Prior to the discussion of the collective results of the relative studies, it is imperative to understand some basic limitations of research in the area of oxidative stress and acute exercise. The multiple body systems, inclusive of the antioxidant defense system, function in a complex and vastly interconnected fashion. Therefore, concrete conclusions regarding precisely how and why RONS are produced during exercise, remains a topic of continued study. To claim a complete understanding of these processes at this time may largely underestimate the complexity of the human body and associated redox systems. This simply means that current understandings and findings relative to RONS and acute exercise should remain open to further interpretation and discovery. Of course, a key element involved in the progression of a given scientific area is a clear understanding and familiarization with current findings and beliefs. It is the intent of this review to provide such information.

Currently, it is clear that both acute aerobic 252627 and anaerobic 282930 exercise has the potential to result in increased free radical production, which may or may not result in acute oxidative stress. As stated earlier, in order for oxidative stress to occur, the RONS produced during exercise must exceed the antioxidant defense system present, thereby resulting in oxidative damage to specific biomolecules 40. Different exercise protocols may induce varying levels of RONS production, as oxidative damage has been shown to be both intensity 4142 and duration 43 dependent. During low-intensity and duration protocols, antioxidant defenses appear sufficient to meet the RONS production, but as intensity and/or duration of exercise increases, these defenses are no longer adequate, potentially resulting in oxidative damage to surrounding tissues 44. Other factors appear to impact the degree of antioxidant defenses present, including age 45, training status 3839, and dietary intake 7. If oxidative stress does occur, detection depends to a large degree on the tissue sampled, the timing of a given sample, as well as the sensitivity and specificity of the biomarker chosen 17. Significant or null findings may be related to the lack of specificity of the chosen biomarker (as has been suggested for TBARS 46), improper sampling protocol (too few measures or too short time course), or improper tissue (blood or urine vs. skeletal muscle). Under these circumstances, it is possible that in investigations where oxidative stress was not observed following acute exercise, oxidative stress may have occurred prior to or after sample collection or in tissue (e.g., skeletal muscle, cardiac, liver, brain) other than that which was sampled (most commonly blood). Taken together, it appears that several factors influence both the onset of oxidative stress (intensity and duration of exercise, age, training status and dietary intake of subjects) as well as the detection of such stress in vivo (biomarker chosen, tissue sampled, timing of sampling). These variables may partially explain some of the inconsistency present within the literature.

The majority of research in the area of oxidative stress and acute exercise in humans has utilized aerobic exercise protocols (> 160 original investigations). Typical protocols have included submaximal or maximal effort aerobic exercise either on a treadmill or cycle ergometer, with the majority of investigations utilizing a graded exercise test (GXT) to induce an oxidant stress. Most laboratory based protocols have involved short to moderate duration exercise bouts (≤ 2 hours), while a few laboratory protocols, and the more common "field" tests, have included much longer times of exercise (> 2 hours). In addition, some treadmill studies have focused on downhill running, involving eccentric bias in order to induce muscle injury. For the purpose of this review, as a means of classification, all exercise protocols discussed in the text will be referred to as maximal or submaximal, as detailing each specific protocol would not be practical due to the variation within each study design. Results will be discussed relative to each specific biomarker utilized, with the initial section providing a brief illustration of the nature of each biomarker that can be referred to throughout for clarification purposes. Studies involving non-eccentric aerobic exercise without antioxidant supplementation will be discussed below and can be viewed in Table 1 of Additional file 1.

Of the above reviewed studies, several investigators have also included a variety of antioxidant treatments in their study design, in an effort to attenuate and/or eliminate exercise-induced oxidative damage. For a summary of such studies, please refer to Table 2 in Additional file 1. Typical treatments have included vitamin C, vitamin E, and beta-carotene, either alone or in combination for a variety of durations, administered chronically (1–8 weeks pre exercise) and acutely (1–2 days pre exercise). Vitamin E is believed to be the most important and effective nutritional antioxidant throughout the lipid phases of the cell, as it contributes to membrane stability and fluidity by preventing lipid peroxidation, whereas vitamin C plays an equally important role of preventing lipid peroxidation in plasma and interstitial fluids 140. Moreover, vitamin C and vitamin E work in conjunction with each other during conditions of oxidative stress, as vitamin C is utilized to regenerate vitamin E following reaction with RONS 140. Although not as commonly utilized, the major carotenoid precursor to vitamin A, beta-carotene, is primarily responsible for quenching singlet oxygen 140. Aside from the common antioxidants above, other investigators have utilized less common antioxidants, including: coenzyme Q10 (CoQ10) 100, N-acetylcysteine (NAC) 63120, uric acid 113, propranolol 101.

CoQ10, also known as ubiquinone, is an essential chemical component of the mitochondria in all animal cells where it functions as a cofactor in the electron transport chain during the synthesis of adenosine triphosphate (ATP) 145. In addition to its role in energy production, CoQ10 has also been shown to provide antioxidant protection either by directly scavenging superoxide produced during oxidative phosphorylation or by regenerating vitamin C, and vitamin E from their oxidized states 146. NAC is a thiol-containing compound which potentially may reduce the impact of RONS-associated damage by directly scavenging RONS and/or supplying cysteine for enhanced glutathione synthesis 120. Uric acid is an abundant aqueous antioxidant that accounts for almost two thirds of all free-radical-scavenging activity in human serum 113. Finally, propranolol is a β-blocking agent that has been shown to possess antioxidant properties in vitro 147.

