Dehydration

It is vital that an athlete intake enough fluids.  It is also a good idea to use some form of sports drink that will replenish electrolytes as well.  Sports drinks such as Gatorade and PowerAde work well, and the glucose in these drinks provides a quick and readily available energy source. 

If an athlete will be competing at an event, it is a good idea to train with the same type of drink that will be served at that event.  This is of particular importance to marathon runners and triathletes.  In Japan many of the sports drinks use fructose instead of glucose, and even Gatorade comes in many different concentration levels.  Try to train with the same type of sports drink at the same concentration level that has been selected for that particular event.  This will ensure that your body is accustomed to that particular drink and reduce the chances of nausea at critical times.