The search for relevant studies was conducted in several databases, including PubMed, Scopus, Web of Science (WOS), EBSCO-SPORTDiscus, and Embase. The search covered the period from the inception of the databases to August 2023. The search strategy employed both subject headings and Boolean operators, primarily focusing on two main concepts: polyphenols and exercise performance. As an example, for the PubMed database, the search equation was as follows: (“Athletic Performance” [Mesh] OR Athletic Performances OR Performance, Athletic OR Performances, Athletic OR Sports Performance OR Performance, Sports OR Performances, Sports OR Sports Performances) AND (“Polyphenols” [Mesh] or Polyphenol or Provinols).

The inclusion and exclusion criteria for this systematic review are well defined according to the PICOS model: Population (P): healthy athletes and sports enthusiasts Intervention; (I): Supplementation with polyphenols or polyphenol compounds, including combinations of several polyphenols, with clear supplementation doses; Comparison (C): Comparison between intervention/experimental groups with similar characteristics and a placebo group; Outcome (O): Studies that include tests of athletic or sports performance before and after supplementation with polyphenols or polyphenol compounds, with available test data; Study (S): Randomized controlled trials with a parallel or crossover design, published in English. In addition, exclusion criteria were specified as follows: (i) Studies with confounding factors other than polyphenols. (ii) Studies with types of exercise programs that were not consistent with the focus of the research. (iii) Supplements containing non-standardized polyphenol compounds or supplements with unknown polyphenol content. (iv) Studies with incomplete or missing subject information, experimental protocols, or procedures. (v) Studies with no data available for extraction. (vi) Inaccessible full-text articles by legitimate means. (vii) Studies conducted in specific environments not relevant to the research.

The search for relevant published literature was conducted jointly by two authors in a standardized manner to assess the eligibility of the literature. Any disagreements that arose during the screening process were resolved through discussion and negotiation. During the screening process, all duplicate articles and non-clinical trial literature were removed. The screening process included the examination of titles and abstracts of the literature, including relevant articles that contributed to the research. Finally, the literature that met the inclusion criteria was selected for full-text reading and review.

After applying the eligibility criteria, the following information was extracted from each included literature: Bibliographic information (author(s) and year of publication), study design: (whether the study design was parallel or crossover, information on blinding of subjects, sample size), population characteristics (age of participants, gender distribution, height and weight data, if available), details of polyphenol supplementation: (types of polyphenol supplements used, dosage of polyphenol supplements, timing and method of supplementation). (i) Performance measures: time to complete the trial, time to fatigue during exercise, power output, distance covered, speed achieved, maximum aerobic velocity (MAV), Rate of Perceived Exertion (RPE), exercise economy, training intensity. (ii) Metabolic metrics: maximum oxygen consumption (VO2max), oxygen consumption (VO2), carbon dioxide production (VCO2), respiratory rate (RR), heart rate (HR), energy metabolism parameters such as carbohydrate oxidation (CHOox) and fat oxidation (FATox), blood glucose concentration (glucose), blood lactate concentration (lactate), blood lactate concentration at certain points (B [La]). (iii) Antioxidant Capacity Metrics: high-density lipoprotein (HDL) levels, low-density lipoprotein (LDL) levels, total antioxidant capacity (TAC), aspartate aminotransferase (AST) levels (U/L), and other relevant markers of antioxidant capacity.

A total of 11 trials were included in this systematic review. Of these 11 trials, seven had male participants and three had mixed-gender groups. The initial search of Scopus, PubMed, Web of Science (WOS), SportDiscus, Embase, and Cochrane yielded 491 relevant articles. After removing 104 duplicate articles, 387 remained. After reviewing the titles and abstracts, 300 articles were excluded as they did not meet the inclusion criteria. A total of 87 studies were reviewed, of which 76 were subsequently excluded (for reasons outlined in the flowchart, Figure 1).

