Taking sugar while you exercise increases the amount of training you can do, and does not lessen the benefits of your increased training. 

Athletes in sports requiring endurance need to train in their sport many hours each day. They damage their muscles by taking a hard workout on one day, feel sore on the next, and then take less intense workouts for as many days as it takes for the muscles to heal and the soreness to go away. The more intense the training workout without injury, the more intensely they can compete. The longer they can go on their less intense recovery days, the tougher their muscles become to withstand the tremendous forces on them during their hard workouts and during competition.

Anything that can increase the intensity of their hard days or amount of work they can do on their recovery days will make them better in competition. Running out of muscle sugar makes you feel tired. So anything that preserves stored sugar in muscles during a workout will help you exercise longer. This study shows that taking sugar regularly during workouts allows you to extend the amount of training without lessening the benefits that you receive from the extra work.

A drink containing both protein and sugar every three miles and at the finish of a 36-mile bicycle time trial was far more effective than a drink containing just sugar in 1) riding faster at the end of the time trial, 2) preventing next-day muscle soreness and 3) lessening muscle damage. A protein-sugar drink taken immediately after intense exercise also hastens healing of the muscles damaged by hard exercise.

Taking refined carbohydrates (sugar or flour) when you are not exercising can cause a high rise in blood sugar that increases risk for diabetes and heart attacks. Contracting muscles remove sugar so fast from the bloodstream that blood sugar usually does not rise too high during exercise and for up to half an hour after you finish exercising.