Oxidative stress (OS) is a general term used to describe the steady-state level of oxidative damage in cells, tissue or organ caused by the reactive oxygen species (ROS). OS initially affects a specific molecule ultimately making the entire organism susceptible to damage. Imbalance can result from a lack of antioxidant capacity or by an over production of ROS. Increased ROS may be related physical, environmental and behavioral stress. Lack of antioxidant capacity may also be caused by multiple mechanism involving various metabolic pathways. Excess ROS can damage lipids, protein, DNA and RNA and other macromolecules thereby altering the normal functioning of the body. Aging process in a healthy population may be related to overproduction of ROS . Apart from dietary habits regular physical activity may decrease OS .
Yoga is one of the six orthodox systems of Vedic philosophy. Maharshi Patanjali, father of yoga, compiled and refined various aspects of yoga systematically in his yoga “sutras” (aphorisms). He described various yogic techniques including Astanga yoga (eight folds/eight limbs of yoga) for the all round development of human personality (Table 1). “Patanjali Yoga Sutra” is the first book on yoga for practical aspects. According to his philosophy, yoga is the “chitta vritti norodha” means yoga is the control over mind, body and activities. Maharshi Patanjali defined Astanga yoga for anyone, who may be saint, saga or general family person. Astanga yoga is also known as Raj yoga. Astanga yoga is divided into two parts Bahirang (external) yoga and Antaranga yoga (Internal) yoga. Bahiranga yoga includes first five limbs up to Pratyahara and antaranga yoga includes last three limbs. Niyama and yama described the precautions during, before and after yogic practices. The third limb of Astanga yoga is posture, also known as asana. It strengthens the body and provides the practitioner more flexibility to sit for much more time for pranayama and dhyana. Breathing maneuvers are generally known as pranayama. According to Pandit Shriram Sharma Acharya Pranayama is the capacity to inhale extra Prana (air). After attaining maturity in pranayama practitioner goes toward antaranga yoga (Dharana, Dhyana and Samadhi).
Yoga is well-known for its beneficial effects on physiological, physical and psychological health [3, 4, 5]. Many studies on healthy and diseased population showed the beneficial role of yoga on central nervous system, respiratory system, neurophysiology and neuropsychology [3, 4, 6]. Studies on different healthy and diseased population showed the beneficial effect of yoga in shifting the practitioner toward a more stable redox state [7, 8, 9, 10, 11, 12, 13]. Another study showed yogic practices to have a beneficial role in slowing down age-related changes in free radical production and its worst impacts on the body . There is a need of systematic review of the relevant literature to determine if regular yoga practice is indeed beneficial to reduce OS. Therefore, in this review, research studies investigating the effects of yoga training on OS and enhancement of the antioxidant levels of practitioners were examined. Specific evidence-based yoga adaptations for OS reduction have been identified.
Materials and methods
Data source and searches
The search articles for this systematic review were identified by accessing the MEDLINE, EMBASE, SCOPEMED and Indian database as well as through the reference lists of relevant journal articles. Each of these databases was searched using the keywords yoga AND OS OR antioxidant. Where full-texts were not available, attempts were made to contact the author. If a reply was not received within 10 days from the corresponding author, abstracts were studied to verify if they had the sufficient information.
For inclusion in this review, studies had to be written in English and published in peer-reviewed journals through the institute’s library database. These studies were separated into two categories: experimental trial studies and non-experimental review-type studies. Article was excluded if it was single-subject case study or an editorial or an immediate effect of yogic practices. Randomized controlled trials (RCTs) were included in the review if they were involved in at least one asana and/or one pranayama and/or meditation and reported at least one OS or antioxidant parameters. Apart from that, studies not categorizing the volunteers were excluded from the review. Figure 1 shows the details of literature search and inclusion exclusion of studies in this review.
Data abstraction and quality assessment
Using the aforementioned techniques, 77 studies were identified and considered for initial evaluation. Of which 31 studies were removed as they were duplicates. Among last 46 studies, 36 were excluded due to their intervention and/or outcome and 1 study was excluded because it was a commentary. A total of nine studies met rigorous criteria for final review. For each RCT, authors extracted the following study details: participant characteristics, details of the intervention of yoga and control groups, blinding, dropout rate, and OS measures. The quality of the RCTs was assessed using Cochrane risk of bias tool  (Table 2). Although meta-analysis has a major impact on systematic review, but it is not done here for this review.
The search yielded a total of 97 trials, of which 11 met rigorous criteria for final review. Among these, 4 RCTs [13, 16, 17, 18] investigated the effects of regular yoga practice on healthy individuals, 2 RCTs [7, 8] on diabetic patients, 1 RCT (Hegde et al., 2013) on prediabetic patients, 3 RCTs [10, 11, 19] on hypertensive patients and 1 RCT  on end-stage renal disease patients.
In healthy young, significant enhancement was noted in reduced glutathione (GSH), ratio of reduced to oxidized glutathione (GSH/GSSG), total antioxidants status (TAS), vitamin C, vitamin E, superoxide dismutase (SOD) activity, glutathione reductase (GR) activity and glutathione –S transferase (GST) activity. Significant reduction was noted in GSSG, glutathione peroxidase (GPx) activity and malondialdehyde (MDA) contents following yogic practice as shown in Table 3 and Table 4 [12, 13].
