As mentioned above, polyphenols are antioxidants. This activity is ascribable to hydroxyl groups operating as electron donors and stabilizing free radicals through the delocalization of unpaired electrons ^[1][60], and is termed a chain-breaking function ^[2][61]. Polyphenols also demonstrate metal chelation activity. Because transition metals can produce reactive oxygen species (ROS) that damage the genome, polyphenols play an antioxidative and DNA-protective role ^[3][62]. In addition, they inhibit certain enzymes involved in ROS production, i.e., xanthine oxidase and NADPH oxidase, whilst upregulating other endogenous antioxidant enzymes, e.g., superoxide dismutase (SOD), catalase, and glutathione peroxidase (GPX) ^[4][63]. The antioxidative property of polyphenols is important for athletes, because it is well established that exercise generates ROS formation and prolonged exercise can promote oxidative damage to active myofibers ^[5][6][7][64,65,66]. Oxidative stress presents an imbalance between ROS-generating and -scavenging systems. ROS contain at least one oxygen atom and one or more unpaired electrons. ROS include free radicals (e.g., superoxide anion (O₂⁻), hydroxyl radical ·OH) and non-radicals (e.g., singlet oxygen (¹O₂), hydrogen peroxide (H₂O₂)). ROS overproduction can lead to tissue injury and initiate inflammation ^[4][63]. Superoxide (O₂⁻) is relatively membrane-impermeable and unreactive ^[8][67] and has a short half-life in the cell ^[9][68]. Superoxide dismutase (SOD) spontaneously converts O₂⁻ to non-radical hydrogen peroxide (H₂O₂) ^[10][69] which is further reduced to water by GPX ^[11][70]. H₂O₂ molecule is considered stable and can therefore diffuse considerable distances in or out of the cell ^[12][2]. O₂⁻ and H₂O₂ are products of NADPH oxidase ^[13][71]. In contrast to H₂O_2, the hydroxyl radical (·OH) is a strong oxidizing agent and is considered the most damaging ^[14][15][72,73]. Another free radical is nitric oxide (NO·), which is a short-lived and weak oxidant produced by the action of nitric oxide synthase (NOS) ^[16][74]. It has been shown that oxidative stress and inflammation are related, because inflammatory response occurs after exercise ^[17][75]. During inflammation, phagocytes undergo a respiratory burst that imposes high oxidative stress on the engulfed pathogens ^[18][76]. The oxidative burst is mediated by NADPH oxidase, which produces O₂- and H₂O₂ ^[19][77], and these products achieve their cytotoxic effects due to reactions with other antimicrobial systems to generate ROS ^[8][20][67,78]. In skeletal muscle cells in culture, two primary NADPH oxidase isoforms (NOX2 and NOX4) ^[21][79] were found to be the main sources of ROS ^[22][23][80,81]. Thus, the role of NADPH oxidase-derived ROS is important in pathogen-mediated and sterile inflammation (i.e., without pathogen involvement). Importantly, ROS can exacerbate the inflammatory process ^[24][82]. Nonetheless, free radicals are crucial for proper cell physiology and function actively as intracellular messengers ^[9][24][68,82]. This means that ROS have both positive and negative physiological effects. ROS are by-products of cellular metabolism, and constitutive antioxidant defenses neutralize the housekeeping production of reactive species, e.g., superoxide dismutase (SOD) converts O₂₋ to H₂O₂ ^[25][83]. Another antioxidant strategy is the scavenging of ROS by numerous molecules of low molecular weight that exist in the extracellular space and also within cells. A further method is to minimize the availability of pro-oxidants via metal-binding proteins. These chelating molecules avert these transition metals from participation in ROS formation ^[12][26][2,84]. The toxic effects of ROS and RNS are also counteracted by non-enzymatic reactions, e.g., ascorbic acid or vitamin E ^[27][85]. However, when the production of radicals overwhelms the antioxidant defenses, oxidative stress proceeds. This can lead to changes in cell membranes and other biological structures including lipoproteins, lipids, proteins, DNA ^[28][86], etc. Polyphenols are of particular interest because they can also protect against oxidative damage through various mechanisms. For instance, resveratrol, which is a stilbenoid and a type of natural phenol found in red grapes, reduced the expression of NADPH oxidase 4 (NOX4) in mice ^[29][87]. Quercetin, a natural flavonoid found in onions, green tea, and apples, decreases lipid peroxidation and inhibits cellular oxidation in erythrocytes in type 2 diabetic patients ^[30][88]. Another flavonoid, myricetin also protected healthy human erythrocytes from oxidative stress in vitro ^[31][89]. The same effect was observed for tea catechins in human diabetic erythrocytes ^[32][90]. Conversely, others showed that catechins provided only modest protection from oxidative damage in healthy men ^[33][91] and showed no effect in soccer players ^[34][12]. Notwithstanding their high training demands, athletes’ diets may not contain sufficient antioxidants to support their physical activity ^[35][36][92,93]. This raises the question of whether they should take dietary polyphenols. Given that the health benefits of exercise are well documented, it seems unlikely that exercise-generated free radicals should negatively impact performance in the long term ^[35][37][92,94].