Several studies have noted an attenuation in oxidative stress following administration of a variety of mixed antioxidant supplements (e.g., vitamin C and vitamin E/vitamin C, vitamin E and beta-carotene) 899899137. However, independent or combined administration of vitamin C, vitamin E, and beta-carotene have been the most commonly utilized treatment option in regards to non-eccentric aerobic exercise-induced oxidative stress. Several studies have reported a reduction in exercise-induced oxidative stress following chronic administration of vitamin C 6270119, vitamin E 215580139, and beta carotene 129 when administered alone. However, attenuation has not occurred for all measured biomarkers 627080139. Moreover, a few studies have reported no effect of independently administered vitamin C 98, or vitamin E 669098. Null findings were also reported following independent administration of CoQ10 100. Disparities in the literature regarding antioxidant supplementation and attenuation of oxidative damage are likely due to several factors including training status of the subject population 135, dietary intake 115, as well as the magnitude and duration of supplementation period, as both vitamin C 70 and vitamin E 148 have been shown to respond in a dose-dependent manner. The null findings in regards to vitamin E supplementation 669098 could have been due to insufficient dosages and or treatment durations, as it has recently been shown in a time-course study, that maximal reduction of oxidative stress (assessed via F₂-isoprotanes) does not occur until 16 weeks of vitamin E supplementation at a dosage of at least 1600 IU per day 148. It is certainly possible, though not reported to date, that other antioxidants respond in a similar manner and could potentially explain a portion of the inconsistency regarding antioxidant supplementation in attenuating exercise-induced oxidative stress.

Acute antioxidant supplementation prior to or during non-eccentric aerobic exercise, although not as commonly investigated, has resulted in more consistent findings when compared to chronic supplementation. This is evidenced by an attenuation in various biomarkers of oxidative stress following treatment, almost without exception 2762636980101113120. Attenuated biomarkers have included PC 69, 8-OHdG 69, DNA damage via Comet Assay 80, GSSG 63120, TBARS 62, MDA 27, LOOH 27, F₂-isoprostanes 113, total antioxidant capacity 63113, and expired pentane 101. These results have been noted following acute administration of a multivitamin 80, vitamin C 2762, vitamin E 80, NAC 63120, uric acid 113, propranolol 101, as well as an antioxidant (black grape, raspberry, red currant concentrates) rich beverage 69. Furthermore, direct detection of exercise-induced RONS production, via electron spin resonance, has also been shown to be eliminated following acute ingestion of 1000 mg of vitamin C 27. It should be noted, that in a similar manner to chronic supplementation, no antioxidant treatment completely eliminated oxidative stress, as attenuation was not consistent across all selected biomarkers for any study 62636980101113120, with one exception 27.

While most investigations have implemented primarily concentric aerobic regimens (e.g., cycling, treadmill walking/running), some have measured the oxidative stress response following aerobic exercise with an eccentric bias, as discussed below. For review, please consult Table 3 in Additional file 1.

Eccentric exercise involves high force during the lengthening portion of muscle contraction. This can occur involuntarily or voluntarily during conditions in which the activated muscle cannot produce enough force to overcome the resistive force (e.g., during heavy resistance training) or during an intentional production of submaximal force in order to control the eccentric (lengthening) movement (e.g., controlled lowering of external load and/or downhill running), respectively. Both damage to the involved muscle tissue and concomitant soreness associated with such damage have been shown to be greatest following eccentric compared to concentric exercise 149. Furthermore, exercise-induced trauma to the musculature has been shown to lead to a proinflammatory migration of phagocytic cells into the affected area, leading to the increased release of RONS (during respiratory burst), designed to aid in the breakdown of damaged tissue 101150151152. Both the post exercise phagocytic migration, as well as the initial increase in RONS during eccentric exercise (due to increased mitochondrial respiration) have been suggested to result in an acute state of oxidative stress during and following an eccentric exercise stimulus 151153154.

Almost without exception, the majority of studies have utilized eccentric protocols in the form of downhill treadmill running. Typical intensities and durations have included running speeds corresponding to 70–75% age predicted heart rate max or 60% VO_2max with a duration of 40–50 minutes of continuous or intermittent (3 bouts of 15 min downhill runs) exercise at a negative 12–20% grade. This form of exercise is often chosen in order to induce muscle tissue damage, evident by reported increases in creatine kinase (CK) 153154155156 and lactate dehydrogenase (LDH) 156 following such a protocol. Along with these markers of cellular damage, studies have reported mixed finding in relation to oxidative stress biomarkers.

Increased lipid peroxidation, measured via TBARS 156, MDA 154157158, F₂-isoprotanes 45154, and LOOH 155, has been reported by several authors following aerobic eccentric protocols in untrained subjects, with elevations typically reaching significance several hours (> 6 h) or days (24–72 h) following the stimulus. This would provide evidence for the increased migration of phagocytic cells following eccentric exercise, resulting in increased RONS production and subsequent oxidative damage. In opposition to the above findings, two similar investigations, utilizing trained subjects noted no changes in MDA 153, conjugated dienes 153155, or glutathione redox status 151. It was suggested that trained individuals may experience an attenuated oxidative stress response following eccentric exercise, perhaps mediated by greater antioxidant enzyme protection and/or lower levels of muscular damage following exercise 153. Furthermore, the null findings of Camus et al. 151 may have been related to sampling time, rather than training status, as samples were only taken immediately and 20 minutes post exercise.

