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How does yoga reduce stress? A systematic review of mechanisms of change and guide to future inquiry

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Yoga is increasingly used in clinical settings for a variety of mental and physical health issues, particularly stress-related illnesses and concerns, and has demonstrated promising efficacy. Yet the ways in which yoga reduces stress remain poorly understood. To examine the empirical evidence regarding the mechanisms through which yoga reduces stress, we conducted a systematic review of the literature, including any yoga intervention that measured stress as a primary dependent variable and tested a mechanism of the relationship with mediation. Our electronic database search yielded 926 abstracts, 71 of which were chosen for further inspection, 5 of which were selected for the final the systematic review. These five studies examined three psychological mechanisms (positive affect, mindfulness, self compassion) and four biological mechanisms (posterior hypothalamus, IL-6, CRP, cortisol). Positive affect, self-compassion, inhibition of the posterior hypothalamus, and salivary cortisol were all shown to mediate the relationship between yoga and stress. It is striking that the literature describing potential mechanisms is growing rapidly, yet only seven mechanisms have been empirically examined; more research is necessary. Also, future research ought to include more rigorous methodology, including sufficient power, study randomization, and appropriate control groups.
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How does yoga reduce stress? A
systematic review of mechanisms of
change and guide to future inquiry
Kristen E. Riley
a
& Crystal L. Park
a
a
Department of Psychology, University of Connecticut, Storrs, CT,
USA
Accepted author version posted online: 03 Jan 2015.Published
online: 15 Apr 2015.
To cite this article: Kristen E. Riley & Crystal L. Park (2015): How does yoga reduce stress? A
systematic review of mechanisms of change and guide to future inquiry, Health Psychology Review,
DOI: 10.1080/17437199.2014.981778
To link to this article: http://dx.doi.org/10.1080/17437199.2014.981778
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How does yoga reduce stress? A systematic review of mechanisms
of change and guide to future inquiry
Kristen E. Riley* and Crystal L. Park
Department of Psychology, University of Connecticut, Storrs, CT, USA
(Received 23 August 2013; accepted 26 October 2014)
Yoga is increasingly used in clinical settings for a variety of mental and physical
health issues, particularly stress-related illnesses and concerns, and has demonstrated
promising efficacy. Yet the ways in which yoga reduces stress remain poorly understood.
To examine the empirical evidence regarding the mechanisms through which yoga
reduces stress, we conducted a systematic review of the literature, including any yoga
intervention that measured stress as a primary dependent variable and tested a
mechanism of the relationship with mediation. Our electronic database search yielded
926 abstracts, of which 71 were chosen for further inspection and 5 were selected for the
final systematic review. These five studies examined three psychological mechanisms
(positive affect, mindfulness and self-compassion) and four biological mechanisms
(posterior hypothalamus, interleukin-6, C-reactive protein and cortisol). Positive affect,
self-compassion, inhibition of the posterior hypothalamus and salivary cortisol were all
shown to mediate the relationship between yoga and stress. It is striking that the literature
describing potential mechanisms is growing rapidly, yet only seven mechanisms have
been empirically examined; more research is necessary. Also, future research ought to
include more rigorous methodology, including sufficient power, study randomisation
and appropriate control groups.
Keywords: yoga; stress reduction; mindfulness; clinical interventions; methodology
Introduction
Accumulating experimental and clinical research demonstrates that yoga reduces stress
(see Chong, Tsunaka, Tsang, Chan, & Cheung, 2011). The term mechanism, which will
be used throughout this paper, refers to underlying psychological, social and neurophy-
siological processes or mediators through which therapeutic change occurs (Kazdin &
Nock, 2003); in this case, mechanism refers to those processes caused by yoga that lead
to reduction in stress. However, the mechanisms through which yoga may alleviate stress
remain unclear and some healthcare professionals are reluctant to recommend yoga to
their patients, in part due to this lack of clarity (Roehr, 2008). Many mechanisms have
been proposed, but few studies have been conducted to assess these pathways (Black
et al., 2012; Michalsen et al., 2005). The psychological benefits include positive affect,
mindfulness and self-awareness; among the biological are lower levels of cortisol and
nitric oxide.
No explicit reviews of the mechanisms through which yoga is purported to affect
stress have been published. However, theories about why and how yoga affects health
*Corresponding author. Email: Kristen.Riley@UConn.edu
Health Psychology Review, 2015
http://dx.doi.org/10.1080/17437199.2014.981778
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have been put forward, and many of these theories address stress and the proposed
linkages between yoga and health. Specifying the paths through which yoga may reduce
stress will increase the likelihood that it is recommended and implemented as a
complement to psychological and pharmacological therapy for stress-related conditions.
Knowledge of these paths will also allow for tailoring of yoga interventions to specific
types of stress and the basing these interventions on the most efficacious components of
yoga. This paper summarises the most commonly cited proposed links of yoga and stress
and then systematically reviews the empirical tests of those mechanisms. Finally, this
paper makes concrete methodological suggestions to improve future inquiry in this area.