The two authors used the Cochrane Collaboration tool from the Cochrane Handbook for Systematic Reviews of Interventions (version 5. 1. 0) to assess the risk of bias in the included randomized controlled trials (RCTs). The assessment criteria included: (i) Sequence generation (selection bias). (ii) Concealment of allocation (selection bias). (iii) Blinding of participants and staff (performance bias). (iv) Blinding of outcome assessors (detection bias). (v) Incomplete outcome data (attrition bias). (vi) Selective reporting (reporting bias). (vii) Other biases. Each criterion was rated at three levels: low risk, unclear risk, and high risk, to assess the risk of bias in each study (Figures 2, 3).

Blackcurrants are a fruit rich in polyphenolic compounds that we use every day. In addition to small amounts of flavanols and flavonols, they also contain anthocyanins and various glycosides (Cook et al., 2015). Cyanidin is a type of flavonoid with certain anti-inflammatory (Zhu et al., 2013) and antioxidant (De la Cruz et al., 2013) properties. Besides blackcurrants, several food sources are rich in cyanidin, main fruits such as grapes, Montmorency cherries, and others (Bowtell and Kelly, 2019). Several trials included in this review used cyanidin-rich polyphenolic compounds as interventions. These included blackcurrant extract capsules (Cook et al., 2015; Willems et al., 2015), Montmorency cherry capsules (Morgan et al., 2019), Haskap berries (Howatson et al., 2022), grape and apple extract capsules (Deley et al., 2017) and grape juice (de Lima Tavares Toscano et al., 2020). Previous research consistently indicates that anthocyanins and polyphenol-rich foods have a positive impact on oxidative stress, inflammation, and muscle recovery indices (Howatson et al., 2010; Bell et al., 2014; Bell et al., 2015; Bowtell and Kelly, 2019). However, in previous reviews or meta-analyses, there has been a relative scarcity of data or analysis concerning exercise performance (Kimble et al., 2023). Looking at the combined literature, we find that supplementation with these cyanidin-rich polyphenols has shown improvements in several exercise performance-related indicators. In de Lima et al. (de Lima Tavares Toscano et al., 2020), athletes ran for an average of 59.2 ± 27.8 min and covered an average distance of 12.6 ± 6.3 km after consuming a placebo drink until exhaustion. However, after ingesting purple grape juice, they exhibited significantly better performance, running on average 9.2 min longer than the placebo group, representing an 18.7% improvement, and covering an additional 1.9 km compared to the placebo group. Similarly, in Nho et al. (Nho and Kim, 2022), the time to exhaustion during exercise was 128.9 ± 53.0 s in the placebo group and significantly increased to 134.4 ± 58.4 s in the green tea extract group. Gaelle Deley et al. found a noteworthy increase in exercise-to-fatigue time (+9.7% ± 6.0%) in the Vinitrox™ group compared to the placebo group in the endurance test (Deley et al., 2017). In Morgan et al. (Morgan et al., 2019), a randomized controlled trial showed that time trial (TT) completion time based on Montmorency cherries (1,506 ± 86 s) was 4.6% ± 2.9% faster than that based on the placebo (PL) (1,580 ± 102 s), indicating that polyphenol supplementation could reduce race completion time.