Aside from lipid peroxidation, other biomarkers have been utilized by a few investigators, including markers of DNA damage (8-OHdG), as well as changes in antioxidant capacity (ORAC) and/or circulating levels of antioxidants (vitamin C, vitamin E). Only one study to our knowledge has investigated oxidative damage to DNA, as well as changes in antioxidant capacity following eccentric exercise, noting no increase in 8-OHdG and a decrease in ORAC evident at 72 h post protocol 154. In regards to circulating antioxidants, blood levels of vitamin C and vitamin E have been shown to exhibit no change when corrected for changes in plasma volume 153154158. Although, other work opposes these findings, noting a transient decrease in the plasma concentration of vitamin C 151 and vitamin E in skeletal muscle 158.

As with non-eccentric biased aerobic exercise, a few investigations have included antioxidants within the research design involving eccentric aerobic work. Due to the relatively small number of such studies, these can also be reviewed in Table 3 of Additional file 1. Vitamin E, provided at a dosage of 1000 and 1600 IU per day for 12 weeks or 48 weeks was reported to attenuate the increase in F₂-isoprostanes following eccentric exercise 154, as well as eliminate the increase in urinary MDA which was observed 12 days post exercise in the placebo group 158, respectively. In support of Meydani et al. 158 a similar study, utilizing vitamin C (dosage of 1 g/day), noted no increase in MDA following eccentric exercise, compared to a significant increase 3 and 4 days post exercise in the placebo group 157. It should be noted however that the treatment effect observed by Sacheck and coworkers 154 was only evident in older subjects, with young healthy subjects experiencing no additional benefit of supplementation. In such a case, it may be that antioxidant supplementation only provides additional protection in those individuals at an increased risk of oxidative damage due to the presence of old age and/or disease 159. Moreover, a reduction in exercise-induced oxidative stress by way of antioxidant supplementation may not be beneficial, as it has been suggested that increased RONS during and following exercise may be necessary in order to bring about adaptations in antioxidant defenses, as well as other physiological parameters 193136.

The idea of exercise-induced oxidative stress representing a potential contributor to the development and/or progression of ill health and disease receives considerable attention when applied to acute long duration (> 2 hours) aerobic exercise, (for review see 140). In fact, epidemiological data suggests that a very high volume of exercise is associated with an increase in the risk of developing cardiovascular disease 160161. Moreover, increased oxidative stress has been suggested to be the link, connecting the above association between excessive exercise and disease risk 140. At first glance, this statement appears to be highly contradictory to common beliefs regarding regular exercise and health benefits, as current recommendations suggest that individuals should accumulate at least 30 minutes of moderate-intensity physical activity each day in order to improve and maintain their health 162. These recommendations are made in spite of the fact that numerous studies have reported increased oxidative stress in response to acute aerobic exercise of various intensities and durations (for review see relevant section above). Collectively, disease risk has been shown to decrease as a function of exercise up to a certain point, at which the disease risk begins to increase, suggesting that an optimal level of exercise may exist 140. Because oxidative stress appears connected to the relationship between disease and exercise, it is certainly possible that an optimal level of increased RONS production during exercise gives to way to improved health, potentially via an upregulation in antioxidant defenses. However, because RONS production is known to be a function of both exercise intensity 41 and duration 43, exacerbated prooxidant production that exceeds the currently undefined optimal level, may in turn overwhelm antioxidant defenses in such a way that irreparable oxidative damage may occur, potentially resulting in ill health and or disease. More research is needed before definitive conclusions can be established, however, several studies have investigating the oxidative stress response following long duration aerobic exercise. For the purpose of this review, long duration aerobic exercise will be defined as aerobic activity maintained for a duration of greater than two hours and/or performed in a field setting (e.g., half or full marathon). Additionally, the impact of acute overtraining on oxidative stress will also be included in this section. These studies will be reviewed in detail below and will be presented in Tables 4 (without antioxidant supplementation) and 5 (with antioxidant supplementation) in Additional file 1.

