A number of factors contribute to success in sport, and diet is a key component. An athlete's dietary requirements depend on several aspects, including the sport, the athlete's goals, and the environment. The importance of individualized dietary advice has been increasingly recognized, including day-to-day dietary advice and specific advice before, during, and after training and/or competition. Athletes use a range of dietary strategies to improve performance. As an example, maximizing glycogen stores is a key strategy for many. Dietary supplements are used for a variety of reasons by athletes and recreational exercisers (1,2). The role of polyphenols in athletic performance is only now beginning to emerge. Polyphenols operate at several levels including gene activation which leads to increased mitochondrial efficiency and increased blood flow to deliver more oxygen to the mitochondria. In particular, replacing damaged mitochondria while simultaneously replacing them with newly synthesized mitochondria is a key function of polyphenols. Since mitochondria supply 85% to 95% of the energy to a muscle cell, the more efficient the mitochondria are, the greater the athletic performance.
Exercise-Induced Oxidative Stress
Humans and other aerobic organisms constantly produce free radicals as part of normal metabolic processes. Free radicals are defined as molecules or molecular fragments with one or multiple unpaired electrons in the outermost orbital. Free radicals derived from oxygen are called reactive oxygen species (ROS). However, ROS also includes nonradicals, and are closely related to other free radical families, such as reactive nitrogen species. Superoxides (O2−) and nitrogen monoxide are the primary free radicals that trigger a chain reaction of hydrogen peroxide (H2O2), hydroxyl radicals (OH−), peroxynitrite (ONOO−), and hypochlorous acid (HOCl). Although these free radicals have positive effects in immune reactions and cellular signaling, they also are known to have negative effects, such as oxidative damage of lipids, proteins, and nucleic acids (3).
Organisms are equipped with antioxidant defense systems that protect cells from the toxic effects of free radicals divided into enzymatic antioxidants, such as superoxide dismutase, catalase, and glutathione peroxidase, and nonenzymatic antioxidants, such as vitamin C, vitamin E, glutathione, and bilirubin (4).
The oxidation-reduction (redox) balance is maintained in vivo by a complex regulatory mechanism; however, various physiological stimuli (i.e., radiation, alcohol use, smoking, and exercise) disturb this balance toward oxidation, thus inducing oxidative stress. Oxidative stress was defined as “disturbance of the oxidation-reduction balance in favor of oxidants, leading to a disturbance in redox signaling and control and/or molecular damage” (5).
Free radicals and ROS are the main oxidizing agents in cellular systems and are involved in aging and the onset of many types of diseases (6,7). They are physiologically produced in different cellular biochemical reactions occurring in the body, such as in mitochondria for aerobic oxygen production, in fatty acid metabolism, in drug metabolism and during activity of the immune system. ROS, within physiological concentration, are important signaling molecules that regulate growth, proliferation, and differentiation, and are responsible for some key adaptations to exercise performance at the tissue and cellular levels, for example, antioxidant enzyme regulation, mitochondrial biogenesis, and skeletal muscle hypertrophy (8).
Exercise may indeed increase oxygen consumption (V̇O2) up to 20 times above resting values. In the mitochondria of muscle cells, this translates to 200-fold greater oxygen utilization and the subsequent production of a large amount of ROS (9,10).
During exercise, ROS are produced by skeletal muscle from a range of sources, including phospholipase A2 and enzymatic sources such as nicotinamide adenine dinucleotide phosphate oxidase and xanthine oxidase (9). ROS are important signaling molecules and have been implicated in contraction mediated increases in muscle glucose uptake and the control of skeletal muscle blood flow. For instance, hydrogen peroxide has been shown to cause vasodilation in exercising muscle. It appears that under conditions of low oxidative stress and redox perturbation, that is, during rest or low-intensity exercise, ROS promote optimal vasodilation and hyperemia in exercising muscle. In contrast, excessive development of ROS derived during intense physical exercise can impair blood flow and vasodilatory capacity. Moreover, excess ROS generation has been shown to impair calcium handling and sensitivity resulting in reduced contractile force development, thus impairing exercise performance (11).