Additionally, Cook et al. (Cook et al., 2015) found that supplementing with New Zealand blackcurrant, compared with the placebo group, reduced the 16.1 km running time from 1722 ± 131 s in the control group to 1,678 ± 108 s in the supplement group, resulting in a 2.4% performance improvement. Furthermore, Howatson et al. (Howatson et al., 2022) found that the Haskap group, supplemented with Haskap berries, improved their 5 KM time trial performance by approximately 21 s compared to the placebo group, equivalent to an increase in average running speed of 0.25 km/h, representing a performance improvement of more than 2%. In 2015, Roberts et al. (Roberts et al., 2015) recruited 14 men to participate in a randomized controlled trial by randomly assigning either a supplement capsule containing dGTE or a placebo for 4 weeks, followed by a 40-min performance trial at weeks 0, 2, and 4. The use of dGTE led to a gradual increase in distance covered, from 20.23 ± 0.54 km in week 0–21.77 ± 0.49 km in week 2, and finally, to 22.43 ± 0.40 km in week 4, showing a significant increase of 10.9%. Similarly, a significant increase in average power output was observed. The average power output increased by 17.9% from week 0 (162.06 ± 10.08W) to week 2 (191.08 ± 10.85W), and from week 0 to week 4 (198.91 ± 8.61W) increased by 22.7%. This suggests that supplementation with polyphenols, primarily cyanidin-rich compounds, benefits both recreational and professional athletes in terms of aerobic endurance performance, despite variations in supplement form and dosage. The mechanism behind the improvement in exercise performance with cyanidin supplementation is related to the improvement in endothelial function. The mechanism by which supplementing anthocyanins improves exercise performance is related to the enhancement of endothelial function. NO is an efficient vasodilator capable of mediating and regulating vascular function and blood flow during exercise (Ferguson et al., 2013; Lee et al., 2015). By increasing the production of endothelium-derived vasodilator NO, supplementation of anthocyanins can mediate endothelium-dependent vasodilation induced in the thoracic aorta of rats (Nakamura et al., 2002). Another non-glycosylated anthocyanin in blackcurrants, delphinidin, can also increase endothelial NO production to vasodilate blood vessels by elevating intracellular Ca²⁺ concentration in endothelial cells. This increase in NO, along with a reduction in NO free radical breakdown, contributes to enhanced peripheral blood flow (Martin et al., 2010).

Recruiting 12 elite athletes, Nho et al. (Nho and Kim, 2022) conducted a study to compare the effects of GSE and placebo on endothelial function during 14 days of progressive cycling. Brachial endothelial function was assessed using Flow-Mediated Dilation (FMD). The results indicated that GSE led to an increase in brachial artery diameter induced by FMD (14.4% ± 5.2% vs. 17.6% ± 4.5%). The study demonstrated that long-term supplementation of GSE improved endurance performance, possibly attributed to the vasodilation of active skeletal muscle mediated by enhanced endothelial functio. (Howatson et al., 2022). Haskap berries containing cyanidin-3-O-glucoside (C3G) have been shown to increase mitochondrial biogenesis, improve muscle function, and enhance exercise performance in rodents (Matsukawa et al., 2017; Saclier et al., 2020). In the literature we examined, a study on Haskap berries supplementation caught our attention. The research involved 30 male recreational runners in a double-blind, placebo-controlled, independent group design. The participants were randomly assigned to either the Haskap berries intervention or an isocaloric placebo control to investigate the impact of Haskap berries on parameters related to endurance running performance. The study observed slight changes in heart rate and VO_2max at submaximum intensity. Notably, during the VO_2peak test, the Haskap group showed a 20-s extension in exercise-to-fatigue time, representing a meaningful improvement in the context of human running performance (Deley et al., 2017). During intense exercise, upregulation of antioxidant genes and protein expression mediated by Nrf2 through C3G helps maintain muscle function. Therefore, Haskap berries may potentially enhance performance by modulating endothelial function pathways through C3G mediation (Xu et al., 2004; Edwards et al., 2015). It is therefore speculated that the improvement in vascular endothelial function leads to increased O₂ utilization at low intensities. This may explain changes in associated fatigue times and TT race performance. Future research should refine this concept more systematically and strive to comprehensively investigate various aspects of the proposed mechanism to gain a more enriched understanding of the impact of dietary anthocyanins on exercise performance. Attempting to elucidate a singular mechanism through simplified approaches may not be practical.

