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Could sports drinks be improved with the addition of protein? That question has long gripped physiologists and nutritionists. It’s well established that the carbohydrates (sugars) that sweeten most sports drinks aid performance. They provide immediate fuel for straining muscles, keep blood-sugar levels stable and allow you to work out for a longer period of time or at a higher intensity, or both, than if you don’t swallow any extra fuel. But why wouldn’t taking in protein, together with carbohydrates, during a workout or race make you even more speedy and durable? Protein, after all, is what muscles fundamentally are made of, so it seems reasonable to imagine that adding it to sports drinks could provide some additional benefit.

But to date most studies have not shown that to be true. Some earlier experiments that did find athletic-performance benefits from protein-enhanced sports drinks used protein beverages that contained more calories than the carbohydrate-only versions and, as some critics pointed out, the extra calories rather than the protein, per se, probably provided the benefit. More typically, most studies came to the same conclusion as one published earlier this year in the journal of the American College of Sports Medicine. For that study, cyclists consumed a carbohydrate sports drink or one enriched with whey protein while they warmed up with two hours of steady-state cycling and then completed a strenuous one-hour time trial. The researchers found no benefit from the protein drink. The cyclists downing it didn’t produce more power while pedaling or cover more miles during the time trial, nor did they harbor fewer markers of muscle damage the next day. The protein didn’t hurt their performance, but it didn’t help in any measurable fashion, either.

The researchers themselves weren’t surprised, said Dr. Asker Jeukendrup, a professor in the school of sport and exercise sciences at the University of Birmingham in England and senior author of the study. “No one has a clue why protein would work,” he said. “Protein is a poor fuel.”

But a few scientists (and athletes and sports-drinks-makers) can’t shake the notion that protein can and should do something during exercise, and a new, narrowly defined study published last month in the Journal of Strength and Conditioning Research may give them some ammunition. For this study, researchers at the University of Texas at Austin recruited 15 competitive cyclists and had them complete two grueling exercise sessions in a lab. During each session, the cyclists rode for three hours, with the pedaling varying, on a set schedule, from languid to draining. At the end of the three hours, the pedaling resistance was upped considerably, and the volunteers were told to ride until they barely could turn the pedals.

Throughout the sessions, the riders swallowed either a typical sports drink or one that had been supplemented with whey protein. Importantly, this protein-enriched drink was low-calorie, with half as much of the carbohydrate sweetening as the sports drink. “Not everyone needs or wants all of those calories from sports drinks,” pointed out Dr. John Ivy, a professor of kinesiology and health education at the University of Texas and senior author of the study. “I personally think we take in far too many calories that way.”

But can you still get benefits from a low-calorie drink? In Dr. Ivy’s experiment, a certain subset of the riders did. Those who pedaled during the final few, exhausting minutes of the experiment at a pace just below their ventilatory threshold (the point at which the muscles simply cannot get enough oxygen) and who swallowed the low-calorie protein drink rode a precious few moments longer than riders at the same intensity who drank only carbohydrates. The riders, however, who strained through the final ride to exhaustion above their ventilatory threshold got no clear benefit from protein.

What does this finding mean for those of us trying to decide on a sports drink? Frankly, no one knows. “We don’t know” why or how protein would operate on muscles during exercise, Dr. Ivy said, or why it might provide benefits when calories are cut or why it was effective only for riders who pedaled at certain intensities. “Protein is composed of amino acids, and they have many different effects on metabolism,” he said.

Other scientists remain skeptical. “I just don’t see a viable mechanism” for protein to work as a fuel during exercise under almost any conditions, said Dr. Martin Gibala, chairman of the department of kinesiology at McMaster University in Hamilton, Ontario. His lab has studied protein ingestion during exercise extensively and found no discernible performance benefits. “It’s possible that if you reduce calories substantially,” so you’re taking in “suboptimal” levels of carbohydrates, maybe, somehow “the protein is converted” to fuel, he said. But that’s a very artificial situation. “You don’t do endurance exercise that way,” he said. Someone who can ride a bike vigorously for three hours or more is probably not someone who needs to worry unduly about weight loss. “The science just isn’t there for an acute effect” of protein in sports drinks, he said.

Which means that, for now, you can probably skip adding protein powder to your Gatorade. (The flavor would be ghastly anyway.) “I have a hard time” believing “that protein would have any effect,” Dr. Jeukendrup concluded. The topic may continue to draw attention from scientists and athletes hoping for a simple, liquid means of getting themselves to the finish line faster, but he said, “I am convinced that protein in carbohydrate drinks during exercise will soon be out of fashion.”