Does a cold plunge boost your immune system? The markers barely move

Nobody has tested whether a cold plunge boosts your immune system in the sense people mean it: fewer colds, fewer sick days. What has been tested is narrower - whether weeks of cold water immersion change the immune cells circulating in the blood - and the answer there is that they barely do.

In the most recent of those studies, 12 healthy men in their twenties sat in water at 7C (about 45F) for 12 minutes, four times a week for three weeks [6]. Their total white blood cell count dropped, but so did the control group's, and that group did no cold at all. The only change unique to the cold group was a small fall in neutrophils, the first-responder white cells that swarm bacteria, and the authors' own conclusion was that three weeks of cold water had no relevant effect on immune cell counts [6].

An older study went gentler on temperature and far harder on duration: athletic young men were immersed in water at 14C (about 57F) for a full hour, three times a week for six weeks [3]. A single immersion did almost nothing. After six weeks there were small rises in monocytes, a white cell that clears debris and calls in the rest of the immune system, and in lymphocytes, the white cells that mount targeted defence, carrying a marker that shows they have been switched on. Antibodies, the proteins that recognise specific germs, did not change, and neither did total white cell counts, neutrophils or the standard blood markers of inflammation [3].

The study everyone quotes is a different design again. 24 healthy volunteers were split in two; 12 spent 10 days learning meditation, a forced breathing pattern and immersion in ice cold water, and then all 24 were injected with a dose of bacterial toxin that reliably produces a few hours of flu-like illness [1]. The trained group produced more interleukin-10, the body's main inflammation-off signal, less of the three main inflammation-on signals, and reported feeling less ill [1]. That is a damped reaction to a provocation rather than a stronger immune response to a germ, and because the 10 days taught three practices at once, the cold cannot be pulled out from the breathing.

The closest thing to a real-world infection result is a survey. Among 77 regular practitioners, those who did cold immersion alone reported shorter colds and sore throats than matched people who did neither, and those who added breathwork reported shorter ones still [4]. People who choose to sit in ice water and then fill in a questionnaire about it are not a random sample, and nothing here was measured in a laboratory. It was recalled.

Cold water sets off a large, fast surge of adrenaline, the fight-or-flight hormone. In the bundled training study that surge did the work: adrenaline levels before the toxin injection tracked how much interleukin-10, the inflammation-off signal, each person produced afterwards [1]. Adrenaline tells immune cells to quieten down, which is why the mechanism with the best evidence behind it points at suppressing inflammation rather than raising immunity.

Weeks of repeated immersion do something slower. Shivering raises the rate at which the body burns energy and keeps adrenaline and its close relative noradrenaline elevated, and across six weeks that came with a mild version of the response the body mounts to injury: more haptoglobin and haemopexin, two proteins the liver releases when tissue is under stress [3]. The authors read that as an immune system being prodded by a stressor that is not an infection, which is not the same as being trained against one.

There is also a cell-housekeeping effect, and it depends on how far the body actually cools. When core temperature was pushed down by half a degree Celsius, immune cells taken from young men showed more autophagy, the process by which a cell breaks down and recycles its own damaged parts, along with more heat shock proteins, which help other proteins keep their shape under stress [2]. Push the cooling to a full degree and the same cells instead showed more self-destruct signalling [2]. Neither reading tells you whether the person who owns those cells catches fewer infections.

The human evidence is small and short. The two cold-only studies ran 12 men per group and a few dozen young men respectively [6][3], and none of this work followed anyone through a winter to count actual infections.

Who the evidence does not apply to:

• Older adults, whose immune cells in the cooling study showed almost no autophagy response and more self-destruct and inflammation signalling instead [2], so results from men in their twenties should not be read across.
• Women, who were not enrolled in any of the cold-only immersion studies [6][3][5].
• Anyone already unwell, since every volunteer was healthy and the toxin injection in the bundled study is a controlled provocation given to fit young people, not an infection [1].

Individual response also varies more than a protocol suggests. Among 40 men immersed at 14C, some cooled quickly and some slowly, and the two groups showed different hormone and immune patterns [5], so the same tub does not produce the same internal response in two people sitting in it.

Two sources get pulled into this question that do not belong to it. The work showing cold damaging immune tissue, with colon inflammation and a leaky gut lining, was done in mice kept at 4C for three hours a day for 10 days [8], which is chronic ambient cold rather than a plunge. And whole body cryotherapy is a separate exposure, air at -110C to -140C for about two minutes: a review in athletes found it did not increase bone marrow output and caused only low-level oxidative stress, the wear that reactive oxygen molecules inflict inside cells [10].

Longevity clinics and recovery studios list cold plunge therapy as a service, usually a tub held somewhere between 4C and 15C (39F to 59F) for two to ten minutes, often alternated with a sauna. A first visit is normally a short screening conversation about your health history, then a supervised few minutes in the water with staff present and the temperature monitored.

Two protocols sit behind the immune claim, and they are worth comparing with what a session actually involves. One held men at 7C for 12 minutes, four times a week [6]. The other sat them at 14C for an hour, three times a week, for six weeks [3]. The hour-long version bears no resemblance to a drop-in session, and neither produced a measurable immune gain. If a provider's immune claim rests on the ice-water-and-breathing study, that programme taught meditation and a breathing pattern alongside the cold, and it tested a toxin injection in a laboratory rather than a cold season [1].