In this review, we found that consuming polyphenols may have an impact on fat metabolism. In Cook et al. (Cook et al., 2015) well-trained endurance athletes who supplemented with blackcurrant extract capsules for 1 week showed a significant difference in the outcome measure FATox between the experimental group (0. 44 ± 0. 12) and the placebo group (0. 37 ± 0. 15). This suggests that supplementation with blackcurrant extract capsules may enhance whole-body fat oxidation during moderate-intensity exercise. The appearance of lactate may have been influenced by the effect of anthocyanins on substrate oxidation, potentially leading to an increased contribution of fat oxidation at relatively low intensities (Willems et al., 2015). Tsuda et al. (Tsuda et al., 2005) in 2005 found that glycoside-treated adipocytes upregulated genes related to fat metabolism and signaling. Similarly, Benn et al. (Benn et al., 2014) found that long-term intake of blackcurrant extract increased energy metabolism-related genes and mRNA levels in C57BL/6J mice. Therefore, the increase in fat oxidation may result from the combined effects of multiple pathways including the upregulation of genes related to fat oxidation, transportation of fatty acids to mitochondria, enhanced availability of nitric oxide, and increased peripheral blood flow. Similar findings were replicated in another study involving green tea extract. Roberts et al. (Roberts et al., 2015) enrolled fourteen recreationally active males and randomly administered either green tea extract or a placebo. The results revealed a noteworthy increase in the total fat oxidation rate within the dGTE group, rising from 0.241 ± 0.025 g/min to 0.301 ± 0.009 g/min, marking a substantial increase of 24.9%. The benefits of green tea extract on the human body are primarily associated with the catechin polyphenols, a significant proportion of which is EGCG (Randell et al., 2014), EGCG upregulates cell signaling not only through antioxidant protective mechanisms but also via the mediation of PGC1α (Hodgson et al., 2013), SIRT1 and the mAPK pathway (Vincent et al., 2013; Kim et al., 2014). Over a longer period (>4 weeks), moderate doses of EGCG may promote the upregulation of genes associated with fat metabolism during exercise, thereby enhancing whole-body fat oxidation. Some studies also suggest that EGCG may improve glucose tolerance, insulin sensitivity, and adiponectin levels, further supporting this notion (Potenza et al., 2007; Venables et al., 2008). In contrast, Martinez et al. (Martinez-Noguera et al., 2019) in 2019 showed that acute supplementation with hesperidin increased activation of the intracellular signaling pathway AMPK, induced changes in PGC1α activity, and promoted the use of fat as an energy substrate. Increased FATox implies a degree of glycogen sparing during moderate-intensity exercise. Howatson et al. (Howatson et al., 2022) found that HR and VO₂ were lower at a certain exercise intensity (lactate threshold). As fat oxidation consumes more oxygen than carbohydrate oxidation, fat may not be the preferred energy substrate under these circumstances.

Similarly, at moderate intensity (50% VO_2max), subjects’ FATox increased from 0. 241 ± 0. 025 to 0. 301 ± 0. 009 g-min-1 after supplementation with green tea extract, an improvement of approximately 24. 9%. However, the magnitude of the increase was more significant than in the previous study. The subjects in this study were recreationally active, so it has been suggested that the combined effect of exercise training and polyphenol supplementation may be more appropriate for untrained individuals (Randell et al., 2014). In the studies considered, grape seed extract exhibited no significant impact on the performance of primary basketball players (Nho and Kim, 2022). However, among non-endurance sports athletes, supplementation with carob demonstrated a significant improvement in maximal aerobic velocity (MAV) and performance in the yoyo test compared to baseline for professional taekwondo athletes. Gaamouri et al. (Gaamouri et al., 2019) recruited 23 taekwondo athletes for their experiment. Prior to the study intervention, there were no significant differences between the groups. After the intervention, the carob supplement group showed significant improvements of 92.43% in distance and 12.18% in MAV, whereas the placebo group only exhibited improvements of 40.37% and 4.95%, respectively. This result indicates that polyphenol supplementation can effectively enhance the athletic performance of professional athletes. Carbohydrates and fats are the most important substrates for energy metabolism during exercise. However, the proportion of energy contribution from these substrates varies with different exercise durations and intensities. During low to moderate-intensity exercise (up to 60% of VO_2max), the absolute value of fat oxidation increases (Dandanell et al., 2017). As exercise intensity further increases, the absolute rate of fat oxidation decreases, and carbohydrates become the primary energy substrate (Dandanell et al., 2017). In the process of exercise, the ability to oxidize fats at a high rate is considered an advantage for endurance-trained athletes. Muscle glycogen stores are relatively small; therefore, theoretically, any intervention that enhances skeletal muscle fat oxidation capacity could lead to glycogen sparing and thereby enhance endurance (Randell et al., 2014). Endurance exercise training leads to skeletal muscle adaptations that favor fat metabolism (Tunstall et al., 2002). Trained individuals exhibit higher absolute fat oxidation rates compared to untrained populations (Nordby et al., 2010). However, the increase in fat oxidation following polyphenol intervention is relatively smaller in trained individuals compared to untrained ones, explaining the more significant impact of polyphenols on exercise performance in untrained individuals or those engaged in non-endurance activities. In recent years, the topic of olive-derived supplements has gained popularity. In Roberts et al. (Roberts et al., 2023)in 2023, an olive-derived supplement rich in hydroxytyrosol, an important polyphenol, may support endogenous antioxidant mechanisms related to mitochondrial respiratory capacity, such as upregulation of PGC-1α (Feng et al., 2011; Wood Dos Santos et al., 2018), This study showed no difference in VO_2max performance during 75% moderate-intensity exercise, which may be related to regular aerobic exercise habits. Further research is needed to refine the types of polyphenols, dosages, and exercise protocols for different populations.