Long duration exercise-induced oxidative stress has typically been assessed following either a half 163164165166167 or full 131168169170171172173174175176 marathon, an ultramarathon 177178179180181182, or a triathlon 183184185186187188. Although other findings have been reported in reference to a duathlon 189190191192, a long duration run 71169193194195196197, cycle ride 198199, march 200, or bike race 71201202203204. Studies investigating the impact of overtraining have also been conducted 191192205206. Collectively, it would appear that acute long duration aerobic exercise promotes an acute state of oxidative stress, evident by reported increases in lipid peroxidation (TBARS 190191, MDA 44163164166168203204, F₂-isoprostanes 177178180181187188194197199207 CD 71169193 LOOH 177178197, susceptibility of LDL to oxidation 170172175193), protein oxidation (PC) 203, oxidative damage to DNA (8-OHdG 168182201, DNA damage (Comet assay) 168174200208), as well as changes in GSH redox status (decreased GSH 165173190191192195 and increased GSSG 165173190191192195202203). However, a few exceptions have been noted such as no change in TBARS 71165171176183192195, MDA 164196, F₂-isoprostanes 179, CD 165166170, LOOH 184187188, susceptibility of LDL to oxidation 167, PC 200, 8-OHdG 185188, and glutathione redox status 183. Exercise-induced changes in antioxidant defenses follow a similar pattern as the results presented in the previous section on non-eccentric aerobic exercise, with antioxidant capacity typically experiencing an increase immediately post race 163164169170172175187192197199. Varying results for specific antioxidant enzymes, as well as circulating antioxidants have also been reported by several authors, noting a transient increase (GPX 176189196, GR 189202203, SOD 166203, CAT 202, vitamin A 165, vitamin C 165172175207, vitamin E 169181202), decrease (GPX 175176181207, SOD 44, CAT 44166, vitamin A 184, vitamin C 170176181, vitamin E 175176181207), or no change (GPX 165166, SOD 165173189192202, CAT 165189203, vitamin A 170176202203, vitamin C 170184, vitamin E 165170172184203) following exercise. Null findings for any of the above biomarkers have been suggested to be related to the highly trained nature of the subject populations, intensity of exercise, biomarkers utilized, the timing of tissue sampling 140, as well as the uncontrolled intake of carbohydrates 140 and nonsteroidal anti-inflammatory drugs (NSAID) 179. On average, subjects participating in the above investigations trained approximately 20–30 hours/per week and thus likely experienced decreased RONS production, as well as increased antioxidant defenses 140183. It was suggested that while the duration of some exercise protocols may have been sufficient for the induction of RONS production, the intensity was likely so low (in order to maintain the long duration activity), that such highly trained individuals may have possessed sufficient antioxidant defenses to combat such radical production, thus masking any potential accumulation of oxidative stress biomarkers 183. Similar to aerobic eccentric exercise, long duration protocols are known to result in substantial muscle damage (evident by increased CK 165168182200201), subsequently resulting in phagocytic migration to the affected area, increased respiratory burst activity and oxidative stress. Therefore, if sampling was not carried well into the recovery period, oxidative stress may not have been identified. Moreover, the lack of sampling during the actual protocol itself may also have impeded investigators ability to detect an oxidative stress, as elevations have been reported during such protocols 163181. Finally, as mentioned above, lack of control for both carbohydrate and NSAID intake during exercise may also have influenced results as both have been shown to attenuate 199 and exacerbate 179 oxidative stress, respectively.

A few studies have investigated the impact of overtraining for a period of days or weeks on various markers of oxidative stress. Overtraining protocols have included some form of vigorous exercise, performed for a defined length of time, such as 10 205, 28 192, or 30 206 days, typically reporting an increase in oxidative stress following cessation of training (8-OHdG 205206, TBARS 206). However, in opposition to the above findings, one study reported no increase in markers of lipid peroxidation, DNA damage or glutathione redox status following a period of overtraining in trained men 192.

Numerous studies have investigated the impact of antioxidant supplementation on long duration exercise-induced oxidative stress. These studies are presented in Table 5 of Additional file 1. Treatments have typically consisted of the common antioxidants (vitamin A, vitamin C, vitamin E) administered in combination 174176190191196198207 or separately 177180187188189209, with the exception of a few studies utilizing CoQ10 175, as well as acute administration of carbohydrate-rich beverages, with 178 or without 197199 additional vitamin C.

Unlike the results of antioxidant treatment and short duration aerobic exercise discussed above, the majority of investigators have noted no attenuating effect of supplementation on markers of lipid peroxidation, DNA damage, and/or glutathione redox status following long duration protocols 175177191197. However, some exceptions exist, with authors reporting reductions in F₂-isoprostanes 180199207, TBARS 198209, as well as DNA damage (Comet assay) 174 following supplementation with vitamin C and vitamin E administered in combination (174198207, as well as vitamin C 199 and vitamin E 180209 given separately. While the lack of enhanced protection against oxidative stress may be related to the issues discussed above (e.g., training status, dosages, time course of supplementation), it may be that the increase in RONS observed during and following long duration protocols may be so great that the prooxidants produced overwhelm both the endogenous and exogenously consumed antioxidant defenses, thereby masking the benefit of supplementation. It is possible that larger dosages and or longer durations of treatment may be necessary in order to provide significant protection against long duration exercise-induced oxidative stress 148.

It has been shown that exercise of various intensities and durations serves as a sufficient stimulus to invoke increased RONS production in both animals 25 and humans 92. While the body does possess a complex antioxidant defense system that serves to provide protection against RONS, defenses are often not sufficient to eliminate oxidative damage during and following exercise, evident by numerous findings of increased lipid, protein, DNA and glutathione oxidation following acute aerobic exercise (both short and long duration protocols) in humans and animals. Antioxidant supplementation does appear to provide some degree of protection, typically observed with short duration protocols; however, precise dosages and durations of treatment remain to be determined. Both the oxidative stress experienced following exercise, as well as the impact of antioxidant supplementation appears affected by several factors including intensity and duration of exercise, training status, age, and health status of the subjects tested, in addition to the specific biomarkers chosen, timing of tissue sampling, and the amount and duration of antioxidant treatment. Therefore, it is recommended that future investigations employ sufficiently stringent exercise protocols, and utilize a wide array of oxidative stress biomarkers and take multiple samples post exercise (through several hours of days of recovery) in an attempt to provide valid and meaningful findings.

