High-protein meals eaten immediately after hard exercise have been shown to help athletes recover faster, but the data that taking protein during exercise improves an athlete's performance is extremely weak.
Researchers from the University of Birmingham, UK, showed that adding protein (19g/hour) to a sugared drink does not improve one-hour cycling time trial, maximum power; or post exercise isometric strength, muscle damage (CPK) or muscle soreness (Medicine & Science in Sports & Exercise, June 2010). Protein also does not help athletes cycle faster in a 50-mile time trial (Medicine & Science in Sports & Exercise, August 2006). Most studies showing that adding protein to a carbohydrate drink improves performance were in people working at a fixed rate of effort over a long time, rather than using spurts of energy as athletes do in competition.
Just about everyone agrees that taking in a carbohydrate drink helps improve performances in athletic events lasting more than an hour. In events lasting more than three hours, you also need salt. Calories come from carbohydrates, fats, and proteins. During highly-intense exercise, your muscles use carbohydrates far more efficiently than proteins or fats. So carbohydrates are the calorie source of choice during intense exercise.
All sugared drinks except those with added artificial sweeteners contain eight percent sugar because that is the concentration at which the drinks taste best. You can increase endurance equally with fruit juice, special energy drinks or sugared carbonated soft drinks. Adding caffeine to the drink increases endurance even more because it helps to preserve your stored muscle sugar.