This review suggests that prolonged use of specific polyphenol supplements may promote whole-body fat oxidation during moderate-intensity exercise, which may benefit aerobic endurance performance in both professional athletes and sports enthusiasts. This may have implications for how people utilize energy substrates and plan their dietary strategies. In terms of supplementation strategies, only three out of the 11 trials included acute supplementation before exercise. Acute polyphenol supplementation was found to improve exercise performance compared to continuous supplementation, although the effect was not statistically significant. In Martinez et al. (Martinez-Noguera et al., 2019), acute ingestion of 500 mg of 2S-hesperidin (Cardiose^®) was administered to investigate its impact on athletic performance. The best data from repeated sprint trials showed significant differences between Cardiose^® and placebo in mean power (+2.27%), maximum speed (+3.23%), and total energy (+2.64%). While acute polyphenol supplementation improved performance in repeated sprint tests, the average performance did not differ significantly from the placebo group. Similarly, in a separate study, Deley et al. (Deley et al., 2017) explored the effects of acute intake of grape and apple polyphenols on endurance exercise capacity. Volunteers were randomly assigned to either take 500 mg of polyphenols or a placebo the night before and 1 h before the test. The mean duration of the maximum endurance test significantly increased in the polyphenol group compared to the placebo group (+9.7% ± 6.0%). The maximal perceived exertion was reached later with polyphenols (+12.8% ± 6.8%). Another study (de Lima Tavares Toscano et al., 2020) investigated the effects of a single dose of grape juice on runners’ physical performance. In a running test up to exhaustion (80% VO_2peak) after consuming a placebo drink, athletes ran for an average of 59.2 ± 27.8 min and covered an average of 12.6 ± 6.3 km. During the course after ingestion of purple grape juice, they demonstrated significantly enhanced performance, running for an average of 9.2 min longer, representing an 18.7% improvement, and a 1.9-km increase in distance compared to the placebo group. Among the studies on acute supplementation included, this one exhibited the most substantial improvement in athletic performance. However, in terms of endurance exercise distance, long-term polyphenol intake yielded a greater enhancement in athletic performance. Gaamouri et al. (Gaamouri et al., 2019) designed a 6-week double-blind randomized parallel fully controlled training study with pre- and post-measurements. Analysis of aerobic activity for 6 weeks before and after carob supplementation showed that the total distance covered increased from 847.2 ± 473.9 m to 1,494.9 ± 619.2 m, representing an increase of 92.43%. Regarding average power output, there was no significant change in average power output in Martinez et al. (Martinez-Noguera et al., 2019) results which is acute polyphenols supplementation. On the other hand, Roberts et al. (Roberts et al., 2015) found in his 2015 study that supplementation of green tea extract capsules for 4 weeks resulted in a notable increase in average power output with dGTE by 17.9%, or 29.02 ± 5.53W from week 0 (162.06 ± 10.08W) to week 2 (191.08 ± 10.85W), and by 22.7%, or 36.85 ± 3.20W from week 0 to week 4 (198.91 ± 8.61W). Metabolic parameters also showed no significant differences in these two trials. This may be due to the small sample size, as only two articles existed. Because the bioavailability of phenolic compounds in different supplements has not been well established, the availability of supplements cannot be determined before starting an exercise program. Caution should be taken when comparing studies involving different types of polyphenols, as their bioavailability may vary and subsequent interactions with other nutrients taken at the same time may influence the results of the comparison (Myburgh, 2014).

