Does the Wim Hof Method have a beneficial impact on physiological and psychological outcomes in healthy and non-healthy participants? A systematic review

Introduction Wim Hof, also known as the iceman, developed a method called Wim Hof Method (WHM) which he claims to have several benefits on physical and mental health. The aim of this systematic review is to identify and synthesise the results of the studies conducted on WHM on physiological and psychological health-related outcomes. Materials and methods This systematic review followed the PRISMA guidelines for systematic reviews. The protocol was registered in the International Prospective Register of Systematic Reviews (PROSPERO), registration number CRD42022333209. Medline and Web of Science were searched and included studies from January 1, 2014, to July 4, 2022. Studies were included if they met the predetermined inclusion/exclusion criteria. Inclusion criteria included RCTs and cohort studies published in peer-reviewed journals, studies conducted on healthy individuals and people with pre-existing medical conditions (adolescents and adults over the age of 14), studies that included all three pillars (breathing, cold exposure, and commitment) of the WHM, and studies that only focused on Wim Hof breathing method (WHBM). Exclusion criteria included studies that discussed WHM but are not original experimental research or are not peer-reviewed, studies that included children under the age of 14, and studies that used methods similar to WHM, but not actually WHM, such as tummo meditation. The articles were assessed for risk of bias using RoB 2.0 and Scottish Intercollegiate Guidelines Network (SIGN) tools. The effects of WHM were categorised into physiological or psychological related outcomes and narrative synthesis was conducted. Results Nine papers were included in this review which consisted of eight individual trials. The findings suggest that the WHM may reduce inflammation in healthy and non-healthy participants as it increases epinephrine levels, causing an increase in interleukin-10 and a decrease in pro-inflammatory cytokines. Additionally, effect of WHBM on exercise performance showed mixed findings. Effects on respiratory parameters of minute ventilation, tidal volume, and breathing frequency were mixed following bouts of exercise. Conclusion Taken together, the findings of this review show promising use of WHM in the inflammatory response category. The focus of future studies should further investigate the benefits of WHM in non-healthy participants with inflammatory disorders and explore the use of Wim Hof breathing method to enhance exercise performance.


Introduction
The Wim Hof Method (WHM) is a lifestyle change that claims to enhance a person's quality of life which consists of three pillars: Wim Hof breathing method (WHBM), cold therapy, and commitment.According to Wim Hof, the creator of this method, a combination of these three pillars enhances a person's autonomic nervous and immune systems, thus strengthening a person's health and mentality.
Wim Hof is recognized for his ability to resist extreme cold temperatures.He holds 21 Guinness World Records for some of his remarkable human achievements, including climbing Mount Kilimanjaro while wearing shorts, swimming 66 metres beneath ice, standing for two hours in a container filled with ice cubes, and running a half marathon over the Arctic Circle, only wearing shorts and barefoot [1].Wim Hof credits his achievements to the WHM.
The first pillar of the WHM is WHBM which consists of hyperventilation 30-40 times then holding the breath voluntarily at low lung volume [2].Secondly, cold therapy involves taking daily cold showers or sitting in an ice bath [3].The final pillar, which is commitment, is the foundation of the other two pillars; mastering both mindful breathing and cold exposure takes patience and persistence [4].To support his claims, Wim Hof has submitted his method and himself for research.Many studies, including case studies, randomised control trials (RCTs), and observational trials have been conducted on him and his method [5][6][7][8][9][10][11][12][13][14][15][16].
To identify some of the benefits people are experiencing, Wim Hof collaborated with RMIT University by conducting a survey inquiring about the impact WHM had on an individual's health and well-being [17].Over 3,200 people answered the survey, and the majority of the answers were positive.Respondents reported a good mix of physical and mental benefits such as an increase in their tolerance to resist cold, as well as an increase in energy, mood, mental focus, and general health.The survey findings also claimed that the WHM had benefits for specific conditions such as stress, tiredness and fatigue, anxiety, depression, back pain, insomnia, arthritis, and chronic pain [17].
Collectively, these findings suggest that the WHM may be of importance to healthy and non-healthy individuals and that the WHM approach may be given consideration in the public health and lifestyle medicine fields.The notion of lifestyle medicine refers to the study of how actions and habits affect illness prevention and treatment [18].However, caution should be applied insofar as the aforementioned survey [17] has not been peer-reviewed or published, and Wim Hof's own involvement in this study and others [5,9] may give rise to conflicts of interest in the pursuit of both an evidence base and a commercial opportunity.Wim Hof has attained a level of celebrity as a result of his achievements, method and associated media career.Due to all his remarkable achievements, his charisma, and authenticity, Wim Hof might be considered to be both achieved and micro celebrity [19], where achieved celebrity refers to the attainment of fame based on recognized talent, achievement, or ability and micro celebrity describes an individual that achieves status and social media presence by self-broadcasting about niche subjects to a small community of followers.Wim Hof is an example of a celebrity that embodies the para-social relationship, exchanging the allure of intimacy, integrity, and authenticity.The growing number of celebrities giving lifestyle and health advice and scientific knowledge to the public indicates the need to critically examine such advice [20].Nunan et al. urge caution for the potential of "the unintended consequences of uncritical endorsement and application of lifestyle medicine including the infiltration of pseudoscience, profiteering, and the potential for widening health inequalities by a continued focus on the 'individual'" (p229) [21].Therefore, an independent, systematic synthesis of this evidence is warranted to evaluate the positive health claims of the WHM.
A systematic review has not yet been conducted on the WHM body of evidence.Therefore, an independent, systematic synthesis of this evidence is warranted to evaluate the positive health claims of the WHM.Hence, this review aims to identify and synthesise the results of the studies conducted on WHM on physiological and psychological health-related outcomes.