Stress: definition and measurement
Aldwin (2007) suggested that stress refers to the quality of experience, produced through
a person-environment transaction that, through either overarousal or underarousal, results
in psychological or physiological distress (p. 22). Stress has been measured in myriad
ways; these ways can be broadly categorised as either subjective or objective (see Li &
Goldsmith, 2012, for a review). Subjective measures of stress are self-report measures
that assess perceived stress, the degree to which one appraises situations in ones life as
stressful (Cohen, Kamarck, & Mermelstein, 1983). Objective measures of stress consist
of biological markers (biomarkers) of stress (Li & Goldsmith, 2012). Juster, McEw en,
and Lupien (2009) suggested 25 biomarkers that reflect a stress response (specifically, in
studies assessing allostatic load). Because the number of biomarkers that may reflect
stress processes are potentially innumerable, and also because biomarkers often reflect
many other biological processes, we limited our yogastress empirical review of the
objective meas ures to these 25 biomarkers that have been identified as particularly
associated with stress. These stress biomarkers can be categorised as neuroendocrine,
immune, metabolic, cardiovascular and anthropometric (see Appendix 1).
How yoga reduces stress
A recent review of yogas effect on stress reported that 25 of 35 published articles
reported a significant reduction in stress after a yoga intervention (Li & Goldsmith,
2012). The authors attributed the inconsistent or null findings of the other 10 studies
largely to methodological flaws, including small sample size, lack of a control group,
non-randomisation, lack of exclusion criteria and large standard deviations (Li &
Goldsmith, 2012).
Proposed mechanisms: psychological
Psychological mechanisms that have been proposed as ways through which yoga
ameliorates stress include increases in positive attitudes towards stress (Malathi &
Damodaran, 1999; Taylor, 2003; Woodyard, 2011), self-awareness (Arora & Battacharjee,
2008), coping mechanisms (Heilbronn, 1992; Kinser, Bourgiognon, Whaley, Hauenstein,
& Taylor, 2013; Rizzolo, Zipp, Stiskal, & Simpkins, 2009), appraisal of control (Bonura,
2008; Roehr, 2008), calmness (Brown & Gerbarg, 2005b; Sherman, Wellman, Cook,
Cherkin, & Ceballos, 2013), spirituality (Evans, Cousins, Tsao, Sternlieb, & Zeltzer,
2011; Moadel et al., 2007), compassion (Braun, Park, & Conboy, 2012) and mindfulness
(Brown & Ryan, 2003; Chiesa & Serretti, 2009; Evans et al., 2011).
Many researchers suggest that mindfulness is a powerful link between yoga practice
and stress reduction (Dunn, 2008). Brown and Ryan (2003) define mindfulness as the
2 K.E. Riley and C.L. Park
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state of being attentive to and aware of what is taking place in the present. Its association
with yoga practice, as well as its ability to combat stress, has been widely demonstrated
(see Chiesa & Serretti, 2009, for a review). Studies suggest mindfulness is associated
with measures of lower stress (Gilbert & Waltz, 2010). Yoga has been associated with
higher levels of mindfulness. For example, Brisbon and Lowery (2011) found that
advanced yoga practitioners had higher levels of mindfulness (and lower levels of stress)
compared to new yogis.
Proposed mechanisms: biological
Proposed biological mechanisms through which yoga may reduce stress include the
autonomic nervous system (Brown & Gerbarg, 2005a; Evans et al., 2011; Field, 2012;
Malathi & Damodaran, 1999; Michalsen et al., 2005), hypothalamicpituitaryadrenal
(HPA) axis (Innes, Vincent, & Taylor, 2007; Purdy, 2013; Ross & Thomas, 2010),
endothelial function and release of nitric oxide (Dunn, 2008; Michalsen et al., 2005),
endogenous cannabinoids and opiates (Michalsen et al., 2005), inflammatory and
endocrine responses (Kiecolt-Glaser et al., 2010; Yadav, Magan, Mehta, Sharma, &
Mhapatra, 2012), limbic system activity (Riley, 2004), the peripheral nervous system
including GABA (Streeter, Gerbarg, Saper, Ciraul o, & Brown, 2012) and gene expression
(Black et al., 2012). There is a growing literature regarding the possible mediating effects
of the autonomic nervous system, the HPA axis and inflammatory and endocrine
responses.
Autonomic nervous system. Both sympathetic nervous system (SNS) and parasympathetic
nervous system (PNS) are often posited as the mechanism through which yoga reduces
stress (e.g., Brown & Gerbarg, 2005a; Evans et al., 2011;Malathi&Damodaran,1999;
Michalsen et al., 2005). Yoga training has been shown to decrease sympathetic response
(systolic pressure, diastolic pressure, mean pressure, heart rate and rate pressure product)
after experiemental stressors (Vijayalakshmi, Madanmohan, Patil, & Kumar, 2004), though
no mediation analyses were conducted. Field (2012) hypothesised that enhanced vagal
activity caused by the stimulation of pressure receptors due to yoga would lead to decreased
stress hormones such as cortisol.
The vagus nerve regulates heart rate, blood pressure, gastric acid secretion and
functions in many other organs. Innes et al. (2007) pointed out that yoga has immediate
beneficial effects on heart rate variability and baroreflex sensitivity, which are thought to
reflect direct stimulation of the vagal nerve. Sengupta, Chaudhuri, and Bhattacharya
(2013) posited that yoga decreases vagal stimulation, which causes parasympathetic
activation, decreasing perceived stress while simultaneously decreasing activation and
reactivity of the sympathoadrenal system and the HPA axis.