In diabetic patients, significant enhancement was noted in GSH, SOD activity and vitamin C contents. Significant reduction was noted in MDA contents following the yogic practices as shown in Table 3 [7, 8]. In prediabetic patient, significant reduction in SOD activity and MDA contents was noted following yogic practices as shown in Table 3 . In hypertensive patients, MDA content was decreased and ferric-reducing ability of plasma (FRAP) was increased following yogic practices as shown in Table 3 [10, 11]. End-stage renal disease patient showed enhancement of SOD activity and reduction of protein oxidation (POX) following yogic practice as shown in Table 3 . Effects of yogic practices on OS parameters are also described in Table 4.
Details of yogic practices provided by researchers are depicted in Table 5.
Regular yogic practices have shown consistently to improve antioxidant and redox status and reduce OS. Herein, this article reports the effects of yogic practices on reduction of OS, as measured by MDA content and POX; enhancement of antioxidants as measured by GSH, GSH/GSSG, TAS, vitamin C, vitamin E and improvement of antioxidant defense enzymes as measured by SOD activity, GPx activity, GR activity, GST activity and catalase activity. Table 3 and Table 4 summarize the results from studies investigating the effects of regular yogic practice on these parameters of antioxidant and redox status in practitioners. Table 5 shows the list of yogic practices provided by the researchers.
Yogic practices decreased the activity of GPx and increased the activity of GR. The GSH level increased significantly. GSH/GSSG, another important and sensitive marker of the antioxidant systems, has been shown to increase significantly following yogic practices. These may be due to the improvement of the antioxidant defense mechanism by yoga. Antioxidant defense mechanisms improved by the increase in antioxidant defense enzymes such SOD activity, GPx activity, GR activity, GST activity and catalase activity. Hydrogen peroxide (H2O2) is generated directly from superoxide (O2−) though a rapid dismutation reaction that can occur either enzymatically with SOD or spontaneously during aerobic metabolism or exercise. SOD catalyzes the formation of H2O2 from two O2−, whereas catalase catalyzes the formation of oxygen (O2) and water (H2O) from H2O2 molecule. Studies on healthy individuals , diabetic patients  and end-stage renal disease patients  showed that SOD activity increased significantly following yogic practices. Reports showed that catalase activity is increased after yogic practices. This will convert more H2O2 into O2 and H2O. GPx has a role in oxidizing hydroperoxides, including H2O2, and functions to protect the cell from oxidative damage. GSSG, produced after oxidation of hydroperoxide by GPx, is recycled to its reduced state (GSH) by GR using reduced nicotinamide adenine dinucleotide phosphate (NADPH) as hydrogen donor. Earlier researchers showed that the yogic practices increased GSH and decreased GSSG. This may be due to alteration of activity of antioxidant defense enzymes GPX and GR. GSTs are multifunctional enzymes which play a key role in cellular detoxification. The enzymes protect cells against toxicants by conjugating them with glutathione, thereby neutralizing their electrophilic sites, and rendering the products more water-soluble . The glutathione conjugates are metabolized further to mercapturic acid and then excreted. GST also acts to catalyze the reaction between the Thiol group of GSH and possible alkylating agents, allowing GSH to carry out its detoxifying functions. A study in healthy young subjects found that GST activity is increased significantly after 3 months yogic practices . This may reduce ROS production. Level of vitamin C and vitamin E is increased following yogic practices [7, 13]. Vitamin C acts to regenerate vitamin E by accepting the electron from the vitamin E radical. The vitamin C radical which is formed is either excreted in the urine or reconverted to vitamin C via electron donation from GSH. It is also reported that both vitamin E and vitamin C function to scavenge superoxide radicals and hydroxyl radicals in lipid and aqueous phase, respectively. This inhibits lipid peroxidation and oxidative damage to other macromolecules. Significantly improved TAS after yogic practices denotes a marked improvement in the overall cellular antioxidant level . Increased FRAP following yogic practice in hypertensive patients may be due to increased vitamins, GSH level and overall TAS . OS generally causes damage to the membrane polyunsaturated fatty acids leading to the generation of MDA, a thiobarbituric acid reacting substances. Studies also showed that the yogic practices decreased MDA level in healthy young diabetic patients, prediabetic patients, hypertensive patients and end-stage renal disease patients [7, 8, 9, 10, 11, 12, 13, 17] .
This review, however, has identified only one study from each category i. e. prediabetes  and end-stage renal disease patient . Some studies did not elaborate the yogic practice, which is essential. Yoga was not the only intervention in some studies. A combination of yoga and diet and/or conventional medicine was used [7, 8, 10, 16]. Studies reporting only one or two indicators of OS are unable to provide sufficient information regarding mechanisms of OS reduction and improvement of antioxidant redox status [8, 11, 17]. Only one evidence was identified showing decreased OS in hypertensive women . The findings of this study demonstrate that yogic practices are suitable physical activities for the reduction of OS and improvement of antioxidant redox status. Studies on healthy individuals provide sufficient information regarding the significant reduction of OS and improvement of antioxidant status following yogic practices. Significant reduction in MDA contents following yogic practice by many researchers also indicates reduction of OS in diabetic, prediabetic, hypertensive and end-stage renal disease patients.
Although a great deal has been learned about the yoga during the last few decades, much additional information is necessary if we want to serve adequately for human health and wellbeing. Some key areas these reviews are identified as critical areas for future research. There is a need to fully correlate the features of yoga with specific outcome of antioxidant redox system. Most of the studies are done on healthy young males. We do need to know more about the impact of yoga in various disease states, aged population and females. More experimental studies are needed to find out the potential mechanism that may explain the changes in antioxidants that are elicited by yoga.
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About the article
Published Online: 2017-10-25
Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
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Employment or leadership: None declared.
Honorarium: None declared.
Competing interests: The funding organization(s) played no role in the study design; in the collection, analysis and interpretation of data; in the writing of the report; or in the decision to submit the report for publication.