Materials and methods
This systematic review followed the PRISMA guidelines for systematic reviews [22].The protocol was registered in the International Prospective Register of Systematic Reviews (PROS-PERO), registration number CRD42022333209.

Literature search
MEDLINE and Web of Science databases were used to search for published studies for potential inclusion in this systematic review.The following search phrases were used in combination: Wim Hof; breathing exercise OR breathing technique; meditation OR commitment; cold temperature OR cold exposure OR ice bath OR cold shower.The filters English language, year = "2014-July 4, 2022", and journal articles or observational studies or randomised controlled trials (RCTs) were applied.A filter for the year was applied because the first study performed on WHM was in 2014 [10].

Study selection
Duplicates were deleted before abstracts were screened.The screening of titles, abstracts and full texts was conducted by two independent reviewers, with any disagreements resolved by consensus.Screening was performed in Covidence (title and abstract screening, full text review, and extraction of the chosen articles).Studies were included according to the predetermined inclusion/exclusion criteria.Inclusion criteria included RCTs and cohort studies published in peer-reviewed journals, studies conducted on healthy individuals and people with pre-existing medical conditions (adolescents and adults over the age of 14), studies that included all three pillars (breathing, cold exposure, and commitment) of the WHM as defined above, and studies that only focused on WHBM.Articles just covering WHBM were permitted as it has been deemed more important and studied more frequently than other pillars.Furthermore, exclusion criteria included studies that discussed WHM but are not original experimental research or are not peer-reviewed, studies that included children under the age of 14, and studies that used methods similar to WHM, but not actually WHM, such as Tummo meditation.Both WHM and Tummo methods allow for the controlment of body temperature, and both have similar breathing techniques, however Tummo meditation consists of breathing and visualisation techniques which are intended to summon spiritual insight, whereas WHM is not religious [23].After deciding on the included studies, the reference lists were handsearched for other relevant articles.
Data extraction comprised the name of the study, place where the study was conducted, research design, participant demographics, study context, description of intervention and control, and key findings.One reviewer completed the extraction while a second reviewer checked it for accuracy.Furthermore, study quality was assessed using the RoB 2.0 risk bias tool [24] for RCTs and the Scottish Intercollegiate Guidelines Network (SIGN) risk bias tool [25] for cohort studies.Two reviewers independently evaluated the possibility of bias.The answers were then compared, and any disagreement was discussed between the reviewers.

