Comment on: “Effects of Carbohydrate Mouth Rinse on Cycling Time Trial Performance: A Systematic Review and Meta-analysis” and Subsequent Comment/Author Reply from Li et al.

Abstract

Recently, Brietzke et al. [1] presented a systematic review and meta-analysis on the effects of a carbohydrate (CHO) mouth rinse on cycling performance. This review received some criticism concerning the effects of the CHO mouth rinse in subjects in the fed and fasting states [2]. The authors replied to this criticism by including a new subgroup analysis for subjects who fasted overnight versus those in a restricted alimentary state [3]. Unfortunately, the review and letters contained substantial miscalculations and misinterpretations that could confuse the reader and lead to inaccurate conclusions. In the present letter, we discuss our three main concerns about the outcomes of Brietzke et al. [1, 3] and Li et al. [2]. Firstly, the authors stated that “A carbohydrate mouth rinse improves mean power output, but not the time to complete a cycling time trial” [1]. This statement could have been true if the time trial had been conducted outdoors, where environmental conditions such as wind may have impacted the results. However, all of the studies were carried out in the laboratory. Therefore, this statement could confuse readers who are unfamiliar with athletic performance measures. Nevertheless, the authors state in their discussion (Sect. 4.2) that these results are from Lane et al. [4], a study that only presented the highest standardized mean difference (SMD) for power output, but not for time taken to complete the time trial (we have some concerns about this SMD, as discussed below). The key to time-trial performance is the completion of a set distance or work in the shortest time possible, so the cyclist should produce the highest average power. This relationship can be expressed as power = k(speed)x, where k and x are constant for a particular mode of exercise and for a specific ergometer in cycling [5, 6]. Therefore, ∆% for power = (∆% for speed, distance, or time)x, and x = 2.2 for the ergometer used by Lane et al. [4] (https ://www.cycle ops. com/post/blog-15-cycle ops-scien ce-resis tance -curve s). For example, a change of 1% in speed/distance or − 1% in time is equivalent to a change in power of 2.2%. The inverse of this relationship is also applicable. Although the power output is the best parameter to use to measure performance effects [5–7], a simple conversion that is commonly employed in meta-analysis [5, 8–11] could have been used for the data provided by Lane et al. [4]. This would have changed the conclusions of Brietzke et al. [1, 3] and Li et al. [2] that CHO mouth rinse improved power output but not the time taken to complete the time trial. Secondly, when we converted the results of Lane et al. [4], we checked the data in the original study and discovered that Brietzke et al. [1] used standard errors of the mean (SEM = √SD ÷ n) to calculate the SMD, not between-subject SDs (e.g., Cohen’s d = (M1 − M2) ÷ (√((SD 2