Furthermore, ROS also orchestrate the activation of redox-sensitive signal pathways that control cytokine production and muscle adaptation, such as those involved in the activation of nuclear factor (NF)-κB, nuclear factor of activated T cells, nuclear factor erythroid 2-related factor (Nrf2) and heat shock proteins, and peroxisome proliferator-activated receptor-gamma coactivator-1α. Furthermore, several studies have clearly indicated that muscle ROS and NF-κB also activate other important cell signaling pathways, leading to skeletal muscle adaptations to exercise, such as mitochondrial biogenesis and endogenous antioxidant defense (9,12).
Prolonged, high-intensity, strenuous, or unaccustomed bouts of exercise have been experimentally associated with increases in contractile-induced damage and inflammation reactions in skeletal muscle (10,13). Therefore, while exercise-induced inflammation is necessary for muscle repair and adaptation, the uncontrolled proliferation of inflammatory cells and oxidants can exacerbate muscle damage and impair muscle function (10,13).
The growing evidence on exercise-induced oxidative damage and impairment of athlete performance has spurred intense research on the evaluation of muscle protection by antioxidant supplementation in exercising individuals.
Although exercise-induced ROS production is an important signaling pathway to induce biological adaptations to training, ROS over production also could have a deleterious impact on cells and tissues. This concern has led some experts to suggest consuming more dietary antioxidants and antioxidant-containing supplements to mitigate the ROS production that can cause excess oxidative stress during and after exercise.
This belief that dietary supplements are helpful, or at least safe, when used in conjunction with an exercise program; however, has recently been questioned (14). For example, the supplementation with vitamin C (1000 mg·d−1) and E (400 IU·d−1) blunted some of the beneficial effects of exercise, such as improved insulin sensitivity, mitochondrial biogenesis level, and antioxidant enzyme activity, in 19 untrained and 20 pretrained healthy young individuals, and that exercise alone produced a better outcome (15). At present, it is unclear whether antioxidant supplements enhance or attenuate exercise training adaptive biological responses in either healthy adults or lower-functioning older adults.
What Are Polyphenols?
Polyphenols are nutrients found in plants and more colorful plants, generally, have the higher polyphenols content. Fruits, especially berries, and vegetables, like cauliflower, are good sources of polyphenols. They comprise a wide family of molecules bearing one or more phenolic rings and are present in many food sources like wine, green tea, grapes, vegetables, red fruits, and coffee. Polyphenols are described as “secondary metabolites,” that is phytochemicals synthesized through secondary metabolism, and they are involved in a wide range of critical processes in plants, including growth, pigmentation, pollination, and resistance to pathogens and environmental stressors. In recent decades, special attention has been paid to the antioxidative or antiproliferative role of polyphenols (16–21) in the human diet, with evidence supporting the contribution of polyphenols in the prevention of cardiovascular diseases, diabetes mellitus, cancers, and neurodegenerative diseases (22–24).
Total daily dietary intake of polyphenols can be as high as 1 g·d−1, which is ~10 to ~100 times higher than the intakes of other “phytochemicals” and known dietary antioxidants, that is, vitamin C, vitamin E, and carotenoids. Using the Phenol-Explorer database, Godos et al. (25) estimated that an Italian study population had a mean intake of approximately 660 mg of polyphenols per day, obtained from nuts, tea, apples, coffee, cherries, citrus fruits, vegetables, chocolate, and red wine all regular constituents of the Mediterranean diet, included in the list of the Intangible Cultural Heritage of Humanity by the United Nations Educational, Scientific and Cultural Organization (UNESCO). However, major dietary sources of polyphenols may vary depending on the traditional diets adopted in various countries; thus, in Northern and Eastern European countries, the main dietary sources of polyphenols are represented mostly by beverages, such as coffee and tea, while in Southern European and Mediterranean countries, important dietary sources may be nuts, olive oil, fruits, and vegetables (22).
