Nutritional Ergogenic Aids in Cycling: A Systematic Review

Two literature searches were conducted on 18 March 2024, using the PubMed (MEDLINE), Scopus, and Web of Science databases. The terms used in the first search were (“caffeine supplement*” OR “creatine supplement*” OR “sodium bicarbonate supplement*” OR “nitrate supplement*” OR “beta-alanine supplement*” OR “glycerol supplement*”) AND “cycling”. The terms used in the second search were ((“caffeine” AND “creatine”) OR (“caffeine” AND “sodium bicarbonate”) OR (“caffeine” AND “nitrate”) OR (“caffeine” AND “beta-alanine”) OR (“caffeine” AND “glycerol”) OR (“creatine” AND “sodium bicarbonate”) OR (“creatine” AND “nitrate”) OR (“creatine” AND “beta-alanine”) OR (“creatine” AND “glycerol”) OR (“sodium bicarbonate” AND “nitrate”) OR (“sodium bicarbonate” AND “beta-alanine”) OR (“sodium bicarbonate” AND “glycerol”) OR (“nitrate” AND “beta-alanine”) OR (“nitrate” AND “glycerol”) OR (“beta-alanine” AND “glycerol”)) AND “cycling” AND “supplement*”.

The present systematic review was prospectively registered in OSF (Registration Protocol: https://doi.org/10.17605/OSF.IO/6YN53↗) and conducted following the guidelines outlined in the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) 2020 statement [9]. The inclusion criteria were defined according to the PICOS criteria. Within this framework, the study aimed to enroll healthy adults as the target population. The intervention of interest encompassed the supplementation of caffeine, creatine, sodium bicarbonate, beta-alanine, nitrates, glycerol, or a combination thereof. Placebo was designated as the comparator for eligible studies. The outcomes of interest centered on the measurement of time or power output on a cycle ergometer or bicycle, comprising assessments such as the Wingate test, time trial, or exercise test to exhaustion. Study designs eligible for inclusion involved Randomized Controlled Trials (RCTs) or crossover design studies (CSs).

The inclusion criteria for the first search were studies on humans, conducted on adult populations, published in English, and published between 2019 and 2023. The inclusion criteria for the second search aligned with those established in the first; however, in the second search, the temporal range was expanded to include any studies published from inception to 2023. Literature review studies were excluded.

Two authors (AVM and PR) independently screened the articles obtained through the search following the established inclusion criteria. Any potential disagreements were resolved by a third reviewer (AL). Initially, duplicate studies were removed. Then, the titles and abstracts of the articles were assessed, followed by a review of the full texts of the studies that met the initial criteria. The entire screening process was performed using Zotero^® version 6.0.35.

The data from the included studies were independently extracted by two reviewers (AVM and PR) and thoroughly reviewed by a third reviewer to resolve any potential discrepancies (AL). Data extraction from the selected articles was performed using a standard table designed in Microsoft Word^® version 16.83. The following data were collected from the eligible studies: authors and publication date; study design; participant characteristics (sex, age, Body Mass Index (BMI), maximal aerobic capacity (VO₂ max), and peak maximal capacity (VO₂ peak)); supplementation protocol (dosage and duration); exercise protocol; and outcomes obtained (magnitude of improvement and level of significance).

The methodological quality of the included studies was independently assessed by two reviewers (AVM and PR), with a third reviewer (AL) providing oversight to resolve any potential discrepancies. The “Physiotherapy Evidence Database (PEDro)” scale was used (see Table 3) [10]. A standard table was designed using Microsoft Word^® version 16.83. The PEDro scale comprises 11 items for evaluating the methodological quality of clinical trials. Each item is scored as 0 (criterion not met) or 1 (criterion met). The maximum score is 11. Studies scoring from 9 to 11 were considered of high methodological quality, those scoring from 6 to 8 were considered of moderate quality, those scoring from 4 to 5 were rated as low quality, and studies scoring < 3 were considered of very low quality.

A total of 326 results were identified after conducting searches on PubMed (n = 60), Scopus (n = 86), and Web of Science (n = 180). Subsequently, following the exclusion of duplicates (n = 144), the remaining total was 182. After the inclusion criteria were evaluated, 141 articles were excluded based on title and abstract screening, resulting in a total of 41 articles. Additionally, a total of five articles were excluded after full-text screening due to combining supplemental treatment with hypoxia [11], lacking a control group consuming a placebo [12], not measuring performance variables such as time or power output [13], and not meeting the inclusion requirements for study design, as they were not randomized studies [14,15]. Finally, 36 articles were selected for this systematic review (Figure 1).

