It is not clear whether the effects were merely a result of additional calories from the protein or because of specific protein-mediated mechanisms. The small study has also been criticized for failing to use a placebo.
However the findings give some early indications of potential benefits of adding proteins to sports drinks.
The study of 15 male cyclists compared Accelerade, a sports drink containing carbohydrates and whey protein marketed by US-based PacificHealth Laboratories, with a carbohydrate-only drink, similar to many products on the market today.
The researchers based at James Madison University in Virginia, US asked cyclists to pedal to the point of exhaustion while replenishing with either the protein-added or carb-only drink every 15 minutes. The athletes performed a second, more demanding ride the next day. One to two weeks later, they went through the process again, this time with the other drink.
Writing in this month's Medicine in Science and Sports Exercise (36(7):1233-1238), the researchers report that the men lasted 29 per cent longer during the first test and 40 per cent longer during the second test when they drank the protein-containing drink.
There were also signs of less exercise-induced muscle damage, according to the researchers. After the exercise tests, the cyclists' blood levels of creatine phosphokinase - an enzyme released from muscles under stress - were lower when they consumed protein during the workout.
Sports drinks with added carbohydrates and electrolytes are thought to rehydrate athletes faster, reducing potential for fatigue. But formulators are increasingly looking at whether added protein can have extra benefits, and help repair the muscle damage that occurs during exercise.
For example, ingredients producer DSM has developed a sports drink containing protein in the form of casein fragments that are said to stimulate insulin release in the body, allowing glucose to be absorbed faster from the blood into the muscle cells. Once absorbed by the muscles, glucose is converted into glycogen, which acts as a muscle fuel. The faster this process takes place following high-intensity exercise, the faster athletes can perform again at their optimum level, says the firm.
But a statement from the US National Athletic Trainers' Association urged caution in interpretation of the result of the new study, noting that a placebo control had not been used to measure the performance benefits properly.
Also, the same research group presented a study at the 2004 American College of Sports Medicine (ACSM) meeting that showed contrasting results - no additional performance benefit of a carbohydrate-protein mixture, although there was some evidence of reduced muscle damage (Med. Sci. Sports Exerc. 36(5):S126, 2004).
"There are benefits of consuming protein after exercise - to increase muscle protein synthesis by providing essential amino acids that are good for muscle recovery. However, when consumed in a sports drink, the presence of protein might slow emptying and absorption and actually reduce the primary benefit of a sports drink - to prevent dehydration by rapidly replacing the fluid and electrolytes lost through sweat," said the organisation.
It continued: "Scientific exploration of the benefits of protein intake before, during, and after exercise is in its early stages. Before advising athletes on the best course of action, more research must be done on this topic."