Oxidative stress occurs when the production of reactive oxygen species (ROS), often referred to as "free radicals," exceeds antioxidant defense (Halliwell and Cross, 1994; Sies, 1997).
The antioxidant system is comprised of both endogenous (within the body) and exogenous (outside of the body) defense mechanisms. Oxidative stress may progress to oxidative damage involving cellular proteins (contractile, structural, and enzymatic), lipids, DNA, and other molecules in ways that might lead to abnormal cellular function.
The degree of oxidative damage, as well as the time course for elevation in oxidative stress biomarkers has varied across studies, and appears dependent on the type, intensity, volume and duration of exercise, the exercise training and nutritional status of the research subjects, and the tissues being investigated (Bloomer and Goldfarb, 2004).
It should be noted that despite the potential for excessive ROS production to be problematic, low levels of ROS appears necessary for important physiological functions such as cell signaling, immune response, and apoptosis (Volaard et al., 2005).
What Is Apoptosis? ///
Apoptosis is a form of programmed cell death in multicellular organisms. It is one of the main types of programmed cell deaths (PCD) and involves a series of biochemical events leading to a characteristic cell morphology and death, in more specific terms, a series of biochemical events that lead to a variety of morphological changes, including blebbing, changes to the cell membrane such as loss of membrane asymmetry and attachment, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation. Processes of disposal of cellular debris whose results do not damage the organism differentiate apoptosis from necrosis.
While ROS are constantly produced in small quantities within biological systems, their presence increases when exposed to both environmental and physical stressors (Halliwell and Cross, 1994). Exercise is one such stressor; hence, the use of antioxidant agents to combat ROS has been popular in recent years.
Exercise And ROS Production
The topic of exercise-induced oxidative stress has received much attention over the past 30 years since it was first reported that lipid peroxidation was increased following 60 minutes of cycling exercise (Dilliard et al., 1978).
Based on the available evidence, it is clear that exercise of sufficient intensity (typically >60% VO2 max or 50% one repetition maximum) and duration increases the formation of ROS, having the potential to create an imbalance between oxidant and antioxidant levels.
Interestingly, while acute exercise appears to increase ROS transiently, this same exercise stimulus is needed to allow for an up-regulation in endogenous antioxidant defenses (Powers et al., 1999). In this way, the generation of ROS acts as the "signal" to allow for these important adaptations in antioxidant defense, which may have significant implications for protection against future elevations in ROS.
The specific sites of ROS generation with acute exercise have been previously discussed in detail (Bloomer and Goldfarb, 2004; Jackson et al., 2007). These include both primary sources in which ROS are generated in direct response to a given condition, as well as secondary sources in which ROS production may occur in response to damage induced through other mechanisms, such as eccentric muscle actions which are common with strength/bodybuilding type exercise.
A major pathway for ROS generation during exercise involves oxygen, where oxygen is ultimately used for ATP production. Under normal physiological conditions, most of the oxygen consumed by cells is reduced to water in the mitochondria.
What Are Mitochondria? ///
The spherical or elongated organelles in the cytoplasm of nearly all eukaryotic cells, containing genetic material and many enzymes important for cell metabolism, including those responsible for the conversion of food to usable energy. Also called chondriosome.
However, some of the oxygen (1-5%) passing through the mitochondrial respiratory chain may give rise to superoxide, which may lead to other harmful ROS. This is especially apparent during acute sessions of strenuous exercise when oxygen uptake may increase 10-20 fold.
In addition to generation through mitochondrial electron transport, ROS can be produced through other primary sources including prostanoid metabolism, and enzymatic reactions involving NADPH oxidase and xanthine oxidase, which are radical species generators (Jackson, 2007).
Secondary sources of ROS initiation can arise from exercise which involves muscle injury, such as high force eccentric muscle actions. This involves to a large extent the invasion of phagocytic cells into damaged tissue in an attempt to promote healing. In addition, muscle injury may be accompanied by disruption of iron containing proteins such as erythrocytes and myoglobin, which can lead to an increase in free iron which is known to catalyze radical reactions.
Therefore, exercise that creates significant trauma such as high impact aerobic exercise or high force eccentric actions, may lead to destruction of these proteins, allowing for increased free iron availability to aid in the production of ROS. Finally, any imbalance in calcium handling, such as excessive intracellular calcium accumulation, may lead to ROS production. This appears to occur through the activation of phospholipase and proteolytic enzymes.
In the above ways, intense physical activity has been reported in several investigations to lead to an oxidative stress, as previously reviewed in detail (Bloomer and Goldfarb, 2004; Finaud et al., 2006; Volaard et al., 2005).
One concern that many athletes have is the impact of ROS formation on physical performance. While few direct human investigations have focused on this area of research, the following section provides a brief summary for consideration.
Association Between ROS And Physical Function
While human data are scarce, animal studies have noted impaired contractile function, reductions in muscle force output, and greater fatigue rates in isolated skeletal muscle as a function of increased ROS (Reid et al., 2001).