The bioavailability of a substance is the fractional extent to which the active moiety is absorbed and rate at which the dosage reaches the therapeutic site of action ^[38][39][95,96]. The issue of bioavailability presents a major challenge for clarifying the therapeutic effects of polyphenols and may account for large inter-individual variations in clinical trials ^[40][97]. Many factors influence the bioavailability of dietary polyphenols in humans including external factors (sun exposure, degree of ripeness of plants), factors related to food processing (homogenization, lyophilization), interactions with other compounds (bonds with proteins (e.g., albumin), polyphenol-related factors (i.e., chemical structure, concentration in food)) ^[41][42][98,99].

The in vitro approach to studying polyphenols’ absorption, metabolism, and bioavailability has mostly used the Caco-2 cell line ^{[43][44][45][46]}[100,101,102,103], because the line reliably reflects biological barriers and the ability to perform phase II biotransformation by adding hydrophilic groups. Nonetheless, other cells have also been used ^[47][104]. The in vivo approach mostly uses rodents ^{[48][49][50][51]}[105,106,107,108] and humans ^{[52][53][54][55]}[109,110,111,112]. Absorption of polyphenols in mammals occurs in the small intestine after cleaving off sugar moieties ^[56][113]. This process occurs by either passive diffusion or with the use of specific transporters localized in the enterocyte membrane ^[42][99]. Only approximately 5–10% is absorbed, and the remainder accumulates in the large intestine with the bile conjugates, being thereby subjected to digestion ^[57][114]. Importantly, polyphenols of low molecular weight such as gallic or caffeic acids are partially absorbed into the body directly or after conversion. In contrast, polyphenols of higher molecular weight, e.g., proanthocyanidins, are very poorly absorbed ^[58][115]. Phenolic compounds absorbed in the small intestine are further transported to the colon, where they are transformed by the gut microbiota. These microorganisms perform a myriad of activities including hydrolysis, dihydroxylation, demethylation, and decarboxylation ^[42][99] thereby converting phenolic compounds into bioavailable metabolites. Meanwhile, polyphenol-rich extracts can also modulate gut microflora ^[59][60][116,117]. Importantly, the position of the hydroxyl groups might influence the polyphenols’ breakdown. For instance, flavonoids without hydroxyl groups at the C5, C7, and C4′ positions are degraded more slowly ^[56][61][113,118]. This implies that the slow-degrading compounds might be more bioavailable because their absorption can occur more quickly than those degraded more rapidly at the colon level ^[62][119]. When polyphenol sub-products are absorbed by the small intestine or colon, phase I and II transformation begins within enterocytes and in the liver to be distributed through the blood ^[63][64][65][120,121,122]. However, the metabolites may encounter enterohepatic circulation, whereby they are returned to the small intestine and transported back to the liver ^[40][42][66][97,99,123]. Polyphenols can stay in the blood or be delivered to other tissues. Indeed, polyphenol metabolites have been found in various tissues, e.g., glucuronidated and methyl-glucuronidated derivatives of catechin and epicatechin in the muscle of rats fed with grape pomace extract ^[67][124] or catechin-glucuronide, methyl catechin-glucuronide and methyl catechin-sulfate in kidneys, intestine, lungs, spleen, and thymus of rats receiving hazelnut extract ^[68][125]. Nonetheless, their targeted delivery is a major problem relating to polyphenol-rich supplements and their low bioavailability. So far, many systems have been developed to address the issue. However, these approaches mainly concentrate on achieving delivery that minimizes systemic diffusion and degradation of phenolic compounds, because some phenolics are poorly absorbed in the digestive tract while others are extensively metabolized to derivatives with a lower activity or are degraded ^[42][69][99,126]. This means that when polyphenols are orally administered, their therapeutic concentration might not be achieved ^[70][127]. Temperature, light, oxygen, acidic pH, and enzymatic activity in the digestive system ^[70][127] can all affect the beneficial effects of polyphenols ^[71][128]. As mentioned above, polyphenols follow phase I and II metabolic pathways. The major pathway is phase II, during which glucuronidation and sulfation lead to especially hydrophilic conjugates, and methylation to similar if not slightly more lipophilic metabolites. The methylated products are conjugated into glucuronides and sulfates, as long as there is a functional group available for conjugation into more hydrophilic metabolites ^[63][120]. Moreover, unabsorbed and hydrophilic polyphenol conjugates can undergo transformation triggered by gut microflora, which can even result in ring fission. Bacteria can dramatically reduce polyphenols’ bioactivity ^[72][129]. Therefore, various research efforts have been focused on the problem of polyphenol delivery. Of note, bioavailability is a key determinant of polyphenols’ potential health-promoting applications ^[42][99], and is also important in establishing dietary reference intake (RDI) ^[73][130]. This aspect is reviewed in the discussion of polyphenol supplementation strategies.

