Water makes up about 60 percent of bodyweight more in muscular people (about 70 percent) and less in obese people (about 40 percent). Water is critical for life. Even a small water loss of 1 quart makes you thirsty; a 5 percent loss makes you sick, and a 20 percent loss kills you.
Your ability to maintain a near-constant body temperature at rest or during exercise depends on body water that supplies sweat and holds heat. Sports drinks that promote fluid retention and prevent fluid loss in the urine are beneficial because they help the body manage heat better. Researchers from St. Cloud State University in Minnesota found that sports drinks containing proteins improved fluid retention. They compared drinks containing carbohydrates and proteins, carbohydrates, or plain water. The carbohydrate-protein drink retained water 15 percent better than the drink containing carbohydrate and 40 percent better than plain water.
In general, water is the best fluid replacement. However, specialized sport drinks that maximize fluid retention may be important during prolonged exercise - particularly in the heat.