This section outlines the authors and publication date, study design, participant characteristics, supplementation protocol, exercise protocol, and results obtained. Out of the 36 identified trials, 5 studies examined the effectiveness of caffeine supplementation [16,17,18,19,20]. Two studies focused on analyzing the effects of creatine as a supplement [21,22], while six others focused on sodium bicarbonate [23,24,25,26,27,28]. Beta-alanine supplementation was evaluated in three studies [29,30,31], and eight studies assessed nitrates [32,33,34,35,36,37,38,39]. Notably, none of the trials specifically addressed glycerol supplementation.

Additionally, three studies evaluated the combined supplementation of caffeine and creatine in the context of cycling [40,41,42], while three studies investigated the combined use of caffeine and sodium bicarbonate [43,44,45]. One study examined the concurrent consumption of caffeine and nitrates [46], and another focused on supplementation with creatine and sodium bicarbonate [47]. Furthermore, four studies focused on combined supplementation with sodium bicarbonate and beta-alanine [48,49,50,51].

A total of 701 participants were included in the studies. Participant characteristics varied between 19 and 52 years of age, with BMIs ranging from 20.6 to 29.7 kg/m², the VO₂ max ranging from 41 to 71.1 mL/kg/min, and the VO₂ peak ranging from 41.4 to 63.2 mL/kg/min. Regarding the design of the studies included in the systematic review, there were a total of nine Randomized Controlled Trials and 25 crossover studies.

In Table 4, an analysis of the methodological quality of the studies included in the systematic review is provided using the “Physiotherapy Evidence Database (PEDro)” scale. The methodological quality of the studies examined in this review ranged from 6 to 10 points, with an average score of 7.75 points. A total of 9 articles exhibited high methodological quality, while 25 articles demonstrated moderate methodological quality. No articles were rated as having low or very low methodological quality.

A total of five studies investigated caffeine supplementation during cycling [16,17,18,19,20]. Four of them utilized the lowest recommended caffeine dosage (3 mg of caffeine per kg of body weight), while the remaining study employed a dosage of 6 mg of caffeine per kg of body weight [17]. Regarding the timing of the supplementation, the majority of the studies recommended consuming caffeine 60 min prior to exercise, while only one suggested doing so 90 min beforehand [16].

The efficacy of caffeine was assessed through time trials of various intensity levels and distances [16,17,20], as well as an adapted Wingate test [19] and an intermittent sprint test [18]. All studies included in the review revealed significant improvements in cycling performance, reflected in a 1.7 to 4.6% improvement in time needed to complete time trials and an increase of 2.53% in average and maximum power output for sprinting. Notably, although three of the five studies included participants of both sexes [16,19,20], no significant differences were observed between them (Table 5).

Among the studies included in this review that focused on investigating the effectiveness of caffeine supplementation in cycling, they demonstrated significant improvements in athletic performance in short-distance, high-intensity, and long-duration tests at moderate intensity.

Two studies on creatine supplementation were reviewed [21,22]. Both studies implemented a rapid loading protocol (Table 6).

In the study by Gordon et al. [22], 39 women performed an intermittent sprint test on a cycle ergometer. On the other hand, Schäfer et al. [21] conducted four to five constant load tests (80 rpm) on a cycle ergometer with 11 trained men, measuring the time to failure, which was approximately 3 min in both groups. The first study did not report significant improvements, while the second study observed a prolongation of time to fatigue by 11% in a constant load test.

Thus, the reviewed studies revealed mixed results in short-duration, high-intensity tests. Additionally, it is important to note that only two studies were included in this review, both of which focused on short-duration tests.

Among the six studies included in the review, a dose of 0.3 g of sodium bicarbonate per kilogram of body weight was used in four studies [23,24,25,26], while the lowest dose within the recommended range (0.2–0.4 g of sodium bicarbonate per kg of body weight) was used in two studies [27,28]. The supplementation protocols showed wide variability, ranging from 60 to 180 min prior to exercise (Table 7).

To evaluate the efficacy of sodium bicarbonate, high-intensity tests were conducted, including tests for exhaustion [23], time trials with distances between 2 and 4 km [24,27], and various high-intensity interval exercises [26]. In three out of the six analyzed studies, non-significant results were obtained regarding performance in the tests conducted [24,26,28]. In the study by Thomas et al. [26], which involved eight elite cyclists, although no significant improvements in performance were found, an improvement in perceived effort was observed among cyclists who consumed sodium bicarbonate compared to the placebo group. On the other hand, three studies reported significant improvements, with the magnitude of improvement ranging from 1.6 to 12% [23,25,27].

Regarding sodium bicarbonate supplementation, variable results were observed in its effectiveness for improving cycling performance, with half of the studies showing significant improvements.

The three selected articles for the review implemented the same supplementation protocol, in which 6.4 g of beta-alanine were administered daily over a period of 4 weeks to recreationally trained adult men in endurance sports [29,30,31].

