Ice Plunges: What the Randomized Evidence Actually Shows

A meta-analysis of 11 trials and 3,177 adults found cold plunges cut stress at 12 hours, raised inflammation acutely, and did nothing measurable for immunity.

By Hydra Editorial

A 2025 meta-analysis in PLOS One pooled 11 randomized trials covering 3,177 healthy adults and found cold-water immersion increased inflammation immediately and one hour afterward, then reduced stress twelve hours later. It found no significant effect on immune function or mood. The gap between what ice plunges are marketed for and what has been measured is unusually wide.

What did the meta-analysis find?

The review included trials using baths or showers at 7°C to 15°C, from 30 seconds to 2 hours, immersed at least to chest level, in healthy non-elite adults. Seven studies were rated moderate quality and four high quality on the PEDro scale.

The findings, in order of confidence: inflammation rose acutely and then settled; stress was significantly reduced at the 12-hour mark but not immediately; immune function showed no significant change either immediately (SMD -0.16) or at one hour (SMD -0.18). Narrative synthesis of individual trials pointed to longer-term possibilities, including a 29% reduction in sickness absence among people taking cold showers, plus improvements in sleep quality and quality of life. The sleep trial ran five days. The quality-of-life improvement was no longer significant at three months.

Why is the evidence still this thin?

Three structural reasons. The trials are few and small. You cannot blind someone to being in an ice bath, so expectation effects are unavoidable. And the study populations lack diversity, which the authors name directly as a limitation.

There is also a timing consideration that the mainstream coverage tends to skip: the acute inflammatory rise means people with pre-existing cardiovascular conditions have a reason to be cautious rather than casual, and cold-water immersion carries real cardiac and drowning risks separate from any benefit question.

The most defensible reading is that cold-water immersion has time-dependent effects, that stress reduction at 12 hours is the best-supported one, and that most other claims are running ahead of the data. Runners weighing cold water immersion specifically for recovery, rather than for these general wellbeing outcomes, should read the adaptation-blunting angle separately.

The bottom line

  • Across 11 RCTs and 3,177 adults, cold-water immersion reduced stress at 12 hours but not immediately, and raised inflammation acutely
  • No significant effect on immune function was found immediately or at one hour post-immersion
  • One trial found a 29% reduction in sickness absence with cold showers, but the wider evidence base is small, unblindable and short-duration

Frequently asked questions

Do ice baths boost your immune system?
The 2025 meta-analysis found no significant immune function change immediately (SMD -0.16) or one hour after immersion, with low certainty of evidence. A separate trial in the same review reported 29% fewer sickness absence days among cold shower users, though the number of illness days reported did not differ significantly from controls.
How cold and how long do the studies use?
The included trials used water between 7°C and 15°C, for durations from 30 seconds to 2 hours, with immersion at or above the level of the xiphoid process. The review did not identify an optimal protocol, and the authors call for research specifically on dose-response.

Sources

  1. Cain T et al., 2025. Effects of cold-water immersion on health and wellbeing: A systematic review and meta-analysis. PLOS One 20(1):e0317615.
Hydra Editorial

Hydra's editorial team. We write from primary research and cite every study we rely on, with its year and journal. We quantify claims wherever the evidence allows and state plainly where it does not: where a finding rests on a small or single trial, we say so in the text rather than in a footnote.