In this review, the average intake of polyphenols was 229. 04 mg/day, which is equivalent to about 66 g of dark chocolate, 83 mg of green tea, and 99 mg of mixed berries (blackcurrants, strawberries, and blackberries). In these 11 trials, specific individuals and combinations of polyphenols were used as interventions, and the forms of intake varied. Five trials used capsules (Cook et al., 2015; Roberts et al., 2015; Deley et al., 2017; Morgan et al., 2019; Nho and Kim, 2022), three trials used water-soluble forms (Gaamouri et al., 2019) or yoghurt (Howatson et al., 2022) as interventions, and there was also oral consumption through snacks or canned forms (de Lima Tavares Toscano et al., 2020; Roberts et al., 2023). Of these, only one study supplemented carob pods in a water-soluble form, and the participants in this study increased their endurance running distance by 92. 43%, which was a significantly greater improvement compared to other supplementation methods (Gaamouri et al., 2019). However, this supplementation method was only mentioned in one study in the literature, making it difficult to draw qualitative conclusions. Further research is needed to address the complex issue of the relative effects of different forms of polyphenol intake on athletic performance. Analysis of the performance indicators in the 11 studies showed that long-term polyphenol supplementation can significantly improve endurance exercise performance. The most significant indicator was a 92. 43% increase in endurance running distance and this trial had the longest duration of polyphenol supplementation of all 11 trials, lasting 6 weeks (Gaamouri et al., 2019). In contrast, another study found that acute polyphenol supplementation resulted in an increase in endurance running distance of approximately 18. 7% (de Lima Tavares Toscano et al., 2020). This suggests that long-term polyphenol supplementation may have a more pronounced effect than acute supplementation.

It is important to note that existing research suggests that high doses of antioxidant supplementation appear to have a detrimental effect on endurance performance (Paulsen et al., 2014). High doses of antioxidant supplements can shut down those cell signaling pathways sensitive to redox changes, thereby reducing the synthesis of new muscle mitochondria and the production of endogenous antioxidants (Kang et al., 2009; Hawley et al., 2011; Feng et al., 2013). Crucially, the health benefits and performance enhancements brought about by endurance training seem to be somewhat related to this cellular adaptation (Coffey and Hawley, 2007; Ristow and Zarse, 2010). The results showed that supplementation with high doses of antioxidants reduced the increase in cytochrome c oxidase subunit IV(COX4) induced by endurance training in the vastus lateralis muscle (Paulsen et al., 2014). While antioxidant supplementation does not affect the short-term improvement in endurance performance, it may negate the beneficial long-term adaptive effects of endurance training on cells. This suggests that individuals who frequently engage in endurance training should be cautious about using high doses of antioxidants. In contrast, in this study, long-term supplementation with polyphenols had more pronounced effects than acute or short-term ones. This may be related to the dose of polyphenols included in the 11 studies and the different bioavailability of polyphenols compared to other antioxidants (Di Lorenzo et al., 2021), necessitating future research to explore whether high-dose polyphenol supplementation affects those signaling pathways related to improvements in endurance performance.

