Wim Hof Method RCT Found No Benefit, but Could Not Settle It

The first randomized trial of the complete Wim Hof Method found no measurable improvement in resting heart-rate variability, blood pressure, arterial stiffness, perceived stress, affect, or vitality after 15 days. That result deserves attention. It does not, however, settle whether the method can produce smaller or longer-term benefits.

Ketelhut et al.'s 2023 randomized controlled trial tested the combined method rather than one isolated ingredient: progressive cold showers, cyclic hyperventilation with breath retention, and Wim Hof meditation. In 42 healthy young men, the intervention produced no statistically significant between-group difference on any measured cardiovascular or psychological endpoint versus usual daily activity.

The trial answers a narrow question: this 15-day program did not generate a detectable effect in this population. It cannot reliably answer the broader question that attracts most people to the practice: whether consistent Wim Hof training changes cardiovascular health or psychological well-being over months, especially in people who have elevated blood pressure, impaired autonomic function, obesity, or high stress at baseline.

What the trial tested

The researchers randomly assigned 42 healthy men, with a mean age of 26.7 years and mean body-mass index of 22.8 kg/m², to the Wim Hof program or a passive control group. Participants had no cardiovascular conditions and took no cardiovascular medication. The intervention lasted 15 days.

Before and after the program, investigators measured resting cardiac autonomic function with five-minute recordings of RMSSD and SDNN, two heart-rate-variability measures. They also measured brachial systolic and diastolic blood pressure and pulse wave velocity, a surrogate marker of arterial stiffness. Psychological outcomes included the Perceived Stress Scale, the Positive and Negative Affect Schedule, and a vitality scale.

None of those outcomes differed significantly between the groups at follow-up. The study therefore provides no evidence that this specific short program improved those resting measures in healthy young men. It also provides no evidence of a psychological benefit on the questionnaires used.

That is more useful than testimonials, but it is a modest result from a modest trial. The published report does not provide a confidence interval or effect estimate in the information available here, so readers cannot judge whether the results leaned toward benefit, harm, or near-zero change. "Not statistically significant" is not a synonym for "proved ineffective."

A null result with a high detection threshold

With about 21 participants per group, the trial had roughly 80% power to detect only a very large effect, about Cohen's d=0.87. That threshold matters. A smaller effect could be real yet fail to reach statistical significance in a study this size.

For context, a clinically meaningful improvement in resting RMSSD might be 8 to 15 milliseconds; a systolic blood-pressure change might be 3 to 5 mmHg; and a pulse-wave-velocity change might be 0.3 to 0.5 m/s. The concept document estimates that effects in that range would correspond roughly to d=0.3 to 0.6 and would require about 45 to 175 people per group for adequate detection. Those figures describe a study-design problem, not proof that the Wim Hof Method produces those changes.

The physiological context also argues against expecting a dramatic resting shift in two weeks. A 2022 systematic review and meta-analysis of long-distance races found that marathon and ultra-endurance events can acutely alter heart-rate variability and arterial stiffness in young healthy athletes, but those effects were largely transient. If extreme endurance events do not reliably produce lasting resting changes in this group, a 15-day lifestyle protocol would need unusually large effects to stand out in a small trial.

Healthy participants leave little room to improve

The enrolled men were young, lean, healthy, and normotensive. That is a reasonable group for a safety or feasibility study, but a difficult one in which to demonstrate lower blood pressure, improved vascular function, or better resting autonomic regulation. Baseline values already sit near the favorable end of the distribution. A measurement can only improve so far before ordinary day-to-day variation swallows the signal.

Other intervention trials illustrate why population and duration matter. In an eight-week resistance-exercise trial, blood pressure and endothelial function improved in people with elevated blood pressure or stage-1 hypertension. An eight-week randomized trial in adolescent boys with obesity found improvements in cardiac autonomic and vascular function with exercise training. Neither study establishes that Wim Hof training would work in those populations. They do show that cardiovascular trials more often detect change when participants have an abnormal baseline and enough time for adaptation.

The same issue applies to pulse wave velocity. It reflects arterial properties and vascular tone, but it is still a biomarker rather than a clinical outcome. A 15-day cold-shower program has no established mechanism for structural arterial remodeling, and the trial did not show a significant pulse-wave-velocity change. Treating this endpoint as likely to move quickly in healthy 26-year-olds set a demanding target.

A 2021 study of mind-body practice in college students likewise reported cardiovascular autonomic changes with sustained practice. That is the closer analogy for the breathing and meditation elements of Wim Hof training. It supports a simple inference: if these practices affect resting cardiovascular measures, 15 days may be too brief to show it.

Cold exposure has acute effects. Chronic benefit remains unproven.

The cold component is not physiologically inert. A 2024 systematic review and meta-analysis of cold-water immersion and cryostimulation documented acute cardiovascular and cardiac-autonomic responses in healthy people. Yet most included protocols used full immersion or whole-body cryostimulation, which deliver a stronger thermal stimulus than progressive cold showers.

Acute autonomic perturbation does not automatically become a durable improvement in resting HRV, blood pressure, or arterial stiffness. That leap needs direct evidence. The 15-day trial was intended to test precisely that possibility, but its sample size and duration limit what its null findings can exclude.

Prior controlled Wim Hof research, summarized in Ketelhut et al.'s introduction, found that trained practitioners had attenuated inflammatory responses during experimental human endotoxemia. Those findings concern an acute immune challenge and sympatho-adrenal activity. They do not demonstrate chronic resting cardiovascular adaptation, and they should not be used as a shortcut to that conclusion.

Problems that weaken confidence in either direction

The trial was registered on ClinicalTrials.gov on August 6, 2023, although data collection occurred from May through July 2022. Retrospective registration creates uncertainty about whether outcomes and analyses were specified before researchers knew the results. That does not invalidate the study, but it weakens confidence in both impressive positive findings and clean-looking null findings. Prospective registration is how readers can tell which tests were planned before the data arrived.

The researchers also did not objectively verify cold-shower duration or water temperature, completion of breathing sessions, or meditation time. Self-reported adherence cannot establish the dose participants actually received. If some participants only partly followed the program, a null result could reflect inadequate exposure rather than an ineffective intervention.

A passive control group adds another limitation. It cannot separate effects of the specific protocol from the effects of expectation, a new daily routine, or social engagement. The all-male sample also limits generalizability, since cold thermogenesis and autonomic responses can differ by sex. Finally, five-minute supine HRV recordings have more day-to-day variability than 24-hour ambulatory monitoring, which further reduces sensitivity to gradual changes in a small study.

What the evidence supports now

For healthy, normotensive young men, there is currently no randomized evidence that 15 days of the complete Wim Hof Method improves resting HRV, blood pressure, arterial stiffness, perceived stress, affect, or vitality. That is the direct finding.

There is also no established evidence that the complete method produces chronic resting cardiovascular benefits in healthy adults. The one available trial was too short and too small to rule out moderate effects, enrolled participants with little physiological headroom, and used uncertain dose delivery. The more defensible conclusion is unresolved, not miraculous and not disproven.

A useful next trial would prospectively register an 8- to 12-week protocol, enroll at least 50 participants per group, include women and men, use objective adherence measures and an active control, and pair resting HRV with 24-hour ambulatory monitoring. Participants with mildly elevated blood pressure, overweight status, or measurable autonomic impairment would offer a better chance of detecting a meaningful change.

Until such a trial reports results, people can practice Wim Hof breathing or cold exposure for enjoyment, challenge, or subjective benefit, while treating claims about durable cardiovascular improvement as claims awaiting evidence rather than established facts.