Although much has been uncovered regarding oxidative stress and exercise, it is currently unclear as to whether exercise-induced RONS production and subsequent oxidative damage represents a necessary or detrimental stimuli to physiological function that should be utilized or minimized, respectively. It may be that a currently undefined optimal level of RONS production and oxidative damage is necessary for adaptations in antioxidant defenses and other physiological parameters that lead to the improvement of proper health. If so, this may provide insight into the relationship between regular physical activity, diminished disease risk, and increased life expectancy 160161210. However, excessive RONS production and oxidative damage via chronic long duration exercise and/or overtraining may exceed the aforementioned optimal level, thereby leading to irreparable oxidative damage, potentially resulting in the development or progression of ill health and/or disease. If such was the case, this finding may provide insight into the relationship between excessive exercise, increased disease risk, and decreased life expectancy 160161210. Clearly, more research is needed in this area in order to generate firm answers related to these issues.

Although the term anaerobic means "without oxygen", resistance training does result in increased oxygen consumption both during and following acute exercise. However, the magnitude of increase in VO₂ is far less than what is observed following acute aerobic exercise 211. Despite the comparatively low increase in VO₂, it has been shown that acute anaerobic exercise serves as a sufficient stimulus to elicit an increase in RONS formation 2829. Furthermore, unlike aerobic exercise, where increased mitochondrial respiration is thought to be the primary target of increased RONS, it has been suggested that the increased radical production and subsequent oxidative stress observed during and following resistance exercise may be meditated to a large degree by the activities of certain radical generating enzymes (xanthine and NADPH oxidase), prostanoid metabolism, phagocytic respiratory burst, disruption of iron containing proteins, as well as altered calcium homeostasis 24. Brief periods of ischemia followed by reperfusion, resulting from intense muscular contraction, as well as mechanical stress and/or muscle damage, are thought to be the mechanisms underlying the increase in RONS via triggering the activity of radical generating enzymes as well as initiating the migration of inflammatory cells to the affected area 20. Similar to aerobic exercise, although the mechanisms are not fully understood, anaerobic exercise clearly possesses the ability to result in acute oxidative stress, evident by several studies reporting an increase in oxidative stress biomarkers following exercise 24. For this review, results will be discussed relative to the mode of resistance exercise (e.g., dynamic, eccentric, isometric, sprint/jump), and will be presented accordingly in Tables 6–9 of Additional file 1. Because of the relative infrequency of such studies, those incorporating antioxidant treatment into their design will not be discussed in a separate section, but rather they will be included within their respective section and table.

The majority of studies investigating dynamic resistance exercise-induced oxidative stress (Table 6 of Additional file 1) have utilized an exercise protocol consisting of two or more compound lifts (multiple joint exercises), occasionally performed in a circuit fashion 212213214, for ≥ 3 sets at an intensity of 60–95% 1 RM 212213214215216217218219220221. Other studies have used a single movement, such as the squat 222223224225226227 or knee extension 2829 exercise as the stimulus, with the exception of one study in which isokinetic knee extension was performed following maximal sprints on a cycle ergometer 228.

Similar to aerobic exercise, the majority of studies have reported an increase in oxidative stress, evident by increased lipid peroxidation 2829212214215216218219220221223225, protein oxidation 216224229, and changes in glutathione redox status 217224226, despite a few studies noting null findings for each (lipid 212213222224226227228, protein 226227, glutathione 218219). In regards to DNA oxidation, no study has reported significant increases following dynamic resistance exercise 222224. Assessment of antioxidant capacity, concentrations of circulating antioxidants, as well as the activities of certain antioxidant enzymes has resulted in similar inconsistent results to those observed with aerobic exercise, with authors reporting an increase, decrease or no change for various markers (for more information, consult Table 6 of Additional file 1). Null findings are likely related to the specific biomarkers chosen, time course of sample collection, intensity of exercise 221, dietary intake, as well as the training status of the subject population 212213222224227. As with aerobic exercise, it may be that oxidative stress occurred but it did so preceeding or following the sample collection, in a different tissue other than that utilized (typically blood and urine), or resulted in oxidative damage to cellular constituents other than those measured. Furthermore, trained individuals likely experience attenuated muscular damage in response to exercise compared to untrained subjects, in turn blunting the inflammatory and subsequent oxidative stress response.

In an attempt to decrease the oxidative damage induced by exercise, a few studies have investigated the impact of various antioxidant supplements and/or agents 213214215217220225228. Attenuation of exercise-induced oxidative stress has been reported following administration of exogenous vitamin E 214228, L-carnitine 225, and allopurinol 217, despite no treatment effect being noted following similar vitamin E intake 215216, as well as following acute ingestion of a carbohydrate beverage preceeding and during exercise 213.