The level of polyphenols in the same plant is not constant, but varies with, for example, crop and atmospheric conditions (22,26). This variation of polyphenol content in what appears to be the same plant or fruit makes it difficult to assess the ingested amount by a particular person. All these facts have to be considered in the balance of the potential beneficial roles of polyphenols versus the possibility of intensified accumulation, safe consumption, and toxic effects.
There are thousands of compounds described in plants that have the presence of at least one phenolic ring in common and structurally, polyphenols are characterized by two or more hydroxyl groups attached to one or more benzene rings. The number and characteristics of these phenol structures underlie the unique physical, chemical, and biological properties. Variations in this chemical structure lead to the classification of polyphenols into two main groups: flavonoids and nonflavonoids (lignans, phenolic acids, stilbenes) (27). Table 1 provides a brief summary of example compounds and key dietary sources of the different polyphenol families.
The taste and color characteristics of fruits and vegetables are strongly influenced by the polyphenol content. Both the quantity and variety of polyphenols present are determined by the plant species, growing conditions sometimes termed terror (sunlight, water and nutrient availability, temperature), postharvest processing, and transport and storage conditions (26,27). There is, therefore, considerable variability in the polyphenol content in the many polyphenol-rich fruit-derived supplements that are now commercially available. Phenolic acids and flavonoids are considered to be the most common in the human diet, with main sources including fruits (such as apples, pomegranates, peaches, apricots, plums, sweet cherries, etc.), berries (such as black chokeberry, black elderberry, low bush blueberry, plum, cherry, blackcurrant, blackberry, strawberry, raspberry, prune, black grapes, etc.), vegetables (such as globe artichokes, red chicory, green chicory, red onion, spinach, broccoli, curly endive, etc.), nuts (such as chestnuts, hazelnuts, pecans, almonds, walnuts, etc.), fruit juices (such as blood orange juice, lemon juice, etc.), soy (such as soy tempeh, soy flour, tofu, soy yogurt, soybean sprouts, etc.), black and green tea, coffee, red wine, cereals, and chocolate (27,28).
The bioefficacy of polyphenols depends on their bioavailability. Indirect evidence of polyphenol absorption through the gut barrier is provided by the increase in the antioxidant capacity of the plasma after the consumption of polyphenol-rich foods, for example, tea, red wine, blackcurrant, and apple juice (11). More direct evidence on the bioavailability of a few phenolic compounds has been obtained by measuring their concentrations in plasma and urine after the ingestion of either pure compounds or of foodstuffs with a known content of the compound of interest (11).
The debate regarding the absorption of polyphenols from food is still ongoing. On the one hand, epidemiological studies agree that there are positive health effects of taking polyphenols; on the other hand, there is a low bioavailability of polyphenols in the body, due to low resorption and rapid transformation and excretion. The absorption of polyphenols via the stomach and small intestine is only 5% to 10% of the total intake. The absorption of polyphenols depends on the amount and size of the phenolic compound, previous diet, nutrient matrix, sex, and gut microflora (29,30).
Literature Search Strategy
The databases PubMed and Web of Science were consulted. Key terms that were included and combined were “polyphenols,” “exercise training,” “inflammation,” “oxidative stress,” and “exercise performance.” The final search was carried out in January 2020. Studies in this section needed to fulfill the following criteria: research conducted with human participants and original data from randomized clinical trials on polyphenols ingestion with an acute or long-term exercise intervention. No limits were used concerning the year of publication.
The Impact of Polyphenols on Recovery Times
The one group of polyphenols that has the greatest bioavailability is delphinidins, and they appear to be the only type of polyphenol that can be absorbed intact by the body. A rich source of delphinidins is blueberries; in particular, the Maqui berry grown in the Patagonia region of Chile has the highest known concentration (31). In addition, the most studied flavonoid polyphenol for increasing blood flow are those from cocoa. Thus, combinations of Maqui and cocoa polyphenol extracts may provide the greatest potential for maximum impact on sports performance (31).