HPA axis. Purdy (2013) summarised the burgeoning research demonstrating that yoga
down-regulates the HPA axis and SNS, both of which have been shown to prevent the
release of the stress hormones cortisol and catecholamines (Ross & Thomas, 2010
). Innes
et al. (2007) examined the potential mediators of yogas effect on cardiovascular disease,
including stabilising HPA axis and sympathoadrenal activity, which have a regulating
effect of stress. They pointed to studies that have demonstrated reductions in markers of
sympathetic activation, including cortisol, catecholamines, reninangiotension and basal
skin conductance, and progre ssive atten uation of sympathoadrenal activation, including a
decline in heart rate and a decline in blood pressure.
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Inflammatory and endocrine responses. Yadav et al. (2012 ) found that there was a reduction
in stress, as measured by plasma corti sol and beta-endorphin as well as inflammation
[interleukin-6 (IL-6) a nd tumour necrosis factor] after a short-term yoga lifestyle
intervention, suggesting that reductions in inflamm ation factors may contribute to a
decrease in stress following a yoga intervention. Black et al. (2012) found reduced activity
of the proinflammatory NF-κB factors and increased activity of antiviral interferon
regulatory factor, both of which associated with chronic stress.
Methods
To examine the empirical evidence regarding the mecha nisms through which yoga
reduces stress, we conducted a systematic review of the literature. We followed the
Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA)
guidelines for conducting systematic reviews (Moher, Liberati, Tetzlaff, Altman, & The
PRISMA Group, 2009). For this review, the inclusion criteria were: any study of a yoga
intervention that measured stress as a primary dependent variable and that tested a
mechanism of the relationship between yoga and stress with mediation analyses.
Mediation analyses included those statistics meant to test direct or indirect effects, above
and beyond simple bivariate correlations, such as regression analyses according to Baron
and Kennys(1986) mediation criteria.
Search
We conducted a multi-database literature search utilising the following databases:
Medline via PubMed and PsycInfo, including dissertations. Databases were searched for
the occurrence of keywords/phrases at any place in the record: stress AND yoga AND
(mechanism OR mechanisms OR mediator OR mediators). In order to capture studies of
effects of yoga on markers of stress, we then used the search terms yoga and the
indicators of stress as defined by Juster et al. (2009, Table 2: Biomarkers repeatedly used
in allostatic load studies, see Appendix 1), for example, yoga AND cortisol.We
inspected the reference sections of retrieved studies as well as conceptual/theoretical
articles on yoga and stress for additional manuscripts.
Coding and synthesis
We extracted the following information from articles: mecha nism, mechanism type,
outcome, type of yoga intervention, type of stress outcome and author/article information
(see Table 2) and sample size, control group information and study randomisation
(Table 3). We had two coders at each stage of the search and coding process.
Results
Figure 1 provides a flow chart of our search and selection process. Our electronic
database search yielded 926 abstracts. We examined abstracts for evidence of an
empirical quantitative study of a yoga intervention and a stress outcome, narrowing the
926 abstracts to 71 articles. We downloaded and examined these 71 articles, and narrowed
by the inclusion criteria above, yielding five articles (n = 164; see Ta bles 2 and 3).
4 K.E. Riley and C.L. Park
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Psychological mechanisms
Positive affect
In a study of nov ice and expert yog a prac titioners, po sitive affect increas ed from pre- to
post-yoga in a singl e-yoga session (Kiecolt-Glaser et al ., 2010) and in a study of healthy
college students, a Hatha yoga intervention created significant pre to post changes in positive
affect; further, change in positive affect was negatively correlated with change in cortisol,
suggesting a meditational relationship (West, Otte, Geher , Joh nson, & Moh r, 200 4).
Mindfulness and self-compassion
Gard et al. (2012 ) examined the mediating roles of mindfulness and self-compassion
during a four-month residential yoga programme on perceived stress. They found that the
yoga interventions effect on perceived stress was mediated only by self-compassion.
Biological mechanisms
This review identified four biological mechanisms that have been empirically tested as
mediating the effects of yoga on stress: the posterior hypothalamus (Bagga & Gandhi,
Table 1. Proposed mechanisms through which yoga reduces stress.
Mechanism type Proposed mechanism Authors
Psychological Self-awareness Arora and Bhattacharjee (2008)
Attitudes towards stress Malathi and Damodaran (1999),
Taylor (2003)
Strengthened coping
mechanisms
Heilbronn (1992), Rizzolo et al. (2009),
Evans et al. (2011)
Appraisal of control Roehr (2008)
Calm (specifically from
pranayama/breath work)
Brown and Gerbarg (2005b)
Spiritual well-being Moadel et al. (2007), Evans et al. (2011)
Compassion, self-compassion Braun et al. (2012)
Mindfulness, acceptance Brisbon and Lowery (2011),
Evans et al. (2011)
Biological HPA axis, SNS Gururaja, Harano, Toyotake, and Kobayashi
(2011); Michalsen et al. (2005), Moadel
et al. (2007)
Autonomic nervous system
(vagus nerve) arousal
Evans et al. (2011)
Endothelial function and
release of nitric oxide
Dunn (2008); Michalsen et al. (2005)
Endogenous
endocannabinoids and opiates
Michalsen et al. (2005)
Inflammatory and endocrine
responses (e.g., cytokine
levels, IL-6)
Arora and Bhattacharjee (2008); Kiecolt-
Glaser et al. (2010); Yadav et al. (2012)
Limbic system activity Riley (2004)
Immune cell gene expression Black et al. (2012)
SNS Sieverdes et al. (2014)
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Table 2. Empirically studied mechanisms through which yoga may reduce stress.
