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

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Health Psychology Review

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How does yoga reduce stress? A systematic review

of mechanisms of change and guide to future

inquiry

Kristen E. Riley & Crystal L. Park

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,

9:3, 379-396, DOI: 10.1080/17437199.2014.981778

To link to this article: https://doi.org/10.1080/17437199.2014.981778

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Published online: 15 Apr 2015.

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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 resear ch demonstrates that yoga reduces stress

(see Chong, Tsunaka, Tsang, Chan, & Cheung, 2011). The term mechanis m, 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 be en published. However, theories about why and how yoga affects health

*Corresponding author. Email: [email protected]

Health Psychology Review, 2015

Vol. 9, No. 3, 379–396, http://dx.doi.org/10.1080/17437199.2014.981778

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 experi ence, 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 one’s life as

stressful (Cohen, Kamarck, & Mermelstein, 1983). Objective measures of stress consist

of biological markers (biomarkers) of stress (Li & Goldsmith, 2012). Juster, McEwen,

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 yoga–stress 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 yoga’s 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, incl uding 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 incl ude 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

380 K.E. Riley and C.L. Park

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), hypothalamic–pituitary–adrenal

(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, Ciraulo, & 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 sensi tivity, 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 yoga’s 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, renin–angiotension and basal

skin conductance, and progre ssive attenuation of sympathoadrenal activation, including a

decline in heart rate and a decline in blood pressure.

Health Psychology Review 381

Inflammatory and endocrine responses. Yadav et al. (2012) found that there was a reduction

in stress, as measured by plasma cortisol 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 inflammation 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 literat ure. 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 Kenny’s (1986) mediation criteria.

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: mechanism, 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 selec tion process. Our electronic

database search yielded 926 abstracts. We examined abstracts for evidence of an

empirical quantitative study of a yoga inte rvention 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 Tables 2 and 3).

382 K.E. Riley and C.L. Park

Psychological mechanisms

Positive affect

In a study of novice and expert yoga practitioners, positive affect increased from pre- to

post-yoga in a single-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 , Johnson, & Mohr, 2004).

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 intervention’s 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)

Health Psychology Review 383

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)

384 K.E. Riley and C.L. Park

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

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

Salivary cortisol Michalsen et al. (2005) Yoga = 16

Control = 8

Wait list control No

Health Psychology Review 385

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 hypothalam us, 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.

386 K.E. Riley and C.L. Park

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 movement 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 intervent ion 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 pract ice 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 yoga’s 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 which 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 nu merous 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 yoga–stress

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, mindfuln ess and stress, with other related variables measured

and included in the model. A cross-lagg ed panel analysis or a daily diary design would

allow for more causal inference and insight into these relationships.

Health Psychology Review 387

Self-compassion

Self-compassion is frequently viewed as another quality underlying the transformations

that result from contemplative practices. Self-compassion has been described as ‘being

open to and moved by one’s own suffering, experiencing feelings of carin g 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 their 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 psychological

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 optimistic 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 mig ht help them learn to identify

stress more quickly and develop coping mechanisms early in their career for managing

the high stress of a professional practitioner, thus having long-lasting stress-reduction

effects. A qualitative study of the effect of yoga on women’s 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 one ’s 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 decreas e 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).

388 K.E. Riley and C.L. Park

Evans et al. (2011) mentions that a yoga intervention may promote psychospiritual

resources, which he claims are protective against stress. Moadel et al. (2007) also

suggested that spiritual well-being may underlie yoga’s 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 underac tivity 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

yoga’s 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

Health Psychology Review 389

reduction, an aspect of health. For example, in a study of the mechanisms through which

yoga affects cardiovascular disease, Innes et al. (2007) describe yoga’s reductions in

psychological factors such as fatigue, which also contributes to stress.

Holzel et al. (2011) detail the potential mechanisms through 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 counte r-

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 significant

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 & Ger barg, 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 important 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 small sample sizes, lack of

control groups and non-randomisation. No studies 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 general ly 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

390 K.E. Riley and C.L. Park

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 studies 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 intervent ions 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. Further, different yoga interventions, with varying emphases

on the components of yoga such as asana, pranayama 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, & Nahin, 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 yoga’s 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 yoga–stress 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 foll owing 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-re duction

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

Health Psychology Review 391

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 body’s

responses to stressors. Ensuring that yoga interventions are robust and comprehensive

enough to create measureable changes in measures of stress over time is vital for research

examining mechanisms between yoga and stress, and these particular hypotheses should

be tested. Additionally, appropriate methodol ogy 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 issue. For example, if a health

population’s 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 thera py) with less commonly

utilised components, including mindfulness, physical movement 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 quali ty 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, explore additional mechanisms and then determine 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

392 K.E. Riley and C.L. Park

an additional stress-reduction strategy 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, & Gangadhar, 2007).

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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

396 K.E. Riley and C.L. Park