It should be noted that due to their complex structures and higher molecular weights, polyphenols are not completely absorbed in the gastrointestinal tract. Instead, they are primarily biotransformed into low molecular weight, biologically active phenolic metabolites in the large intestine through the action of the gut microbiota (Hill et al., 2014; Ma and Chen, 2020). These low molecular weight metabolites produce antimicrobial substances, regulate the host immune system, and inhibit the production of bacterial toxins. They also have a certain preventive effect on some chronic diseases (Cerdá et al., 2005; Espín et al., 2013; Markowiak and Śliżewska, 2017). Furthermore, polyphenols can modify the composition of the gut microbiota, promoting the growth of beneficial microbes, such as lactobacilli and bifidobacteria, which are two major probiotics beneficial for human health (Wang et al., 2022). In interaction, the gut microbiota also metabolizes polyphenols, producing biologically active metabolites, such as short-chain fatty acids, which further impact the host’s health (Wang et al., 2022). The bioactivity of polyphenols and their metabolites in the body is likely mediated through these metabolites. These metabolites are generated in the body, and recent studies have confirmed that these molecules may have antioxidant and anti-inflammatory properties (Di Lorenzo et al., 2021). The bioavailability of polyphenols in the small intestine is low, mainly due to their interactions with food matrices, liver-mediated metabolic processes (both primary and secondary metabolism), and metabolic processes in the gut and microbiota (Di Lorenzo et al., 2021). However, the activity demonstrated by these compounds through their metabolites in the organism suggests that long-term intake of polyphenols can stabilize and alter the composition of the gut microbiota, thus promoting more beneficial health effects. The aforementioned supplementation with polyphenols can improve the utilization rate of fats as an energy substrate, and polyphenols are also positively correlated with anti-lipogenesis. Polyphenols have been shown to effectively activate the browning of adipose tissue, reducing obesity and lipid accumulation by inducing the browning of beige fat cells (Hu et al., 2020). Daily intake of beverages rich in catechins can increase the density of brown adipose tissue in healthy young women, supporting polyphenols’ role in brown fat genesis (Nirengi et al., 2016). In mice on a high-energy diet, ferulic acid accelerated thermogenesis and mitochondrial synthesis in brown adipose tissue and inguinal white adipose tissue (Han et al., 2018). Brown adipose tissue consumes energy more efficiently, contributing to weight management and improved body fat distribution, which is very important for maintaining good athletic performance and endurance. In recent years, polyphenols have attracted attention for their neuroprotective effects, showing potential effectiveness in reversing neurodegenerative pathology and age-related cognitive decline. Animal studies have shown that blueberries can improve spatial memory deficits in rats, (−)-epigallocatechin enhanced the retention of spatial memory in mice, curcumin could break down plaques and restore neurites in Alzheimer’s disease models, and resveratrol reduced Aβ aggregation in rat hippocampal cells by activating specific protein kinases (Casadesus et al., 2004; Dasgupta and Milbrandt, 2007; Garcia-Alloza et al., 2007; Praag et al., 2007). However, the direct link between polyphenols and improvements in neural health has not been clearly established.

This review has some limitations that should be considered. Firstly, a meta-analysis could not be performed due to the heterogeneity in study design, populations, training status/types, and supplementation methods. Secondly, some studies used VO_2max as a recognized performance indicator, but some subjects may have reached VO_2peak during testing due to their training habits and routines, leading to inherent measurement errors that could potentially affect the data (McArdle et al., 2006). Additionally, we did not classify the participants, which to some extent affected the comparison of the impact of polyphenols on exercise performance. Each of the 11 articles included in this review had varying durations of intervention and small sample sizes, making it difficult to effectively conduct a qualitative analysis of polyphenol supplementation. In addition, in the articles by Martínez et al. (Martinez-Noguera et al., 2019) and Willems et al. (Willems et al., 2015) the participants were aware of the allocation scheme or were made aware of it, which may have biased the results. Furthermore, in Willems’s article (Willems et al., 2015), there was a lack of complete reporting of the performance scores of each participating subject, which could also introduce bias in the results. There are several important considerations for future research. Firstly, more emphasis should be placed on investigating the bioavailability of polyphenols, as this may help to determine the optimal dosage and supplementation methods for different types of exercise, and thus have beneficial effects on athletic performance. Secondly, considering participant categorization and specialization is important. Further investigations can ascertain the potential applications of polyphenols and their energetic effects on various categories of participants and specialized athletes. In addition, few studies have included comprehensive dietary controls. As the sources of polyphenol intake are diverse, efforts should be made to quantify the intake of polyphenols in participants’ diets using placebo-controlled designs, as individuals with low intakes may show more positive responses to dietary interventions. This will help to better explain the research findings.