However, small levels of polyphenols also can enter into the blood. Once in the blood, they can become gene activators. In particular, they activate the gene transcription factors that cause the increased synthesis of specialized proteins. One of these gene transcription factors is Nrf2, known to increase the expression of antioxidative enzymes that are 1000 times more powerful than standard antioxidants, such as vitamin C or vitamin E, in reducing excess free radical production. This is important for reducing delayed onset muscle soreness and the associated increased recovery times (14,32). That takes time, and that is why you need increased recovery times the more intense you workout.
The other gene transcription factor activated by polyphenols is adenosine monophosphate (AMP) kinase. AMP kinase is the master switch for your metabolism. In particular, it is the key to removing damaged mitochondria (mitophagy) and simultaneously replacing them with new ones (biogenesis). Since the cells in the body need ATP on a constant basis, this is only possible if AMP kinase is working at full efficiency. Another benefit of adequate levels of polyphenols in the blood is to increase blood flow by increasing nitric oxide (NO) production which increases oxygen transfer to the mitochondria for still greater ATP production. The mechanism of polyphenol-induced vasodilation appears to be via the enhanced conversion of dietary nitrates (primarily found in green leafy vegetables) into NO (14,33). The greater the number of hydroxyl groups on the polyphenol, the more efficient the conversion of dietary nitrate into NO (32,33).
Fruit-Derived Polyphenol Supplementation for Athlete Performance
As of today, a lot of studies are on polyphenols and physical exercise and in particular the polyphenolic compounds that have been demonstrated both to exert a significant effect in exercise-induced muscle damage and to play a biological/physiological role in improving physical performance. The effects of different polyphenols have been investigated in a wide range of exercise conditions, using a variety of supplementation strategies, timing, and dosage. Until a few years ago, despite the active search for “natural,” polyphenol-rich extracts that might enhance physical performance and decrease oxidative damage because they are antioxidants, the information was very limited and in some cases they suggested the converse. But in recent years, studies have increased considerably and more information is now available on the effect of polyphenols on sports performance. The ergogenic effects of fruit-derived polyphenols appear to be associated with enhanced vascular function and may result in improved muscle perfusion and enhanced oxygen extraction (32,34). Table 2 shows some examples of the effects of polyphenol supplementation on athletic performance.
For instance, acute ingestion of blackcurrant prior to 30 min rowing attenuated exercise-induced ROS generating capability (35). A greater total running distance, and increased preserved maximal sprint running has been observed following supplementation with New Zealand blackcurrant powder in trained cyclists (36). Positive findings have demonstrated that consumption of 1 g of pomegranate extract, consumed 30 min prior to exercise enhanced running time (37). Following Montmorency cherry concentrate ingestion, increases of ~10% in both peak power and total work during a 60-s all out sprint were observed (38). Additionally, a combined supplement of pomegranate, green tea, and grape extract consumed acutely 1 h preexercise increased total power output, maximal peak power output, and average power output during repeated cycle tests in active individuals (39). Conversely, trained cyclists (40) and moderately resistance-trained individuals (41) did not exhibit performance benefits from consumption of 1000 mg of pomegranate extract. Furthermore, no ergogenic benefit was observed following more chronic pomegranate supplementation regimens in trained participants (42). No difference was observed in marathon finish times following 5 d of cherry juice consumption (43).
Based on these findings, dietary recommendations in exercising individuals should emphasize the consumption of a well-balanced diet and/or natural antioxidant-rich foods, such as fruit rich in polyphenols or chocolate, rather than taking synthetic antioxidant supplements. This “nutraceutical strategy” has been increasingly proposed as a potential suitable tool for preventing or reducing oxidative stress and related inflammation during intensive physical training (44).
In this review, we highlighted the actions of polyphenol intake on exercise-mediated oxidative stress and performance, recovery and inflammation.