In the assessment of eccentric biased exercise-induced oxidative stress the majority of protocols involve eccentric contractions of either the elbow flexor 229230231232233234235 or knee extensor 229231236237238 muscles. The exceptions include those studies in which eccentric exercise was performed on a cycle ergometer 239 or using eccentric bench press 240. These studies can be viewed in Table 7 of Additional file 1. Such protocols have been suggested to result in increased muscle damage/cell membrane disruption, evident by increased CK following exercise 229231233238239240241. Furthermore, in an effort to produce the greatest amount of trauma to the exercising muscle, the majority of studies have recruited untrained subjects 229230233239, with few exceptions 238240.

Such protocols have been shown to result in increased lipid peroxidation 230231237238, protein 230233237238 and DNA 236 oxidation, as well as changes in glutathione redox status 230234235237238. Moreover, values have been shown to peak 48–72 hours post exercise, suggesting that increased migration of phagocytic cells and subsequent increased RONS production via respiratory burst may be the main determinant of the oxidative stress response 230231233237238. However, null findings have also been reported despite similar exercise regimens for markers of lipid peroxidation 229232239240241, protein oxidation 229, and glutathione redox status 233. These findings are likely related to the limitations discussed previously. A lack of significance may also be the result of an inability to induce muscular damage (evident by no increase in CK following exercise 232), or the use of skeletal muscle, rather than blood, to measure oxidative stress 229241. Aside from the biomarkers discussed above, various antioxidant capacity assays, as well as the activity of specific antioxidant enzymes (e.g., SOD, GPx, CAT) have been shown to increase following exercise 231237238241, with few exceptions 231239.

Little information exists concerning eccentric exercise and antioxidant supplementation, however a few studies have noted an attenuation in oxidative stress following administration of vitamin C, vitamin E, and selenium given in combination 230, or vitamin C alone 235. No benefit has also been reported following consumption of a vitamin E, omega-3 free fatty acids or soy isolate mixture 232, a vitamin C and vitamin E mixture 240 as well as following intake of vitamin C and NAC 231. Moreover, the vitamin C, NAC combination was administered following exercise and into the recovery period and was shown to result in an exacerbated increase in oxidative stress compared to placebo 231.

Isometric protocols have typically consisted of handgrip exercises with 242243 or without 81244245246247 thumb adduction at 50–100% of maximal voluntary contraction (MVC) either until exhaustion 242243245246 or for a specified amount of time 81244246247. Other studies have also utilized static knee extension at an intensity of 30 248 or 66% MVC 249. While prolonged isometric exercise is characterized by acute ischemic conditions, one study attempted to exacerbate the ischemic period by placing a blood pressure cuff (inflated to 30 mmhg above known systolic pressure) on the exercising arm during the protocol 247. It is believed that the acute ischemia and rapid reperfusion observed during and following prolonged isometric exercise gives rise to increased RONS formation, perhaps via the radical generating enzyme xanthine oxidase 24. Studies utilizing isometric protocols can be viewed in Table 8 of Additional file 1.

Though data are limited, the majority of the above studies have noted an increase in lipid peroxidation following exercise 81242243244245247, as well as changes in the glutathione redox status 242244246248 and decreased antioxidant capacity 244245. However, changes appear to be transient, rapidly returning to pre exercise levels within minutes following exercise 242247. The highly transient nature of changes in biomarkers may potentially, along with the previously discussed factors, explain some of the null findings 81242248249. Only one study to our knowledge has investigated the impact of antioxidant treatment, reporting an attenuation of glutathione oxidation following handgrip exercise when subjects were given an infusion of 100 ml of NAC during exercise 246.

The majority of studies investigating oxidative stress subsequent to sprinting exercise have utilized some form of fatiguing maximal effort sprint either on a cycle ergometer 30250251252253254 or running surface 255256. Additionally, studies incorporating both an intermittent shuttle run 257258259, as well as a 100 m and 800 m swim 260 will also be discussed in this section. In regards to jumping exercise, one study measured oxidative stress in response to six, 30 second sets of repeated jumping in trained and untrained men 261. These investigations are presented in Table 9 of Additional file 1.

Results for the sprinting studies are much more contradictory than those of the previous section, with a similar number of studies noting both an increase in lipid peroxidation 30251256, protein oxidation 252, and DNA damage 255, as well as no change in lipid 250252253254, protein 250, and DNA 252 oxidation. It may be that the volume of exercise, and/or the resistance applied during sprinting was insufficient to evoke an oxidant stress, as lipid peroxidation has been shown to increase as a function of the resistance applied to the flywheel during cycle sprinting 251. Moreover, a longer duration intermittent shuttle run has been shown to result in increased lipid peroxidation, assessed via increased concentrations of MDA 257258259, with both a null and significant attenuating effect offered by acute 257258 and chronic 259 administration of vitamin C prior to the run, respectively. Null findings have also been reported following supplementation for 20 days with Coenzyme Q10 prior to an intermittent maximal sprint test on a cycle ergometer 254.

In regards to other forms of high intensity anaerobic exercise, both successive jumping exercise 261, as well as intense swimming 260 resulted in no change in lipid peroxidation and a decrease in reduced glutathione, respectively.

