Protein in your carb drink boosts endurance, but maybe only as extra calories

Endurance athletes often face several training sessions in a day, or hard efforts on consecutive days, with limited time to recover between them. In that window the body needs to restock liver and muscle glycogen, the stored carbohydrate that fuels endurance work, and repair muscle damage. Carbohydrate drinks are the standard tool for the job, but adding protein has been proposed as a way to speed restoration and lift subsequent performance. Whether the combination genuinely outperforms carbohydrate alone has been contested, and that is the question this review set out to settle.

The authors searched PubMed and EMBASE for randomised controlled trials comparing carbohydrate plus protein against carbohydrate alone, with endurance performance as the outcome. Thirty randomised controlled trials qualified, comprising 43 separate trials and 326 participants in total. Performance was assessed two ways: time-to-exhaustion, where participants continue until they cannot, and time-trial performance, where they cover a set task as fast as possible. Random-effects meta-analyses pooled the results, with subgroup analyses by supplement timing, recovery duration and whether the drinks were matched for calories or for carbohydrate content.

Overall, carbohydrate plus protein significantly improved both time-to-exhaustion and time-trial performance compared with carbohydrate alone. The time-to-exhaustion benefit appeared when supplements were taken during or after exercise, and when the recovery period between bouts was 8 hours or longer; with recovery of 8 hours or less, no effect was found. Crucially, when the two drinks contained the same total calories, no significant advantage remained. When the drinks were instead matched for carbohydrate content, meaning the protein added extra calories, time-to-exhaustion clearly improved.

The headline result supports carbohydrate-protein co-ingestion as a credible alternative feeding strategy for endurance athletes, in line with the attention these mixtures have attracted. But the null result in calorie-matched comparisons complicates the simplest story: if the benefit only shows up when protein adds energy, the active ingredient may be the calories rather than the protein itself. The analysis also suggests timing and recovery length shape the outcome, with longer recovery windows favouring the combination. What protein contributes beyond energy, including any role in muscle repair here, remains an open question.

For anyone training once a day with plenty of recovery, this offers little reason to expect a protein-spiked sports drink to transform performance. For people stacking long sessions with 8 or more hours between them, it is a reason to think carbohydrate with added protein can help the next effort, though possibly just because it delivers more fuel. Either way, the finding concerns endurance performance specifically, not muscle building or everyday gym work.