Mechanism
type Mechanism Outcome Type of yoga Type of stress Authors
Psychological Positive affect Positive affect increased in
yoga group, but no change in
cortisol, so no mediation
effects
Hatha yoga Cortisol Kiecolt-Glaser
et al. (2010)
Positive affect Positive affect increased after
a yoga session then was
negatively correlated with
cortisol
Hatha yoga Perceived stress,
cortisol
West et al. (2004)
Mindfulness, self-compassion Perceived stress was
mediated by self-compassion
but not mindfulness
Hatha yoga Perceived stress Gard et al. (2012)
Biological Posterior hypothalamus Yoga led to inhibition of the
posterior area of the
hypothalamus, decreasing
blood pressure
Transcendental practice and
Shavasana
Blood pressure Bagga and
Gandhi (1983)
Inflammatory and endocrine
responses (IL-6 and CRP)
There was no difference in
IL-6 or CRP after the yoga
session
Hatha yoga Perceived stress Kiecolt-Glaser
et al. (2010)
Salivary cortisol Salivary cortisol decreased
and mediated yoga and
perceived stress
Hatha yoga (three-month
intensive programme)
Perceived stress Michalsen
et al. (2005)
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Table 3. Methodology of empirically studied mechanisms.
Mechanism
type Mechanism Authors Sample size (N) Control group Randomisation
Psychological Positive affect Kiecolt-Glaser
et al. (2010)
Novices = 25
Experts = 25
(exposed to all three
groups)
Movement control, passive video
control
No
Positive affect West et al. (2004) Yoga = 18
Dance = 21
Lecture = 18
African dance, biology lecture No
Mindfulness/
self-compassion
Gard et al. (2012)33 No No
Biological Posterior hypothalamus Bagga and
Gandhi (1983)
Unknown No No
Inflammatory and
endocrine
responses (IL-6 and CRP)
Kiecolt-Glaser
et al. (2010)
Novices = 25
Experts = 25
(exposed to all three
groups)
Movement control, passive video
control
No
Salivary cortisol Michalsen et al. (2005) Yoga = 16
Control = 8
Wait list control No
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1983) and three inflammatory and endocrine responses [IL-6, C-reactive protein (CRP)
and cortisol; Kiecolt-Glaser et al., 2010; Michalsen et al., 2005].
Posterior hypothalamus
Bagga and Gandhi (1983) demonstrated that the Shavasana pose in a yoga session led to
inhibition of activity in the posterior area of the hypothalamus, decreasing blood pressure,
a stress biomarker, after a single session.
Potentially relevant
studies identified and
screened for retrieval
(N=926)
Studies excluded due to
duplication in multiple
databases (N=424)
Studies retrieved for
more detailed
evaluation (N=71)
Studies excluded due
irrelevant focus (N=63)
Studies retrieved for
more detailed evaluation
(N=5)
Studies excluded due to
inaccessible articles (N=0)
Appropriated studies to
be included in review
(N=5)
Studies excluded due to
irrelevant focus in abstract
(N = 431)
Figure 1. PRISMA article selection process.
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Inflammatory and endocrine responses
Kiecolt-Glaser et al. (2010) administered a stressor (Stroop, cold pressor task) before a
yoga class then measured participants physiological response to stress before, during and
after one of three different interventions: a yoga session, a movem ent control condition
and a video control condition. The researchers did not find any change in IL-6 or CRP in
the yoga group as compared to the other groups following the stressful event.
Including cortisol
Michalsen et al. (2005) posited that salivary cortisol levels would act as a mechanism of a
12-week Iyengar yoga intervention on perceived stress and indeed found that salivary
cortisol levels mediated this relationship. Namely, salivary cortisol decreased significantly
after participation in a yoga class, and changes in cortisol significantly predicted
decreased perceived stress over the course of the intervention.
Discussion
The literature linking yoga practice to lower stress levels is growing rapidly, as is the
literature proposing mechanisms to explain this finding. Our systematic review of studies
that have empirically examined as mechanisms of yogas effect on stress identified only
five studies, three of psychological variables and four of biological variables. This
mismatch between the large number of potential mechanisms and paucity of empirical
exploration of them leaves large gaps in the literature in need of exploration.
Discussion of psychological mechanisms
Positive affect
As evidenced in the systematic review, positive affect increased after 46 minutes of yoga
(Kiecolt-Glaser et al., 201 0 ). However, since markers of stress reduction in this study did
not vary pre- to post-yoga class, positive affect could not be linked to a stress outcome,
and therefore could not be claimed as a mechanism through which yoga affects stress. In
future studies, it will be important to measure perceived stress as well as other biological
stress markers in order to have an outcome variable with whi ch to test positive affect as a
mediator or mechanism . It wi ll perhaps also be important to measure the effects of yoga
on stress outcomes over time to detect the differential effects of shorter versus longer-
term interventions on stress variables.