It has been shown that anaerobic exercise results in increased RONS production and collectively, it appears that all forms of anaerobic exercise possess the ability to result in increased oxidative stress. The mechanisms responsible for the exercise-induced increases in RONS have been suggested to be largely a function of radical generating enzymes (activated in response to ischemia followed by reperfusion) and/or phagocytic immune response following muscle damaging exercise. Similar to aerobic exercise, a variety of factor likely impact the oxidative stress response observed, including, specific biomarkers chosen, time course of sampling, tissues sampled, intensity and volume of exercise, as well as the training status and dietary intake of the subjects. The use of antioxidant supplements has given rise to conflicting results with some studies noting an impact, despite other similar studies reporting no additional benefit of supplementation. Taken together, the results of the anaerobic research are not unlike those of aerobic nature; there are simply fewer data on the former compared to the latter. As with aerobic exercise, it is currently unclear as to whether increased RONS formation observed during anaerobic exercise represents a necessary or detrimental event.

Sporting events often possess components of both an aerobic and anaerobic nature and are typically performed in an outdoor, uncontrolled setting. Thus, such studies are discussed in a separate section and are presented in Table 10 of Additional file 1. A few investigators have examined the oxidative stress experienced following sporting events including football 262, basketball 263, soccer 264265, rugby 266267, motocross racing 268, and professional climbing 269. While most did in fact measure oxidative stress following an acute session 262265266268269, others simply assessed changes in biomarkers at rest following a prolonged period of regular season training 263264267.

Related to football, one study noted an increase in lipid peroxidation (measured via increased total peroxides and antibodies against oxLDL) following a professional American football game 262. Similar increases in lipid peroxidation have also been noted following a rugby match 266 and soccer practice 265, with untrained rugby players experiencing exacerbated increases in lipid peroxidation compared to their trained counterparts 266. Moreover, trained athletes have been shown to possess higher levels of antioxidant protection 267, as well as lower levels of resting lipid peroxidation 264 compared to sedentary controls. Both continuous climbing to exhaustion, as well as a simulated motocross race resulted in an increase in MDA, PC, GSSG, and TAC 268269, with climbing exercise also inducing a decrease in GSH and TGSH 269.

Although various sporting events appear to result in increased oxidative stress, it is likely that the vigorous training accompanied by such events leads to an up-regulation in antioxidant defenses, thereby protecting individuals from excessive oxidative damage. However, as may be the case with long duration aerobic exercise, athletes participating in a high volume of vigorous exercise may benefit from antioxidant treatment, as supplementation has been shown to result in decreased oxidative stress and increased antioxidant defenses in professional basketball players 263.

The data presented thus far has been relative to investigations using human subjects. However, an extensive body of research is available with regards to exercise-induced oxidative stress in animal models. Because of the volume of this work, in addition to the fact that multiple tissues and biomarkers are often studied, each individual investigation will not be presented in table format. Rather, a brief synopsis of this work will be presented below.

First and foremost, it should be noted that the results of research utilizing animal models are not unlike those using human subjects in that most demonstrate an increase in oxidative stress biomarkers with acute exercise. It should also be noted that there exists more consistency in the reported findings with the animal work, likely due to the homogeneity of animals and the great degree of control that can be implemented in these designs. The vast majority of investigators have reported increases in various oxidative stress biomarkers in several tissues following a myriad of both aerobic 25270271272273274275276277278279280281282283284285286287288289290291292 and anaerobic 293294295296297298 exercise protocols. Null findings for lipid 272298299300301302, protein 279300303, and glutathione 290304 oxidation are far more scarce than those seen in human studies, which could potentially be explained by the much more controlled nature of animal research as well as the feasibility of measuring a variety of oxidative stress biomarkers in several biological tissues (e.g., heart, brain, lung, kidney, diaphragm, skeletal muscle, blood).

In a study conducted by Ruiz-Larrea et al. 305, the female sex hormone estrogen was shown to exhibit antioxidant properties in vitro, and because females possess a larger concentration of estrogen compared to males, it was believed that they may be less susceptible to oxidative stress 172186. Evidence in support of this notion has been provided by both animal and human studies, although gender differences appear much more pronounced when utilizing animal models, as female rats run to exhaustion have shown modest if any exercise-induced oxidative stress 306 as compared to male rats 307. In addition to an attenuated response following acute exercise, female rats have also been shown to possess lower resting levels of oxidative stress compared to males 308. However, estrogen may not be the only factor involved in gender comparisons of oxidative stress 306, as vitamin C, vitamin E and glutathione levels were also reported to differ in male and female rats following an acute exercise bout 309 as well as at rest 308. Moreover, estrogen administration to male rats resulted in a decrease in vitamin C levels within the muscle 310, providing evidence that alternative mechanisms other than increased estrogen may play a role in explaining the attenuated oxidative stress response observed in the above investigations.

In regard to studies conducted utilizing human subjects, Chung et al. 86 investigated the role of estrogen in decreasing exercise-induced oxidative stress and found minimal difference in oxidative stress levels of women during both the luteal and follicular phases of their menstrual cycle. In support of Chung and coworkers 86, several other studies have reported no difference in the exercise-induced oxidative stress response between men and women following both submaximal aerobic 4399114, long duration aerobic 172, and isometric 246 exercise. It should be noted that although no differences were reported following acute exercise, women have been shown to possess decreased oxidative stress, as well as increased antioxidant protection at rest compared to men 99114311. In opposition to the above findings, Ginsburg et al. 186 reported a decrease in the susceptibility of plasma lipids to peroxidation in men following a triathlon, with no significant change being noted in women. However, uncontrolled antioxidant supplementation occurred in the study and women were 10 yrs older than men and their activity time was about 150 minutes longer with exercise intensity not matched 186.