It is important to keep in mind that oxidative alteration to proteins in particular can lead to impaired physical performance (Goldhaber and Qayyum, 2000), as proteins are involved in enzymatic reactions as well as actual muscle contraction (actin and myosin filaments).
Two human studies have investigated the relationship between oxidative stress and exercise overtraining/overreaching. In this work, four weeks of aerobic overtraining was linked to decreased blood antioxidant status (Palazzetti et al., 2003).
Findings of increased total peroxides have been noted in professional football players over the course of a five month competitive season (Schippinger et al., 2002). Clearly, additional work using human subjects is needed in this area of research.
Because the potential for impaired performance exists due to heightened oxidative stress, methods to reduce the degree of oxidative stress resulting from exercise have been studied. The most common method is the intake of antioxidant nutrients. The following section describes the antioxidant defense system, which is comprised of both endogenous and exogenous components.
Although ROS are constantly generated and can increase with physical exertion, the extent of oxidative damage is largely dependent on the ability of the body to defend against ROS production. This defense is collectively referred to as the antioxidant defense system, and includes both enzymatic and non-enzymatic antioxidants.
Common antioxidant enzymes include superoxide dismutase (SOD), of which three primary forms are known to exist: a cytosolic copper-zinc enzyme (Cu-ZnSOD), a mitochondrial enzyme requiring manganese (MnSOD), and an extracellular SOD (EC-SOD).
Other antioxidant enzymes include glutathione peroxidase (GPx) and catalase (CAT), both of which function to inactivate hydrogen peroxide prior to reacting with the transition metals.
The major non-enzymatic antioxidant within the body is glutathione, which typically exists primarily in the reduced form (GSH). Dietary intake (via whole food or nutritional supplements) supplies further antioxidants in the form of vitamins (e.g., A, C, E), minerals (e.g., selenium, zinc), flavonoids, carotenoids (e.g., beta-carotene), and phenols.
It should be noted that many of these antioxidants function together to provide cellular protection within the body. For example, vitamin C acts to "recycle" vitamin E when vitamin E forms the vitamin E radical while performing its function as a potent chain-breaking antioxidant, intercepting lipid peroxyl radicals.
As mentioned earlier, regular exercise can increase endogenous antioxidant defense, often coupled with decreased ROS formation (Ji, 2002; Powers et al., 1999). This often leads to a decrease in oxidative stress at rest and following acute exercise. While most studies have focused on adaptations resulting from aerobic exercise, positive findings are also available in reference to anaerobic exercise (e.g., weight training).
Glycine Propionyl-L-Carnitine (GPLC) as an Antioxidant Agent
Antioxidant micronutrient intake has been widely used in an attempt to decrease oxidative stress resulting from exercise (Urso and Clarkson, 2003). One antioxidant nutrient that has received considerable attention is L-carnitine, with the amino acid precursor, glycine, also noted as having antioxidant properties.
In fact, previous work involving animals has noted decreased protein and lipid peroxidation following glycine use (Malyshev et al., 1996; Senthilkumar et al., 2004; Zhong et al., 1996).
More specifically, propionyl-L-carnitine (PLC), a propionyl ester of L-carnitine, has potent antioxidant properties (Reznick et al., 1992; Vanella et al., 2000), which protect tissue from oxidative stress-induced injury. This effect has been suggested to be partly related to the role of PLC to enhance blood flow (Loffredo et al., 2007), possibly mediated by an increase in nitric oxide, a finding that has been replicated in two recent studies using a combination of PLC and glycine (Bloomer et al., 2007; Bloomer et al., in press).
This unique combination of PLC and glycine is a molecularly bonded USP grade nutritional ingredient called glycine propionyl-L-carnitine (GPLC). Aside from the effect of oral GPLC to increase blood nitric oxide production, GPLC has been reported to decrease ROS mediated oxidation of lipids in a recent study (Bloomer et al., in press).
In this study, subjects received oral GPLC at a dosage of either 1.5 or 4.5 grams per day over the course of an eight week intervention period. With both dosages, the level of oxidative damage to lipids measured in a rested state was significantly lower at the end of the intervention period as compared to pre intervention. The same was not true for subjects receiving a placebo.
These findings highlight the significant antioxidant properties of GPLC. While these preliminary results are of interest, future work is needed to investigate the impact of GPLC on decreasing oxidative stress resulting from strenuous exercise.
Practical Applications of GPLC
The dietary ingredient GPLC has been shown to possess both antioxidant properties (Bloomer et al., in press) and to increase blood nitric oxide (Bloomer et al., 2007; Bloomer et al., in press). This dual action role makes this ingredient one to consider for both general health enthusiasts and athletes who are interested in improving antioxidant defense while potentially stimulating an increase in blood flow due to the increased levels of nitric oxide.
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