The efficacy of beta-alanine use was evaluated through high-intensity exercises, including tests at 110% of their individual maximal work (W max) [30] and an adapted Wingate test, which involved repeated 10-s sprints and maximal effort tests [29]. Perim et al. [31] opted to implement a simulated road cycling protocol consisting of a 120-min constant load test on a cycle ergometer, with six maximum intensity sprints every 20 min during the test, followed by a 4-km time trial using a road bike [31]. None of the three studies reported statistically significant improvements in any of the performance measures evaluated during cycling.

In summary, the three studies considered in this review on beta-alanine supplementation did not show significant improvements in cycling performance during short-duration, high-intensity exercise or in a simulated road cycling protocol (Table 8).

Eight studies investigating nitrate supplementation during cycling were examined [32,33,34,35,36,37,38,39]. In one study, a dose of 6.4 mmol of nitrates was administered [34], while doses of 8 mmol of nitrates were used in two studies [36,38], a dose of 9.9 mmol of nitrates in one study [35], doses ranging from 12.4 to 13 mmol of nitrates in two studies [33,39], and a dose of 18.5 mmol of nitrates in another study [37]. One study mentioned only the consumption of 340 mg of nitrates without specifying the equivalence in mmol [32]. The supplementation protocols varied between 2 and 3 h prior to exercise, except in the study by Hennis et al. [37], which opted for chronic supplementation over 4 days.

To assess the efficacy of nitrates, various constant load tests were conducted, covering different intensities and durations [35,36,37,38], as well as incremental load tests to exhaustion [39], in addition to tests at 170% of their individual maximal work (W max) [32], a 10-km time trial [33], and a Wingate test consisting of successive 30-s sprints [34].

Significant improvements in cycling performance were identified in two out of the eight studies included in the review [33,34]. The studies that demonstrated these improvements were by Jodra et al. [34], who conducted a Wingate test on a cycle ergometer, and Rokkedal-Lausch et al. [33], who performed a 10-km test on a cycle ergometer. Both observed improvements in the time required to complete the exercise and in the power output during the tests. However, the remaining studies did not find statistically significant improvements in performance.

In summary, the results observed in this review regarding nitrate supplementation have been inconclusive (Table 9). Of the eight included studies, only two reported significant improvements in cycling performance. These studies included short-duration, high-intensity tests.

Three articles were identified as the result of the search. Of these three articles, two were not conducted on humans, and one did not investigate glycerol supplementation in the context of sports. Therefore, no study examining the impact of glycerol supplementation on performance in the context of cycling met the requirements for inclusion in the present review.

Although both this literature review and the AIS Group A classification address ergogenic aids, our review diverges by concentrating specifically on their efficacy within the context of cycling. This focus facilitates a nuanced examination pertinent to the distinct physiological and performance demands of cycling, assessing the effectiveness of these ergogenic aids in isolation, as well as their potential synergistic effects (Table 10).

The possible synergistic effect of caffeine and creatine supplementation was investigated in three studies [40,41,42]. Two of them, conducted by Lee et al. [41,42], reported significant improvements in cycling performance. An increase in mean power and peak power applied during a high-intensity cycling test on a cycle ergometer was observed, along with a 4.5% improvement in time to fatigue in the group supplemented with caffeine and creatine compared to the group supplemented with creatine and a placebo. However, the study conducted by Vanakoski et al. [40] showed non-significant improvements in variables such as total power applied and maximum pedaling speed during constant-load and moderate-duration cycling tests on a cycle ergometer.

Three articles were included in the review investigating the combined use of caffeine and sodium bicarbonate in cycling [43,44,45]. Among them, only Correia-Oliveira and Lopes-Silva [45] reported improvements in performance in a 4-km time trial. They observe a 2.3% reduction in the time required to complete the exercise and a 4.97% increase in applied power after supplementing with caffeine and sodium bicarbonate compared to the groups that consumed sodium bicarbonate or a placebo. However, in two other studies, no significant improvements in performance were detected when participants performed a 3-km time trial [43] or high-intensity cycling exercises to exhaustion on a cycle ergometer [44].

The only study investigating the combination of caffeine and nitrates in cycling did not find a synergistic effect when performing a 20-km time trial [46], as there were no significant differences between groups (p > 0.05).

Morris et al. (2016) did not observe a synergistic effect when investigating the combined supplementation of creatine and sodium bicarbonate, as significant improvements were only obtained in the group supplemented with creatine but not when incorporating sodium bicarbonate [47].

Four studies investigating the combined use of sodium bicarbonate and beta-alanine in cycling were included [48,49,50,51]. None of them found significant improvements in cycling performance when supplemented with sodium bicarbonate and beta-alanine, either in a four-minute test [49] or in high-intensity, short-duration tests [48,50,51], in parameters such as time to exhaustion, time to complete the test, or average power applied during the exercise.

Consequently, there is a lack of consensus in the results of the various studies regarding the different combinations. The combination of supplements that showed the most favorable results in terms of performance enhancement during cycling was caffeine and creatine (two out of three studies).