Data synthesis
A narrative synthesis of the findings from the studies was included by providing a description and comparison between the different studies.This comprised a discussion of the contexts in which the studies have been undertaken, and the effects of WHM thematically grouped into physiological or psychological related outcomes.In each thematic grouping, different sets of outcomes were included.The data were not suitable for meta-analysis as there were various unrelated outcome measures resulting in heterogeneity across studies.

Study characteristics
Nine papers, seen in Fig 1, published between 2014 and July 4, 2022, were identified that examined the WHM.One of these articles was identified from hand-searching the included studies' reference lists [6].Two of these articles [13,16] were further analyses of a study [10].Table 1 summarises the characteristics of the listed studies.Of the nine papers included, there were eight individual trials.Four of the trials performed the full WHM [7,10,12,15] and four did only WHBM [6,8,11,15].The application of WHBM differed between studies as some studies had one breathing exercise [6,8,11,12] while others had two breathing exercises [7,10,13,15,16].The first breathing exercise was common in all studies.Cold exposure was conducted according to what was accessible to each study.However, all studies agreed on daily exposure to the cold through having cold showers, but the duration of exposure under the shower differed across studies.The meditation technique was the least focused component of the WHM.All studies had the same description of the meditation technique except for Petraskova Touskova et al. [12].All the authors described it as visualisations meant to promote complete relaxation, while Petraskova Touskova et al. described it as a form of focusing to obtain selfawareness and will-power.Out of the nine papers, five conducted a randomised controlled trial (RCT) design [10,11,13,15,16], three conducted a crossover RCT design [6][7][8], and one conducted a prospective cohort design [12].

Study quality
For the RCT studies, the RoB 2.0 overall risk of bias judgement was 'high concerns' for all studies except one study which was judged as 'some concerns' [8] (Table 2).Most of the articles were judged as high concern due to the difficulty in blinding the participants and researchers to the intervention.For the cohort study [12], the SIGN tool was used.The cohort study was scored 'unacceptable' as it was not able to reduce the possibility of bias or confounding.An explanation of the results of the SIGN tool can be found in the S2 Table.

Early intervention
Participants were trained by four trainers and Wim Hof.

Late intervention
The participants in the control group also received exposure to the intervention after the 8-week control phase had ended.Patients were trained in the same way as the early intervention.