When ROS production and related endogenous antioxidant capabilities are imbalanced, a maladaptive biological response may occur, leading to both inflammation and oxidative stress. In muscle cells, aerobic energy production produces a significant quantity of ROS, which can rise by up to 10- to 20-fold during physical activity (9). Although moderate levels of ROS may serve as signaling markers that mediate muscle repair and adaptation, mitochondrial biogenesis, protein turn-over, and the up-regulation of antioxidant enzyme levels, increasing evidence suggests that unbalanced ROS levels are able to deregulate the redox state and induce a high rate of proinflammatory myokine release in muscles, which may lead to contractile muscle alteration, accelerated muscle fatigue, longer recovery time, and reduced exercise performance (45).
A number of elements of vegetal origin have been suggested as potential tools to delay fatigue onset during physical performance and/or to promote the recovery process (10). Polyphenols are among these elements and they might directly act as a stimulant on the central nervous system too. Suggestion on the action of polyphenols on brain functions has been described, especially in terms of delayed perception of fatigue (22,46,47).
But, even if the protective properties of polyphenol supplementation have been largely accepted in humans as well as in animal and in vitro searches, the available data show contradictory information. The data are clearly indicative of a dual action by polyphenols. On the one hand, in a rather homogeneous fashion, they collectively highlight the capacity of polyphenols to exert antioxidant actions. On the other hand, the results do not show a clear improvement in postexercise proinflammatory status, and they are not altogether convincing regarding the action of polyphenols on exercise performance and postexercise recovery. Notably, most studies are not directly comparable because they used different contents of polyphenols (10,48).
On the other hand, finely tuned ROS production during exercise is essential to promote the expression of several proteins that are crucial for exercise-induced adaptation, and the use of antioxidants in supraphysiological doses may be detrimental and/or alter the oxidative stress response in terms of proinflammatory gene induction. Thus, antioxidant supplementation might produce adverse consequences by decreasing the ROS concentration beyond the required homeostatic level. Finally, the additional effects of exogenous supplemental antioxidants on different types of exercise are difficult to predict, because exercise itself is a positive stimulus that generally drives the antioxidant capacity (49).
It also is important to consider different biological responses related to the type of exercise and, also, that antioxidants might be effective during specific periods of training. The requirements may vary according to different seasonal needs. For all of these reasons, a personalized plan that considers all of the specific requirements of athletes during the different phases of training would represent the best option to improve global performance, since the training process is highly variable and dependent on a wide range of factors (10).
Finally, between-studies comparisons are sometimes difficult because of the often-limited sample size, as well as the different populations, types of training and physical activity levels, and background diets of the participants that may influence the effects of chocolate supplementation and exercise (1,50).
It is conceivable that the effects of polyphenols, as well as of other food supplements, may be more evident as the severity and temporal extension of inflammation increases, which mostly results from exhausting low-intensity aerobic exercise. So, the standardization of physical activity intervention, sample size, and administered supplement should be carefully addressed in further studies concerning polyphenols usage in professional and amateur sport (10).
The consumption of extracts, juices, and infusions of certain fruits have been observed to ameliorate various biomarkers of health, possibly attributable to the high and diverse polyphenol content and antioxidant capacity of such fruit products. This potential ergogenic effect of polyphenol supplementation might arise as a result of lower oxidative stress and augmented NO production, and subsequent improvements in vascular function and skeletal muscle perfusion and metabolism. The present paper highlights the need for future empirical research to analyze the possible ergogenic effects of polyphenol ingestion on exercise performance.
However, poor regulatory constrictions of commercial polyphenol supplements and nonpharmaceutical formulations are a concern for their safe use. Therefore, modifying nutritional habits by the regular inclusion of polyphenol-rich fresh foods, like red fruits, tea, and natural juices, rather than with the excessive consumption of concentrated supplements, may have the most beneficial effect by increasing the organism's adaptive natural defenses and modulating several mechanisms involved in athletic performance.
The authors declare no conflict of interest and do not have any financial disclosures.
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