Beyond Belief? Absence of Double-Blinding and Limited Ecological Validity in Carbohydrate Loading Research

Carbohydrate loading (CL) has long been a cornerstone of endurance nutrition, yet its true efficacy remains uncertain. Although CL is commonly associated with improved time to fatigue and modest performance gains in exercise lasting ∼90-240 min, few studies have employed double-blind, placebo-controlled (DBPC) designs. Given that belief and expectation can independently influence performance, it remains unclear whether reported benefits reflect CL or placebo effects. We systematically evaluated the use of blinding and in-exercise carbohydrate provision in CL research. Of 14 identified studies, only two were DBPC trials. Notably, only four provided carbohydrates during exercise. Both DBPC studies included appropriate in-exercise carbohydrate intake and reported no evidence of performance benefit of CL. Of the remaining two studies that provided carbohydrates during exercise, CL showed a positive performance effect; however, the effect size was moderate and comparable to that reported for placebo nutritional interventions (Cohen's d ≈ 0.35). Comparisons across CL studies are heterogeneous in design, including differences in performance test type (time to exhaustion vs. time trial), which points to the need for more research in standardized conditions. The current evidence therefore calls for a re-evaluation of whether CL in the 24-48 hr before competition enhances performance once placebo effects are accounted for and in-race fueling is optimized. This commentary highlights the need for more DBPC studies and the potential role of placebo (such as belief) in observed performance outcomes. We propose future research directions to better isolate the independent contributions of CL and placebo effects in endurance performance.