Stress response
Two trials investigated epinephrine levels during the endotoxemia experiment [10,15] and one trial studied epinephrine levels during WHBM [15].Baseline epinephrine levels in WHM- trained participants were significantly higher compared to non-trained participants (p = 0.007).After starting WHBM, epinephrine levels increased further in this group and peaked just before endotoxemia administration and remained elevated until WHBM was stopped [10].Additionally, in Zwaag et al. endotoxemia study [15], participants had no variations in epinephrine levels throughout the experiment (p = 0.48) in both participants exposed to cold (CEX) and participants not performing the WHM (CON).However, an increase in epinephrine levels began much earlier and was significantly greater in both the cold exposure with breathing exercise without retention (CBR) (p = 0.01) and breathing exercise without retention (BRT) (p = 0.04) compared to the CEX and CON groups [15].Furthermore, in Zwaag et al. breathing exercises study [15], both breathing exercises (WHBM with/without retention) raised epinephrine levels, which were unaffected by training duration (short vs extensive training: p = 0.71) or the trainer (independent trainer vs Wim Hof: p = 0.46).However, epinephrine was slightly more sustained in participants performing WHBM with retention compared to without retention (p = 0.003) [15].Moreover, Kox et al. [10] explored norepinephrine and dopamine levels during the endotoxemia trial and found them to be within the reference range throughout the experiment [10].
The associations of cortisol with WHM was also explored.In Petraskova Touskova et al. study [12], the group not practising WHM had higher, but statistically non-significant cortisol levels at the end of the expedition (p = 0.327) compared to WHM-trained participants [12].Whereas Kox et al. [10] found no variations in serum cortisol levels between the WHMtrained and non-trained groups.The serum cortisol levels were drawn before and throughout the period of WHM training.The only variation between the two groups was that serum cortisol levels normalised faster in WHM-trained participants [10].The difference in the results could be due to the different collection procedures for cortisol.Kox et al. [10] collected cortisol from blood while Petraskova Touskova et al. [12] collected hair cortisol.
Four trials examined HR recordings [6,8,10,15].Kox et al. [10] showed an increase in HR after starting WHBM which normalised faster in WHM-trained group compared to nontrained group [10].Furthermore, Bahensky ´et al. [6] reported that HR tended to be lower after WHBM compared to no WHBM [6].Citherlet et al. [8] also found HR significantly increased during hyperventilation (HV) and significantly decreased during breath hold in participants practising WHBM when compared to resting values [8].Similarly, Zwaag et al. [15] observed a significant increase in HR in both CBR (p<0.001) and BRT (p<0.001) groups, during the first hyperventilation cycle, compared to the CON group.No changes in HR responses between CEX and CON groups (p = 0.89) were found during the endotoxemia experiment.The HR measurements of BRT and CBR groups were comparable to those of CEX and CON groups when WHBM was stopped [15].In conclusion, HR was increased when WHBM was initiated and decreased when WHBM was stopped compared to control group.Prior to entering the cold water, participants took five to ten long, deep breaths while contracting particular abdominal and pelvic muscles.
Participants were asked to carry out the exercises once per day on their own after the first three supervised sessions.
One session was around 20 minutes.

Depressive symptoms decreased over an 8-week training programme.
There was a significant reduction in stress response.
Cortisol reduction was also observed to be non-significant.

Marko et al., 2022
N/A Same as Bahensky ´et al., 2020.N/A There were no significant variations in minute ventilation, tidal volume, or breathing frequency before or after the intervention between the experimental and control groups.Following the 4 week intervention period, there was a non-significant small-to-large effect for an increase in BF and V E in both groups, probably as a result of a decrease in training volume and intensity brought on by the break between competing seasons. https://doi.org/10.1371/journal.pone.0286933.t003

Metabolites response
Zwaag et al. [16] showed that lactate and pyruvate play an important role in the anti-inflammatory response in WHM-trained participants.High pyruvate concentration, but not lactate concentration, increased endotoxemia-induced interleukin-10 production, and the combination of both metabolites resulted in an even more prominent and statistically robust increase.Although these two metabolites were highly intercorrelated (p<0.0001),they were not associated with the highly elevated epinephrine levels at any of the examined time points (p-values>0.15).Additionally, pyruvate reduced the production of interleukin-6.Lactate, pyruvate, and the two metabolites combined also tended to reduce endotoxemia-induced TNF-α production; however, significance was not achieved [16].In Kox et al. study [10], lactate levels were significantly higher but not to a clinically relevant level in WHM-trained compared to non-trained participants [10].

Respiratory parameters
Three papers measured V E [6,8,11] and two of them also measured V T and BF [6,11].All three papers were investigating whether performing WHBM before an exercise will improve performance.Bahensky ´et al. [6] found the difference in mean values at each load to be significant for V E [6] and Citherlet et al. [8] found a significant difference (p = 0.039) between participants performing spontaneous breathing (SB) and WHBM at rest, but the mean difference was not statistically significant after performing an exercise (p>0.05)[8].Additionally, Marko et al. [11] showed that at each load stage, V E depicted statistical insignificance between WHBM and normal breathing (p = 0.138, p = 0.825, p = 0.479, p = 0.489) [11].Furthermore, for V T and BF measurements, Bahensky ´et al. [6] found the difference in mean values at each load to be significant (p<0.001,p<0.001) [6]; however, Marko et al. [11] showed no statistical significance between WHBM participants and normal breathing at any load stage (V T : p = 0.630, p = 0.377, p = 0.688, p = 0.087; BF: p = 0.794, p = 0.917, p = 0.956, p = 0.296) [11].