Mindfulness
Increased mindfulness has been empirically linked with both yoga and a decrease in stress
in numerous studies. While Gard et al. (2012) found there was a significant positive
correlation betwee n yoga and mindfulness and a significant negative correlation between
mindfulness and perceived stress, mindfulness was not found to mediate the yogastress
relationship. This study included only two time points, and studies of mediation would
ideally measure variables at least three time points. Future studies should include a
longitudinal study of yoga, mindfulness and stress, with other related variables measured
and included in the model. A cross-lagged panel analysis or a daily diary design would
allow for more causal inference and insight into these relationships.
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Self-compassion
Self-compassion is frequently viewed as another quality underlying the transformations
that result from contemplative pract ices. Self- compassion has been described as being
open to and moved by ones own suffering, experiencing feelings of caring and kindness
towards oneself (Neff, 2003). Pilot studies suggest yoga participation may increase self-
compassion (Braun et al., 2012). Gard et al. (2012) posit that thei r increases in self-
compassion (and resulting decreases in perceived stress) may have been due to students
in this particular intervention receiving instruction to bring awareness and acceptance to
their unique physical attributes as they moved through yoga postures, rather than
emphasising the achievement of the perfect pose. Research is needed to test whether it
was this unique instruction in this yoga intervention that created increases in self-
compassion.
Psychological mechanisms proposed but not studied
Most of the posited psychological mechanisms have not yet been studied empirically,
including attitude towards stress, coping, self-awareness, appraisals of control, calm from
pranayama and spirituality. It will be important to assess these potential psycholo gical
outcomes in future yoga intervention studies.
Woodyard ( 2011) suggests that yoga practice may create a change in attitudes towards
stress, viewing stress and life in a more optimi stic way. It will be important to conduct a
study investigating the impact of yoga practice on these specific types of appraisals about
stress and stressors.
Similarly, some researchers have hypothesised that yoga practice strengthens coping
mechanisms, which allows practitioners to deal with stressors more effectively, thus
reducing stress (e.g., Heilbronn, 1992; Rizzolo et al., 2009). Heilbronn (1992) suggested
that a Hatha yoga workplace intervention could provide breathing and postural techniques
that could be used as a means of quietly and unobtrusively coping with the crises that
occur during the working day (p. 132). Rizzolo et al. (2009) posited that a yoga
intervention for students in health science programmes might help them learn to identify
stress more quickly and develop coping mechanisms early in their career for managing
the high stress of a professiona l practitioner, thus having long-lasting stress-reduction
effects. A qualitative study of the effect of yoga on womens depressive symptoms found
that women reported having learned more coping skills, which decreased their stress
(Kinser et al., 2013).
Yoga has been posited to increases ones sense of control, thereby reducing stress
(Roehr, 2008). Bonura (2008) suggested that self-control is a skill that is learned through
regular yoga practice.
Brown and Gerbarg (2005b) suggested the ways in which Sudarshan Kriya yogic
breathing affects psychopathology, including post-traumatic stress disorder (PTSD),
pointing to four open unpublished datasets. PTSD symptoms did not decrease until Ujayi
breath (audible whisper breath) was added to the yoga (asana, meditation and
psychoeducation) intervention. These authors suggest that the calming Ujayi breath is
one way in which yoga soothes post-traumatic stress symptoms. Sherman et al. (2013)
conducted a qualitative analysis that suggested the importance of breath work in
decreasing stress but did not quantitatively measure their mediation effects.
Increases in spirituality have been linked to both yoga practice (Bussing, Hedtstuck,
Khalsa, Ostermann, & Heusser, 2012) and decreases in stress (Greeson et al., 2011).
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Evans et al. (2011) mentions that a yoga intervention may promote psychos piritual
resources, which he claims are protective against stress. Moadel et al. (2007) also
suggested that spiritual well-being may underlie yogas effect on perceived stress.
Discussion of biological mechanisms
Only three studies have examined four biomarkers as underlying yoga use and stress
reduction.
Posterior hypothalamus
Bagga and Gandhi (1983) conducted very basic methodology and analysis in their 1983
study and only used the Shavasana pose in their inquiry. More sophisticated technology
and data analytic techniques should be applied to examine the relationship of the
posterior hypothalamic region to yoga and stress. FMRI studies are recommended.
Inflammatory and endocrine responses: IL-6, CRP and cortisol
Kiecolt-Glaser et al. (2010) did not find any change in IL-6 or CRP in the yoga group as
compared to the other groups following the stressful event. A longer intervention may be
important to detect changes in stress. Michalsen et al. (2005) demonstrated a mediation of
cortisol on perceived stress. However, cortisol levels are often used as a measure of stress,
so this finding is not surprising.
Biological mechanisms proposed but not studied
Mechanisms proposed but not yet studied include autonomic nervous system and vagal
nerve activity, nitric oxide and endothelial function, endogenous endocannabinoids and
opiates, cytokine levels and limbic system activity.
