On 2 separate occasions, highly trained women exercised for 3 hours, then performed a timed test to exhaustion on a cycle ergometer. Every 20 minutes during the 3-hour bout of exercise, the subjects ingested either a standard 6% carb drink providing 24 kcal or a low carb / low protein supplement (3% carbs and 1.2% protein) providing 17 kcal.
Those who consumed the carb-only drink fatigued just after 42 minutes compared to those who consumed the carb–protein drink who fatigued after 50 minutes.
These results indicate that consuming a blend of carbs and protein during workouts has a stronger impact on endurance than just carbs alone. Further, when protein is consumed during prolonged endurance exercise, fewer carbs are needed to fuel the exercise.