Blood gas measurements
Two trials examined blood gas measurements [10,15].In Kox et al. study [10], starting the WHBM resulted in an immediate and profound decrease in pCO 2 and an increase in pH, but it was not stated if these changes were statistically significant.Furthermore, during WHBM training, the trained group's oxygen saturation decreased significantly [10].In contrast, Zwaag et al. endotoxemia study [15], found pH and oxygen saturation levels significantly higher in the groups performing WHBM (BRT: p<0.01, p<0.001;CBR: p<0.01, p<0.001 respectively) when compared to CON.Whereas, pCO 2 was significantly lower in the same groups (BRT: p<0.001;CBR: p<0.001) when compared to CON [15].In Zwaag et al. breathing exercises study [15], oxygen saturation significantly decreased at the end of each retention phase, only when WHBM is performed with retention (p<0.001) compared to without retention.While pH and pCO 2 were similar in both WHBM exercises, except for a small but statistically significant difference at the final measurement time point (pH: p<0.001; pCO 2 : p<0.001) [15].

Psychological response
One paper [13] investigated the effect of optimism and neuroticism on WHM.It found that a higher level of expectancy and optimism helped significantly potentiate the effect of WHM while a decrease in neuroticism level was not found to significantly help.Participants in the training group were overall relatively optimistic and low in neuroticism.A higher level of optimism was associated with higher interleukin-10 levels (p<0.05) and epinephrine levels (p<0.01).Neuroticism was not found to be a significant predictor of endotoxin response.Participants' expectations to overcome the endotoxemia experiment significantly increased from before WHM training to after the endotoxemia experiment (p = 0.003) [13].
Another paper [7] explored the quality of life of participants and found that SF-36 physical and mental component scores significantly increased over the WHM-training period (p = 0.004, p = 0.004 respectively) compared to the non-trained group.While EQ-5D and EQ-5D visual analogue scale did not experience a significant change between the WHM-trained and non-trained groups (EQ-5D: p = 0.933, p = 0.102; EQ-5D visual analogue scale: p = 0.090, p = 0.674) [7].
The last paper [8] reported the results of a customised questionnaire, used for the subjective examination of the three sessions, and Borg RPE scale.Positive effects of increased energy, less fatigue, and improved breathing were reported by 87% of the participants.Almost half of the participants (47%) also said they would think about incorporating the WHBM into their own practices in the future.Additionally, when the experiment was performed using the WHBM, 66.7% of participants rated it the best method in terms of perceived performance compared to 13.3% for SB and 20% for HV.Likewise, 73.3% of participants judged the WHBM to be the best method to perform, while 53.3% judged SB as the worst.However, 33% of the participants using WHBM reported negative effects of deafness and heaviness [8].Moreover, Borg RPE score was found significantly lower in Bahensky ´et al. (p<0.001)[6] and Citherlet et al.

Reporting of symptoms
Three papers reported experience of flu-like symptoms following endotoxemia administration [10,13,15].Kox et al. [10] reported lower flu-like and self-reported symptoms and faster recovery from fever in participants performing WHM [10].Similarly, Middendorp et al. [13] showed that a higher expectation of the training's effects was associated with lower flu-like clinical symptoms (p<0.01)compared to non-trained group [13].Likewise, Zwaag et al.
endotoxemia study [15] results showed that only CEX group had significantly lower flu-like symptoms (p = 0.017) compared to CON group.All other groups had comparable peak symptom scores to CON group (BRT: (p = 0.70); CBR: (p = 0.21)).Additionally, when compared to the CON group, symptoms disappeared considerably faster in all three intervention groups (BRT: p<0.001;CEX: p = 0.01; CBR: p = 0.002) [15].Overall all participants reported flu-like symptoms but it was significantly lower in participants practising WHM or any one of its pillars such as CEX.The flu-like symptoms also disappeared faster in these participants.
Two papers also reported depressive symptoms [7,12].Petraskova Touskova et al. [12] found that at the end of the expedition, depressive symptoms were significantly lower in the WHM-trained group compared to non-trained group (p = 0.03), while TSC-40 scores, which measures stess-related symptoms, were higher in the non-trained group, but not significantly [12].Additionally, Buijze et al. [7] found no significant effect on depressive symptoms between WHM-trained and non-trained groups [7].