A review by Dunn (2008) noted that nitric oxide may be a potential mechanism
underlying the relationship between yoga and well-being. Also, Michalsen et al. (2005)
explained that vigorous postures in yoga interventions may cause repeated functional
peripheral vessel occlusion which may modulate blood flow and alter endothelial
function, which in turn may create an environment in which nitric oxide is constantly
produced and released throughout the endoth elium. Nitric oxide presence has been shown
to reduce stress (Michalsen et al., 2005)
Streeter et al. (2012) asserted that yoga-based practices may correct underactivity of
the PNS and GABA systems in part through stimulation of the vagus nerve, the main
peripheral pathway of the PNS, thus reducing allostatic load.
Black et al. (2012) also found that 68 genes were found to be differentially expressed
(19 up-regulated and 49 down-regulated) after a yoga intervention, even after adjusting
for differences in sex, illness burden and body mass index. Although these changes were
not directly connected to a stress outcome in that study, the many studies that have linked
gene expression to stress suggest that gene expression may be an important mediator of
yogas effects on stress.
Other possible mechanisms
Many researchers posited other mechanisms through which contemplative interventions
affect health. Though not specific to stress, these models may be helpful in understanding
the broader mechanisms through which yoga or yoga-like interventions affect stress
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reduction, an aspect of health. For example, in a study of the mechanisms through which
yoga affects cardiovascular d isease, Innes et al. (2007) describe yogas reductions in
psychological factors such as fatigue, which also contributes to stress.
Holzel et al. (2011) detai l the potential mechanisms throu gh which mindfulness
meditation, often a component of yoga practice, may affect stress and well-being. This
model posits that mindfulness meditation acts through psychological factors such as
attention regulation, body awareness, emotion regulation, change in perspective on the
self and neuroplastic changes.
Kuntsevich, Bushnel l, and Theise (2010) also posited a number of biological
pathways through which yoga may affect health. Namely, they claimed that yogic
practices may restore physiologic set points to normal after disease or injury, promote
homeostatic negative feedback loops over non-homeostatic positive feedback loops in
molecular and cellular interactions and decrease abnormal noise in cellular and
molecular signalling networks arising from environmental or internal stresses.
Some studies have compared yoga experts and novices in an attempt to draw
conclusions about the ways through which continued yoga practice affects stress.
Additionally, Kiecolt-Glaser et al. (2010) compared adiponectin and leptin data from
well-matched novice and expert yoga practitioners. These adipocytokines have counter-
regulatory functions in inflammation; leptin plays a proinflammatory role, while
adiponectin has anti-inflammatory properties. Leptin was 36% higher and adiponectin
levels were 28% higher in novices compared to experts, a statistically signi ficant
difference. The authors point to this finding as promising evidence supporting the
processes through which yoga may reduce stress, even though it was not studied
longitudinally.
Several researchers have created comprehensive models proposing the pathways
through which yoga decreases stress, including both psychological and biological
variables (see Brown & Gerbarg, 2005a, 2005b; Evans et al., 2011; Kuntsevich et al.,
2010; Sengupta et al. 2013; Streeter et al., 2012).
Recommendations for future research
In future studies, it will be importan t to empirically asses s the suggested-but-not-yet-
studied constructs as potential mechanisms by which yoga affects stress (Tables 1 and 2).
Earlier we noted the methodological shortcomings of much of the yoga literature;
these shortcomings have also been noted by many of the authors of reviews of yoga
research (e.g., Cooper, 2004; Li & Goldsmith, 2012). The methodology of yoga studies
has generally lacked scientific rigour due to the nature of yoga research as a developing
field and lack of consistency in operational definition (Li & Goldsmith, 2012). For
example, Table 3 demonstrates that even the empirical studies of mechanisms we
identified suffered from methodological limitations including smal l sample sizes, lack of
control groups and non-randomisation. No studi es used randomisation techniques, and
two studies did not use compariso n control groups. All studies had group sizes less than
33 participants, some as low as 8, which generally do not supply sufficient power to
detect statistically significant group differences (Brown & Forsythe, 1974).
Choosing an appropriate control group is important in yoga intervention studies is
also difficult (Park et al., 2014). This group of empirical studies reflects the wide variety
of control groups used in yoga studies. For example, control groups in these five studies
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include: wait list control, movement control, passive video control, African dance and a
biology lecture.
Despite four of five studies describing their yoga intervention as Hatha yoga, there
is wide variation in what might comprise a Hatha yoga intervention. Diversity in yoga
practice makes comparison of findings across studi es difficult and limits our ability to
understand the mechanisms by which yoga affects stress and other aspects of well-being
(Elwy et al., 2014). Nearly every published study testing yoga interventions implemented
a combination of movement, mental exercise, both rigorous and gentle postures, breath
work a nd meditative exercise. This heterogeneity, even within a certain type of yoga (e.g.,
Hatha, Iyengar, Integral) presents a problem: It is difficult to determine which component
or combination of components is responsible for any outcome obtained, or whether it is
the totality of the practice. Fu rther, different yoga interventions, with varying emphases
on the components of yoga such as asana, pranaya ma or meditation, may have differential
effects on physiological stress responses, biomechanics and other pathways through
which yoga may reduce stress. To date, these issues remain unexamined and await some
way of classifying the components of the interventions before they can be addressed. A
tool that should be helpful in classifying yoga in research, the Essential Properties of
Yoga Questionnaire, is being designed to address current limitations in the yoga literature
and advance the research on yoga (Park et al., 2014). Development of a reliable, valid
tool to assess the essential dimensions of yoga should lead to the improvement and
tailoring of yoga interventions that will result in their improved effectiveness with a range
of different health conditions and problems.