Collectively, it appears that both men and women are susceptible to oxidative stress at rest and during exercise. Female resting levels of oxidative stress markers may be lower, but exercise-induced oxidative stress responses appear similar between genders. Women's lower resting levels could in part be due to their higher expression and activity of antioxidant enzymes and could potentially explain their longer life span 308.

Clearly, acute exercise imposes a physical stress on the body, as numerous studies have shown that oxidative stress biomarkers are increased following both aerobic and anaerobic exercise. However, whether this exercise-induced increase in RONS exerts detrimental effects on long term physiological function remains a topic of debate, as an ever increasing body of evidence in the area suggests that biologically-derived RONS act in a hormetic manner 9312313. That is, in response to repeated exposure to toxins and/or stressors the body undergoes favorable adaptations that in turn result in enhanced physiological performance and improved physical health 9313. Thus, an optimal level of RONS production appears conducive to optimal health, whereas too little or too much RONS result in impaired defense capabilities or extensive oxidative damage and inflammation, respectively, both of which would be expected to promote the development of ill-health and/or disease. The above concept is perhaps best exemplified when applied to the effects of exercise-induced RONS production on the intracellular redox balance. Recall from above that the redox state present within individual cells has been suggested as a key component of gene expression, as well as cell function, and that chronic disregulation of such balance in favor of a more oxidizing environment is associated with the development of numerous diseased states, in addition to the aging process 16. Moreover, because a more reducing environment is believed to promote health-enhancing effects 16, interventions designed to shift the redox balance in favor of greater reducing potential via increasing antioxidant defenses appears warranted.

One such method that appears to exert powerful benefits in terms of increasing antioxidant protection is the performance of regular moderate intensity exercise 9. This upregulation in antioxidant defense observed with regular exercise training would be expected to shift the redox balance in favor of more reducing conditions, thereby potentially explaining the pro-health/anti-pathological effects of exercise 16313. The mechanism whereby regular exercise results in an adaptive benefit is well described 9. In brief, exercise-induced RONS appear to serve as the "signal" needed for the activation of MAPKs (p38 and ERK1/ERK2), which in turn activate the redox sensitive transcription factor NF-κB 36, via activation of IκB kinase, which then phosphorylates IκB (the inhibitoy subunit of NF-κB). IκB is then ubiquinated and subsequently degraded via the cytosolic ubiquitin-proteosome pathway, thereby releasing NF-κB to migrate into the nucleus. Several antioxidant enzymes [manganese superoxide dismutase (MnSOD), inducible nitric oxide synthase (iNOS), glumatylcysteine synthetase (GCS)] contain NF-κB binding sites in their gene promoter region and thus are potential targets for exercise-induced upregulation via the NF-κB signaling pathway 9. Therefore, any attempt to attenuate the exercise-induced increase in RONS production (via antioxidant supplementation) may actually blunt the adaptive increase in antioxidant defenses and subsequent desirable shift in redox balance, thereby increasing an individual's susceptibility to disease and prooxidant attack both at rest, as well as during subsequent exercise bouts 3644.

Evidence in support of this notion is provided by the reportedly blunted exercise-induced upregulation in MnSOD, iNOS, reduction in phosphorylation of p38 and ERK1/ERK2, as well as reduced activation of NF-κB in response to allopurinol (a known inhibitor of xanthine oxidase) administration 36. Additionally, in both human and animal models, supplementation with vitamin C has been shown to blunt adaptive increases in VO_2max, as well running to exhaustion 312. Similar results in terms of reduced exercise performance following antioxidant supplementation have been reported with the use of vitamin C 314, vitamin E 315 and ubiquinone-10 316 in greyhounds and humans, respectively. It should be understood that the potential negative effects of antioxidant supplementation may exist only when applied to moderate intensity exercise, as administration of antioxidants during competitive and/or exhaustive exercise training periods has been shown to attenuate markers of muscle damage and lipid peroxidation 317.

Collectively, it would seem that an optimal level of RONS produced during exercise is not only necessary, but advantageous in that it serves to drive the desired adaptive response. In support of this notion, adaptations that occur to the body's antioxidant defense system in response to regular exercise appear to not totally eliminate oxidative damage, but merely reduce potential damage from future acute bouts of exercise 24318, as well as other ROS generating situations. These findings support the idea that complete elimination of exercise-induced RONS would not be conducive to optimal physiological function. On the contrary, the production of RONS above and beyond that currently undefined level, potentially as a consequence to conditions similar to overtraining (chronic performance of vigorous exercise), may serve to overwhelm the defense system in place, thereby resulting in extensive oxidative damage, decreased performance and ill-health/disease, as evidenced by the increase in disease risk associated with ultra-endurance exercise training 44. At present, it would seem prudent for future research within the area of oxidative stress and exercise to focus attention towards further elucidating this critical limit between desirable and detrimental effects of exercise-induced RONS. This information is important in informing athletes and coaches, exercise enthusiasts and trainers, clinical populations and practitioners, as well as the general population as to the need for antioxidant supplementation within the context of regular exercise. This work may also provide information as to the volume of exercise conducive to beneficial health outcomes.