The biodiversity of polyphenols found in food is wide. Therefore, it is extremely difficult to determine their content within food products and measure their daily intake ^[74][131]. Several research articles have extrapolated the daily dietary intake of particular flavonoids among the European population, based on the US database of flavonoid concentrations in particular foods. Many authors have referred to the data published by Kühnau et al., almost 46 years ago, where a daily intake of 1 g of total phenols was established in the US population ^[75][132]. There have been attempts to define the daily consumption of polyphenols within the diets of different populations. So far, Hertog et al., have done this for flavonols and flavones ^[76][133], and Reinli and Block for isoflavones in the Dutch population ^[77][134]. Researchers established the intake of flavonols (mainly quercetin) and flavones as 21 and 2 mg/d, respectively ^[76][133]. The average dietary consumption of isoflavones in the Japanese population was determined to be 30–40 mg/d ^[78][79][135,136]. Interestingly, soy products are less popular in Western countries, which is reflected in the lower polyphenol content of the Western diet ^[80][137], explaining why the daily dietary intake of quercetin and genistein does not exceed 2–4% of total polyphenols ^[74][131]. Some authors report that daily polyphenol consumption above 650 mg decreases risk of death in comparison with those whose daily polyphenols intake is below 500 mg ^[81][138]. Other authors report health benefits ranging from intakes of 500 mg to 1500 mg per day ^[74][131]. Further sources recommend a daily dose of 0.1–1.0 g of polyphenols. Fruits such as grapes, apples, pears, cherries, and blueberries contain up to 200–300 mg of polyphenols per 100 g of fresh weight. Interestingly, a glass of red wine or a cup of tea contains about 100 mg of polyphenols, and the presence of these products in the diet may reduce the likelihood of chronic diseases. In Europe, the main sources of polyphenols are onions, black tea, red wine, and apples ^[81][82][138,139].

Various delivery systems for polyphenols have been developed to improve their efficiency and combat their bioactivity problems. One major issue is to find a way to enhance the penetration of active substances and bring hydrophilic compounds into the tissues. This concept has been explored in numerous studies demonstrating the use of formulations with simple emulsions ^[83][84][85][140,141,142], cyclic glucan oligosaccharides known as cyclodextrins ^[86][87][88][143,144,145], gels ^[89][90][91][146,147,148], nanoemulsion ^[92][93][94][149,150,151], or liposomes ^[95][96][97][152,153,154]. Other solutions are also available, such as micelles, nanocomposites, metal oxide nanoparticles, etc. ^[70][127]. The main reason for encapsulating polyphenols is to tackle the problem of their stability. For instance, maltodextrins preserve the integrity of anthocyanins ^[98][155]. Encapsulation increases biocompatibility and prevents degradation caused by the external environment. It also minimizes interactions with other components of the human body ^[99][156]. In general, coating by microencapsulation in particles up to 1000 μm protects active substances and preserves their antioxidant properties ^{[100][101][102]}[157,158,159]. However, nanotechnology is apparently more effective, because nanoencapsulated polyphenols have been found to increase the protection of active substances and bioavailability as well as improving controlled targeted release ^[103][160]. Particle size is generally seen as a factor strongly related to bioavailability. The development of a nanoscale delivery system is aimed at achieving improved site-selective targeting ^[104][161]. This is possible due to the small molecular size and active incorporation into cells by different endocytic pathways ^[105][106][162,163]. However, targeted delivery is complicated. It can be achieved actively or passively ^[107][164]; active targeting involves the therapeutic agent (in this case a polyphenol) being loaded into a carrier and this conjugate attached to tissue or cell-specific ligands ^[108][165]. By contrast, passive targeting requires loading the therapeutic agent into a nanomolecule that passively reaches the target tissue or organ. This leads to the accumulation of a drug delivery system with a specific size, molecular mass, and charge ^[109][166]. Given that a passive targeting system might utilize specific conditions in the diseased tissues or cells (e.g., low pH) ^[109][166], it is reasonable to assume that this approach is suitable to treat oxidative stress generated by exercise. After all, the higher the exercise intensity, the lower the muscle pH ^[110][167]. Nonetheless, the active approach also seems effective since it is possible to target specific cells or the inside of cells (for targeting intracellular organelles) ^[109][166].

Importantly, approaches should be compatible with another target, that of achieving the concentrations required for systemic therapies. In these circumstances, the hydrophilic part of a phenol carrier must be in balance with the lipophilic part ^[111][168], because the affinity of polyphenols for lipid bilayers partially determines their biological activity in vitro ^[112][169] and is of great significance in the biomedical and dietary fields. Polyphenolic extracts interact with the cell membrane by creating a protective coat around the lipid membrane, through their location on the membrane surface. This effect was shown using liposomes as models of lipid membranes, wherein trans-stilbenes and flavonoids interacted at the hydrophilic interface ^[113][170]. This finding is in agreement with another study reporting that three different types of blueberry extracts changed the arrangement of the hydrophilic region of the liposome membranes ^[114][171]. Liposomes themselves possess an aqueous central section as well as hydrophobic and hydrophilic components comprising a lipid bilayer. The aqueous cores typically encapsulate hydrophilic compounds. By contrast, hydrophobic substances favor lipid bilayers. Thus, liposomes might be used for the delivery of diverse substances, such as hydrophilic and hydrophobic compounds ^[115][172].