Additionally, most studies examine the effects of Hatha yoga only (see Tables 1 and 2).
While this is the most commonly practiced type of yoga (Barnes, Bloom, & Nahi n, 2008),
examining the differential affects and pathways through which various types of yoga affect
stress will be important, as interventions can be tailored by type of yoga as well. Some types
of yoga may be gentle and meditative (Integral, Svaroopa), some may be vigorous
(Ashtanga, Power Yoga), or both (Iyengar, Kundalini) and some use heaters (Bikram). Ross
and Thomas (2010) emphasise that the varying types of yoga may have different effects on
the HPA and SNS axes in response to acute and chronic stress, and that studies to test
relative effects are needed.
Another problem in this nascent research field is that proposed mediators in some
studies are outcomes or measures of stress in other studies. For example, immune
response is examined as both the mechanism and stress outcome in some studies (e.g.,
Arora & Bhattacharjee, 2008). Similarly, Michalsen et al. (2005) measured salivary
cortisol levels as a mechanism of Iyengar yogas effects on perceived stress; however,
cortisol level is very often used as a measure of stress per se (Li & Goldsmith, 2012), and
therefore should not be considered a mechanism of the yogastress relationship. It will be
important to assure that operational definitions remain consistent.
There are some studies for which mechanisms could not be tied to stress-reduction
outcomes because were no differences in stress biomarkers following the yoga
intervention (e.g., Kiecolt-Glaser et al., 2010). In their article, Kiecolt-Glaser et al.
(2010) speculated that, in designing the yoga to be appropriate for novices and experts,
they were unable to include some more advanced and purportedly more powerful poses
like full inversions. This may be a possible cause of the lack of biological stress-reduction
changes; that is, the yoga session may not have been powerful enough to elicit stress-
reduction responses. However, these authors postulated that, because novices and experts
did show a group difference in IL-6, an indicator of inflammation, such that experts had
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less inflammation, and because experts demonstrated a lower cardiovascular response and
inflammation after a stressor test, long-term yoga practice could be responsible for a
decrease in the inflammatory immune response (IL-6), which could dampen the bodys
responses to stressors. Ensuring that yoga interventions are robust and comprehensive
enough to create measureable changes in measu res of stress over time is vital for research
examining mechanisms between yoga and stress, and these particular hypotheses should
be tested. Additionally, appropriate methodology and analyses for testing mediation is
warranted (Zhao, Lynch, & Chen, 2010).
Clinical implications
Understanding more about the ways in which yoga affects stress will allow us to inquire
into the types or aspects of yoga that most affect these mechanisms and to focus
interventions on these aspects of yoga (Elwy et al., 2014; Park et al., 2014).
If researchers know what mechanisms affect which type of stress outcome, we may
customise interventions for populations with that stress issu e. For example, if a health
populations primary issue is a highly activated SNS, we may be able to identify the type
of yoga that most effectively reduces SNS activity. In HIV/AIDS populations, the effect
of stress on specific types of biomarkers that are already weakened by the disease is
particularly harmful (Leserman, 2003). HIV/AIDS disturbs HPA axis function, so a type
of yoga intervention shown to result in better regulation of the HPA axis, this intervention
may be particularly useful for those with HIV/AIDS (Zapanti, Terzidis, & Chrou-
sos, 2008).
Additionally, knowledge of why and how yoga affects stress will provide more
credibility to the field of yoga research and interventions, which has important
implications. Health care professionals may be more comfortable recommending yoga
to patients in need of stress management if the mechanisms through which it operates are
better known. Mental health care professionals comfort in implementing yoga in the
context of psychotherapeutic interventions could be useful as well (Yang, 2007). The use
of yoga or yogic techniques in psychotherapy combines empirically supported benefits of
traditional psychotherapy (e.g., cognitive behavioural therapy) with less commonly
utilised components, including mindfulness, physical movem ent and exercise (Somer-
stein, 2010). This combined method of stress reduction, emphasising different paths
through which to dampe n stress levels, may be more helpful than either alone. The ability
to implement yoga interventions to supplement current standards of care or pharmaco-
logical treatment can be transformative, providing better ways to reduce stress and
improve health. These improvements may reduce the incidence of myriad diseases,
including chronic illness, increase quality of life and provide more cost-efficient care and
care with fewer side effects than expensive medications or procedures (Li & Goldsmith,
2012; Roehr, 2008; Woodyard, 2011).
Conclusions
Much additional more-rigorous research is needed to replicate findings, examine purported
mechanisms, explo re additional mechanisms and then deter mine whether a combination of
these mechanisms underlies the relationship between yoga and stress. Research ought to
include more rigorous methodology from this point forward, including sufficient power,
study randomisation and appropriate control groups. Research must distinguish between
indicators of stress reduction and possible mechanisms of stress reduction. The provision of
14 K.E. Riley and C.L. Park
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an ad ditional stress-reduction stra tegy in the form of yoga has far-reaching effects.
