Bibliographic Details
| Title: |
Beyond Belief? Absence of Double-Blinding and Limited Ecological Validity in Carbohydrate Loading Research. |
| Authors: |
Jones, Robyn O.1,2 (AUTHOR), Louis, Julien1 (AUTHOR), Saunders, Bryan3 (AUTHOR), Morton, James P.1 (AUTHOR), Gejl, Kasper4 (AUTHOR), Nielsen, Joachim4 (AUTHOR), Areta, José L.1 (AUTHOR) j.l.areta@ljmu.ac.uk |
| Source: |
International Journal of Sport Nutrition & Exercise Metabolism. Sep2026, Vol. 36 Issue 5, p553-562. 10p. |
| Subject Terms: |
*Exercise physiology, *Health attitudes, *Skeletal muscle, *Fatigue (Physiology), *Medical research, *Physical fitness, *Research methodology, *Dietary carbohydrates, *Athletic ability, *Body movement, *Glycogen, *Nutrition |
| Abstract: |
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. Carbohydrates stored within muscle as glycogen are an essential fuel for endurance exercise performance. To increase these energy stores, athletes use a strategy called carbohydrate loading, where they consume large (above-normal) amounts of carbohydrates 1–2 days prior to a race. Introduced 60 years ago, this strategy had been shown to improve exercise capacity in running and cycling tests lasting >90 min, where glycogen stores are thought to become a limiting factor. However, historically in carbohydrate loading research, experimental conditions have not been concealed from participants and researchers (termed double blinding). This can introduce bias and influence results as participant belief can directly influence outcomes regardless of the treatments received through the placebo effect (e.g., believing a nutritional intervention improves performance can improve performance even if the actual nutritional intervention is absent). Many carbohydrate loading studies also tested participants after an overnight fast or without consuming any carbohydrates during exercise. This limits the application of these findings to real-world endurance sport scenarios, where athletes are recommended to consume large amounts of carbohydrates before and during exercise. We wrote this commentary to highlight this lack of blinding and limited simulation of endurance sport settings in carbohydrate loading research and to encourage researchers and practitioners to consider the influence of expectancy and belief on performance. To do this, we systematically scoped the scientific literature to identify all relevant studies to evaluate the use of blinding and carbohydrate provision during exercise in carbohydrate loading studies that assessed endurance performance (lasting > 90 min) following either a "normal" moderate daily carbohydrate intake (4–6 g of carbohydrates per kilogram of body mass) or a high "loading" intake (>7 g of carbohydrates per kilogram of body mass). We identified a total of 14 studies following these criteria. The three main findings of the commentary are as follows: (a) only four studies (29%) adhered to in-race fueling guidelines, and 10 studies (71%) failed to provide carbohydrates immediately before and/or during the performance test; (b) 12 studies (86%) lacked adequate participant and researcher blinding; and (c) two studies that implemented double-blind, placebo-controlled interventions did not demonstrate significant performance enhancements. Based on the current evidence, it remains impossible to distinguish the independent contributions of placebo and above normal preexercise muscle glycogen concentration (resulting from carbohydrate loading) to endurance performance. It should be emphasized that we are not stating that glycogen is not important for muscular fatigue or recommending low-carbohydrate high-fat diets in competitive sport settings. Our commentary highlights a specific comparison between normal and above-normal carbohydrate ingestion. Considering the current evidence, there is no direct proof for the existence or, equally, absence of a placebo effect of carbohydrate loading on endurance performance. This commentary re-evaluates whether carbohydrate loading in the days before endurance sports competition truly enhances performance once placebo effects are accounted for and in-race fueling is optimized. There is a need for more double-blind placebo-controlled studies to determine the potential role of placebo (such as belief) in performance outcomes and isolate the independent contributions of carbohydrate loading and placebo effects on endurance performance. [ABSTRACT FROM AUTHOR] |
| Database: |
Academic Search Index |