This can cause fluid to move from your low-mineral bloodstream into your higher-mineral brain, which increases pressure in your brain and can cause seizures and unconsciousness. The swelling can cause permanent brain damage.
Hyponatremia is seen almost exclusively in people who are not exercising near their maximum. The major risk factor is having more time to drink than to concentrate on pushing the pace, no matter what the sport or what the duration of the event. Top athletes drink very little fluid during competitions such as bicycle racing, marathon running or cross country skiing, because it is so difficult to drink while you are exercising near your maximum. On the average, a world-class marathon runner drinks less than a cup an hour during a race. This is far less than the amount recommended by the American College of Sports Medicine just a few years ago. On the basis of our present knowledge, it may not be safe for athletes and exercizers to take in more than 800 cc per hour (3.5 cups).
Fluid Intake for the Competitive Athlete
A person in competition can lose a very large amount of fluid before it impairs performance (Medicine & Science in Sports & Exercise, October 2006). A good rule of thumb is that dehydration does not affect performance until it decreases blood volume. Since lost blood volume is constantly replaced by fluid stored between cells, it takes a long time to reduce blood volume and impair athletic performance. Indeed, fluid losses of three percent of body weight usually do not impair exercise endurance (Scandinavian Journal of Medicine and Science in Sports, June 2015). Elite marathon runners drink very little even in hot weather.
However, several studies show that dehydration can slow you down at a 2–3 percent loss of body weight (2-3 pints of fluid loss). Other studies have reported decreased racing speed at a loss of one percent of body weight because of elevated body temperature in very hot weather (Sportmedizin und Sporttraumatologie, 2003;51(1):25–29). Some studies have found decreased heart rate (with resultant decreased pumping of oxygen to muscles) at a two percent loss of body weight (J of Applied Phys, April 1, 1997;82():1229-1236). One study reports a loss of 5.6 percent of a one-repetition-maximum bench press strength with dehydration of 1.5 percent body weight (Sports Med, 2007;37(10):907-921).
Fluid Intake for Exercisers
Most exercisers do not need to take fluids when they exercise at a casual rate for less than an hour. They can drink a cup of water or a sugared drink during that time if they want. A good rule of thumb is that casual exercisers can drink when they are thirsty or take a cup of fluid every half hour. They certainly should not try to drink very large volumes of fluids or to drink every few minutes.
Fluid During Competition
The consequences of taking in too much fluid almost never affect serious conditioned athletes. During competition, athletes concentrate so intently on pushing the pace that they are unlikely to have enough time or energy to drink too much. However, the novice athlete moving very slowly in an endurance event might take in too much fluid. The position statement of the National Athletic Trainers’ Association (Journal of Athletic Training, 2000;35(2):212–224) recommends the following maximum amounts:
• 2-3 hours before competitions lasting longer than an hour: 500 to 600 mL (2 cups or 16 ounces) of water or sugared drink
• 10-20 minutes before competition: 200 to 300 mL (1 cup or 8 ounces) of water or sugared drink
• during the event: 200 to 300 mL (1 cup: 8 fluid oz.) every 20 minutes.
What Should You Drink During Competition?
Virtually all sodas, sports drinks and fruit juices contain eight percent sugar because that is the sugar concentration that tastes best. Sports drinks that contain lower sugar concentrations usually contain artificial sweeteners to make them taste like a drink with eight percent sugar.
Adding caffeine to a sports drink brings sugar faster into your muscles to help you exercise faster and longer, so you gain maximum advantage from drinking any beverage that contains sugar and caffeine. There is little advantage to special sports drinks since you gain no advantage from salt or other minerals in drinks unless you are competing intensely for more than two hours. If your event lasts longer than two hours, you may prefer to eat salted nuts or potato ships, since heavily-salted fluids usually taste awful. You don't need to take calcium, magnesium or other mineral supplements. If you are competing for more than three hours, you should be eating the food of your choice during the competition; fluids will not provide all of the calories you need.