The Truth About Cold Water Immersion: What This New Meta-Analysis Overlooks
Not so quick!
Dear Thermalists,
A new systematic review and meta-analysis published in PLOS ONE has evaluated the health effects of cold water immersion (CWI) in healthy adults. While meta-analyses are often considered the gold standard for summarizing scientific evidence, their methodology is not always suitable for studying complex physiological interventions like CWI.
In this case, the review only included randomized controlled trials (RCTs)—a design originally developed to assess the effects of pharmaceuticals. This approach works well for testing a single-variable drug intervention but is a poor fit for studying CWI, which influences multiple physiological systems simultaneously. Scientists recognize that this method is not designed for capturing the full spectrum of adaptive responses that occur with regular cold exposure. However, in an attempt to reach the so-called "highest level of evidence," the researchers applied it inappropriately, ultimately leading to an incomplete and potentially misleading assessment of CWI’s true effects.
What This Meta-Analysis Found
The review compiled data from 11 studies with 3,177 participants, analyzing the impact of CWI, including cold showers and ice baths, on stress, inflammation, immunity, and overall well-being. Here are the key findings:
Inflammation Response
The review reported an initial increase in inflammatory markers, particularly IL-6, immediately after cold exposure. However, it failed to acknowledge that this is a normal stress response. Importantly, IL-10, an anti-inflammatory cytokine, also increases post-immersion, creating a balance that contributes to long-term stress resilience and immune regulation. Studies excluded from this meta-analysis have shown that regular CWI leads to an overall reduction in chronic inflammation over time.
Stress Adaptation
While the review noted that acute stress levels did not decrease immediately after CWI, it overlooked research demonstrating that habitual cold exposure enhances the body's ability to handle stress. The delayed reduction in stress observed at the 12-hour mark aligns with findings that cold exposure activates the parasympathetic nervous system, leading to long-term stress relief.
Immune Benefits
The analysis confirmed a 29% reduction in self-reported sickness absence among participants who practiced cold showers regularly. However, it did not explore mechanistic studies showing how CWI enhances immune function by increasing white blood cell activity and improving adaptive immune responses.
Sleep & Well-Being
Some studies included in the review noted improvements in sleep and well-being, but the meta-analysis suggested these effects might fade after 90 days. This dismisses individual variation and the adaptive benefits seen in long-term practitioners of cold exposure.
Key Studies This Meta-Analysis Missed
Meta-analyses can only include studies that fit their strict criteria, which means they often exclude valuable research that provides deeper insights into the long-term benefits of CWI. Here are some examples of studies that were left out:
Metabolic Activation & Brown Fat
Research by Huttunen et al. (2001) and van der Lans et al. (2013) shows that regular cold exposure stimulates brown adipose tissue (BAT), increasing energy expenditure and improving metabolic health. These findings suggest CWI could support metabolic flexibility and glucose regulation—critical factors for long-term health.
Additionally, my 2021 research in Cell Reports Medicine demonstrated how contrast therapy (alternating cold and heat exposure) can enhance brown fat activation and metabolic function. This type of physiological response is crucial for long-term metabolic health, yet it was not considered in the meta-analysis.
Stress Resilience & Hormonal Adaptation
A study by Leppäluoto et al. (2008) found that repeated CWI leads to adaptations in cortisol and norepinephrine levels, resulting in improved stress resilience and a blunted stress response over time. This means that while acute cold exposure triggers stress, long-term practice helps the body become more resilient.
Cardiovascular & Immune Health
Research by Dugué & Leppäluoto (2004) suggests that frequent cold exposure enhances immune function by increasing white blood cell counts and natural killer (NK) cell activity, which help the body fight infections. Their work also highlights how cold exposure improves vascular function and circulation, reducing the risk of cardiovascular disease over time.
Mental Health & Mood Regulation
Studies such as Shevchuk (2008) propose that cold exposure triggers the release of endorphins and norepinephrine, acting as a natural antidepressant and helping with mood disorders. This aligns with anecdotal reports from cold therapy practitioners experiencing enhanced mental clarity and well-being.
Why This Meta-Analysis is Limited
Meta-analyses are powerful tools, but they are not without bias. The strict inclusion criteria in this review excluded many experimental studies that provide deeper insights into the physiological benefits of CWI. Here’s why this matters:
Exclusion of Experimental Studies: Many pioneering studies that explore mechanisms—such as brown fat activation, metabolic improvements, and enhanced cardiovascular function—were left out because they do not fit the rigid framework of RCTs.
Overemphasis on Short-Term Outcomes: The review focused on immediate responses to CWI rather than long-term adaptations, missing the well-documented benefits of consistent exposure over weeks or months.
Narrow Participant Selection: Most included studies involved specific demographics, such as athletes, limiting their applicability to the broader population who practice CWI for overall health and longevity.
Hierarchy of Evidence Bias: While RCTs are the gold standard for studying single-variable interventions, cold exposure influences multiple physiological systems simultaneously. Of course we can do more RCT’s with CWI, but that’s not really the problem. Comparing CWI to a pharmaceutical trial model using "meta-analysis of RCT” is flawed because the outcomes are broad and interdependent rather than singular and isolated, like pharmaceutical RCT’s are. This means, that there will be far to few CWI RCT’s to include as they rarely fit the inclusion criteria of a meta-analysis of RCT’s.
Why Ignoring These Results Creates Bias
By excluding important experimental studies, this meta-analysis presents an incomplete and potentially misleading picture of CWI’s true effects. Here’s why this approach can bias conclusions:
Dismissing Real-World Benefits: Many people experience profound health improvements from CWI, yet the omission of long-term adaptation studies means these benefits are underrepresented in systematic reviews.
Influencing Public Perception: When only a narrow set of studies are included, media outlets and health professionals may assume the evidence is weak, discouraging individuals from trying CWI for themselves.
Restricting Scientific Progress: If research is only considered valid when it fits the RCT model, emerging discoveries from innovative study designs may be disregarded, slowing the understanding of how CWI interacts with multiple physiological systems.
What you can use it for
Cold water immersion is more than a passing trend—it is a scientifically supported practice that fosters resilience, reduces chronic stress, and enhances overall health. While this meta-analysis presents useful data, it does not capture the full picture. The exclusion of experimental studies means it underrepresents new and emerging discoveries that further validate the benefits of CWI.
I have written extensively about these topics in my books, including Winter Swimming and Thermalist, where I explore the scientific mechanisms behind cold and heat therapy and how they contribute to metabolic health, stress resilience, and overall well-being.
I am also happy that I took the time to develop a thorough education and certification program where I teach the science of cold water immersion and sauna therapy to Thermalist Method Instructors. Through this program, I ensure that practitioners and coaches deeply understand the research and can apply these powerful tools safely and effectively. It’s important to me that this knowledge is not just anecdotal but grounded in solid science, which is why education is a cornerstone of my work.
As always, I encourage you to stay informed and trust the growing body of science—beyond the limitations of any single meta-analysis.
Stay cold, stay strong!
Susanna Søberg, PhD