Discussion
This is the first systematic review conducted on WHM.The findings of this review suggest that WHM may affect the reduction of inflammation in healthy and non-healthy participants.In addition, there are currently mixed findings on the effect of WHBM and exercise performance.Despite the statistical significance observed in some studies, it must be noted that the quality of the studies is very low, meaning that all the results must be interpreted with caution.Additionally, the low sample size (15-48 individuals per study) and large proportion of males in the studies (86.4%) make the results non-generalizable to the public.Consideration should also be given that participants might experience the placebo effect, where improvements in patients' symptoms are due to their participation in the therapeutic encounter, with its rituals, symbols, and interactions [27].For example, Middendorp et al. [13] found that a higher level of expectancy and optimism helped significantly potentiate the effect of WHM [13].However, Zwaag et al. [15] who queried the so-called 'guru effect' [28] and whether it is necessary to be trained by the creator of the intervention to influence symptomatology [15], found that observed physiological and immunological effects are independent from the individual who provides the WHM treatment intervention.
Out of all the categories, WHM appears to have the most benefit in the stress and proinflammatory/anti-inflammatory response categories.Inflammation, especially when chronic, can cause severe complications such as cardiovascular diseases, cancer, diabetes, rheumatoid arthritis, asthma, chronic obstructive pulmonary disease, alzheimer, chronic kidney disease, and inflammatory bowel disease, therefore, reducing inflammation is beneficial [29].The WHM reduces inflammation using a different mechanism of action (MOA) than other antiinflammatory interventions.The closest MOA to that of WHM is corticosteroids' MOA as they both increase interleukin-10 production.WHM increases epinephrine, causing an increase in interleukin-10 which leads to a reduction in pro-inflammatory cytokines [10,15].Whereas corticosteroids activate anti-inflammatory genes, including interleukin-10 gene, and inactivate multiple inflammatory genes by inhibiting histone acetyltransferase and recruiting histone deacetylase-2 activity to the inflammatory gene transcriptional complex [30].The findings of this review suggest that WHM may provide some benefits in healthy and non-healthy people as it was suggested to be safe and might decrease inflammation, unlike corticosteroids, which are only given to non-healthy people due to having many side effects.
WHBM effect on exercise performance category showed mixed results.Although it is too early to decide whether WHBM enhances the performance or not, WHBM could have other potential applications in the physical exertion category.For example, a published letter suggests that WHBM may be helpful in preventing and reversing symptoms of acute mountain sickness (AMS) [31].Wim Hof and a combination of healthy and non-healthy participants climbed a mountain using the WHBM in 2 days instead of the usual 4-7 days and none of the participants suffered from severe AMS.These findings suggest that a trial on this topic may be of importance, especially for the rescue teams that must ascend quickly [31].However, although this letter was published in a journal, it was not peer-reviewed and there was no control group; hence the results should be interpreted with caution.Further studies on the effect of WHBM and physical exertion must be conducted before drawing a conclusion.
There are three pillars to WHM: cold exposure, WHBM, and meditation.The latter is a foundation of the other two pillars, thus there were no studies testing it by itself.In this review, cold exposure alone was suggested to have an insignificant effect on epinephrine and cytokine levels [15].Similarly, the wider literature agrees with this finding.For example, cold water immersion (CWI) did activate the immune system and alter systematic inflammation but not significantly [32][33][34][35].Only one study found CWI helpful in reducing inflammation in rock climbers by reducing vessel permeability toward the site of inflammation [36].Furthermore, in this review, WHBM alone was claimed to significantly increase epinephrine levels, regardless of the duration of training or trainer, and that increase was more prolonged in participants performing WHBM with retention [15].On the other hand, cytokine levels were only measured in participants performing WHBM without retention.There were no trials comparing WHBM with