Implementation of yoga interventions has the potential to ameliorate stress and stress-
related conditions (Duraiswamy, Thirthalli, Nagendra, & Gan gadhar, 200 7).
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Appendix 1. Biomarkers of stress response (Juster et al., 2009)
Neuroendocrine
Cortisol
Dehydroepiandrosterone
Epinephrine
Norepinephrine
Dopamine
Aldosterone
Immune
Interleukin-6
Tumor necrosis factor-alpha
C-reactive protein
Insulin-like growth factor-1
Fibrinogen
High-density lipoprotein cholesterol
Low-density lipoprotein cholesterol
Triglycerides
Glycosylated hemoglobin
Glucose
Insulin
Albumin
Creatinine
Homocysteine
Cardiovascular and respiratory
Systolic blood pressure
Diastolic blood pressure
Peak expiratory flow
Heart rate/pulse
Anthropometric
Waist-to-hip ratio
Body mass index
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... Remarkably, it mostly did not matter which component was added or practiced in this regard. Physical Hatha yoga emphasizes becoming aware of the (moving) body and has been found to increase vagal/parasympathetic activity [8,25]. This might explain its balancing effect on affectivity. ...
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Mind–Body Medicine (MBM) includes a broad range of interventions with proven preventive and clinical value, such as yoga and meditation. However, people differ in their preferences and response to different MBM treatments and it remains unclear who benefits most from what type of practice. Thus, finding moderators of treatment outcome seems to be a promising approach. This was the aim of the present study. We conducted a single-case multiple-baseline study investigating the outcomes and moderators of four different MBM treatments. Fifty-seven healthy participants with no prior experience were randomly assigned to three baselines (7, 14, and 21 days) and four eight-week treatments: mantra meditation alone, meditation plus physical yoga, meditation plus ethical education and meditation plus yoga and ethical education. We analysed the data using effect size estimation, multiple regression and cluster analyses. High anxiety, high absorption, low spirituality, low openness and younger age were associated with a range of positive outcomes, such as increased wellbeing or decentering and decreased mind wandering. Receiving ethical education consistently improved wellbeing, while engaging in physical yoga reduced mind wandering. In the cluster analysis, we found that participants with a more maladaptive personality structure enhanced their emotion regulation skills more. Consequently, people do differ in their response to MBM interventions and more vulnerable people, or those high in absorption, seem to benefit more. These findings could support the development of custom-tailored MBM interventions and help clinicians to make scientifically sound recommendations for their patients.
... Various research studies in recent past have supported the fact that yogic interventions have numerous physical and psychological health benefits by downregulating the hypothalamus-pituitary-adrenal axis and the sympathetic component of the autonomic nervous system. Various reported benefits of yoga include reduction in blood pressure, enhanced dominance of the parasympathetic nervous system over the sympathetic nervous system, normalization of endocrinal function and gastrointestinal function, improved musculoskeletal fitness and posture, enhanced energy levels, normalization of body weight, better sleep and immunity, reduced pain, and better quality of life, whereas psychological benefits associated with yoga include enhanced mood, feeling of subjective well-being and self-acceptance, and reduced anxiety and depression (11,12). The literature also reports improved biochemical profile in terms of antioxidant effects of yoga like reduced cholesterol, triglycerides, and glucose levels, increased lymphocyte count, and many more (13,14). ...
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... In addition, mind-wandering and mindfulness were shown to be inversely related, representing opposite sides of the same coin (Mrazek et al., 2012). Fittingly, increased mindfulness has been proposed as a central mechanism of yoga's effectiveness on stress and general health (Riley & Park, 2015;Tellhed et al., 2019). ...
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Mind-Body Medicine (MBM) includes a broad range of interventions of proven pre-ventive and clinical value, such as yoga and meditation. However, people differ in their preferences and response to different MBM treatments, and it remains unclear who bene-fits most from what type of practice. Thus, finding moderators of treatment outcome seems to be a promising approach. This was the aim of the present study. We conducted a single-case multiple-baseline study investigating the outcomes and moderators of four different MBM treatments. Fifty-seven healthy participants with no prior experience were randomly assigned to three baselines (7, 14, and 21 days) and four eight-week treatments: mantra meditation alone, meditation plus physical yoga, meditation plus ethical education, and meditation plus yoga and ethical education. We analysed the data using effect size estima-tion, multiple regression and cluster analyses. High anxiety, high absorption, low spirituali-ty, low openness and younger age were associated with a range of positive outcomes, such as increased well-being or decentering and decreased mind-wandering. Receiving ethical education consistently improved well-being, while engaging in physical yoga reduced mind-wandering. In the cluster analysis, we found that participants with a more maladaptive per-sonality structure enhanced their emotion regulation skills more. Consequently, people do differ in their response to MBM interventions and more vulnerable people, or those high in absorption, seem to benefit more. These findings could guide the development of custom-tailored MBM interventions and help clinicians to make scientifically sound recommenda-tions for their patients.
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