retention alone on cytokine levels.When WHBM was performed without retention, pro-inflammatory cytokines significantly decreased, but interleukin-10 levels were not increased significantly [15].This might be because hypoxia improves interleukin-10 release and reduces the pro-inflammatory response via improved adenosine release [37].Thus, it would be interesting for future trials to investigate the effects of WHBM with retention alone on cytokine levels.Nevertheless, when cold exposure was combined with WHBM, regardless of whether it was with retention or without, statistical significance was achieved on epinephrine and cytokine levels [10,15].Therefore, it is probably best to perform all the pillars of WHM to achieve a significant improvement in immunomodulatory effects.Future trials should investigate whether there is a significant difference between performing WHBM with retention and cold exposure compared to WHBM without retention and cold exposure on epinephrine and cytokine levels.
This review suffers from several limitations.Firstly, only two bibiographic databases were searched to identify suitable papers for this review.Including the database EMBASE, as recommended for inclusion in systematic reviews by Bramer et al. [38] may have increased the reach of the search and identified further relevant papers on this topic.Secondly, the search strategy keywords used such as breathing exercise, breathing technique, cold temperature, cold exposure, and cold shower may have excluded studies that used words combined with breathing and cold other than these words.Using breathing and cold as search words alone may have identified further relevant studies.Thirdly, the outcomes were very heterogeneous as many unrelated outcomes are included.This review could have focused on synthesising specific outcomes such as only focusing on outcomes related to stress and inflammation.Fourthly, all the trials had a very high risk of bias.This was due to the lack of a prior published protocol, small sample size, and complexity of blinding the participants and outcome assessors to the intervention.Furthermore, some outcomes such as psychological outcomes were difficult to measure as they are subjective measures usually assessed using a questionnaire.Since the participants were not blinded, it was very difficult to ensure that the answers were honest and valid to the experience.Lastly, the sample size was very small, sometimes affecting the results as any error in measurement or a loss of follow-up can significantly shift the trial results.More evidence needs to be synthesised about WHM before being recommended to the public.An intervention like WHM presents a challenge for rigorous experimental research as participants cannot be blinded to a WHM intervention, therefore the gold standard approach of a doubleblinded RCT is not possible.It does appear to offer benefits in attenuating inflammation with minimal serious adverse events and good positive effects of increased energy, less fatigue, and improved breathing [7,8,10,15].Thus, WHM can probably be used within lifestyle medicine to decrease inflammation in people suffering from inflammatory disorders.However, to be able to recommend WHM in an unwell population, future trials should publish a protocol outlining their experiment before starting the trial, increase the sample size, and make sure to account for loss to follow-up.Additionally, researchers should make sure to blind outcome assessors, and ensure that the outcomes assessed are objective and not subjective.

Conclusion
Considering all the studies, the WHM may produce promising immunomodulatory effects but more research of higher quality is needed to substantiate this finding.The combination of cold exposure and WHBM appeared to most effectively reduce the inflammatory response.Hence, all the pillars of the WHM are important to extract the benefits.The focus of future studies should further investigate the benefits of WHM in preventing or treating diseases, such as inflammatory disorders, in non-healthy participants and explore the use of WHBM in enhancing exercise performance.Studies about WHM have not yet investigated all the beneficial claims the WHM states to have.Future studies may provide valuable insights about WHM as there is still much to explore.

Table 1 . Characteristics of the listed studies. Study Country Study Design Population Description WHM Intervention Description of time over which WHM method practised Control's Group Intervention Additional Experimental Exposure
Same as Kox et al., 2014 Same as Kox et al., 2014.Same as Kox et al., 2014.Same as Kox et al., 2014.Same as Kox et al., 2014.