The ancient tradition of the frigidarium — a brief dip in cold water after a hot sauna or bath — continues to be practiced today in many parts of the world, including among many sauna-lovers here in Minnesota.

But is it beneficial to health, as practitioners have long claimed?

Well, a study published earlier this fall in the journal PLOS One by a team of researchers in the Netherlands offers some interesting, if not definitive, new findings on the topic.

The study reports that people who spent four weeks ending their daily hot shower with at least 30 seconds of cold water took, on average, almost a third fewer sick days from work over a three-month period than did their peers whose showers were hot from start to finish.

In fact, ending showers with a short spray of cold water had almost the same effect as regular exercise on reducing illness-related absenteeism from work.

Four different groups

For the current study, the researchers used online advertisements and social media to recruit 3,018 Dutch men and women, aged 18 to 65. None of the participants had serious medical conditions, and none had a prior history of routinely taking cold showers. All were employed.

The participants were randomized to three “intervention” groups and one “control” group. The intervention groups were instructed to shower daily with warm water for as long as they wanted, but to end the shower with 30, 60 or 90 seconds of cold water. (Timers were provided.)

Cold water was defined as “the average ground temperature at the level of running water in The Netherlands,” which during the intervention period of the study (Jan. 1-30, 2015) was 50-53 degrees Fahrenheit.

After 30 days, the participants in the three intervention groups were told to continue showering daily for an additional 60 days in any way they preferred, including — if they wanted to — with cold water.

The control group was instructed to shower daily for all 90 days, but only with warm water.

Primary findings

The researchers looked at two primary outcomes in their study: illness days (ones during which participants reported feeling ill, including with symptoms of cold and flu) and sick days from work. The study also included several secondary outcomes, including changes to quality of life, work productivity, anxiety, perception of body temperature and adverse reactions to the cold showers.

The main finding was that, compared to standard warm showering, hot-to-cold showering was associated with an increased reduction — by 29 percent — of self-reported sick leave from work. Hot-to-cold showering was not associated, however, with a reduction in the days people reported feeling ill.

This finding suggests, say the study’s authors, that “the intensity rather than the duration of symptoms is modulated by the intervention.”

The study also found that it didn’t matter if the cold part of the shower was 30, 60 or 90 seconds long. The results were the same.

Influenza was the most common reason cited by participants who took five or more days off from work — a not-surprising finding given that the Netherlands was in the midst of its longest-lasting flu epidemic in 40 years during the study’s run.

Interestingly, the only other factor in this study that came close to having the same effect as hot-to-cold showering on sick leave was regular exercise, which was associated with a 35 percent reduced incidence of work absenteeism.

And when hot-to-cold showering was combined with regular exercise, the participants were 54 percent less likely to call into work sick than those who didn’t do either activity.

Secondary findings

As for the secondary outcomes, only one showed a beneficial effect — quality of life — although the improvement was considered too small to be statistically significant.

Most of the participants who took the hot-to-cold showers reported some level of discomfort during the exposure to the cold water, but, remarkably, 91 percent of them said they were willing to continue the new showering routine past the mandatory 30 days — and 64 percent of them actually did.

The most commonly reported reason for liking the cold showers was a perceived increase in energy, which some of the participants compared to the effects of caffeine. The most commonly reported adverse reaction from the cold showers was a lingering after-sensation of being cold, which was reported by 13 percent of the participants.

No serious adverse effects directly related to the study were reported, although it should be pointed out (again) that the participants reported being healthy at the study’s start. Medical experts generally recommend that individuals with heart disease and other chronic medical conditions avoid extreme temperature changes.

Limitations and implications

Indeed, one of the study’s limitations is the fact that its participants were significantly more healthy than the Dutch population overall. Another significant caveat was that 12 percent of the participants dropped out of the study during the first 30 days. They may have quit the study for a reason (illness? adverse effects from the cold?) that, if known, might have altered the results. In addition, all illness days, sick days and other outcomes were self-reported by the study’s participants. Such reports may or may not have been accurate.

So, although these findings are interesting, they are not the last word on the topic. The study’s Dutch authors also point out that no one knows yet why a dousing of cold water daily may protect against illness (or, at least, against illness symptoms so intense as to require taking sick leave). One theory is that the cold has some kind of beneficial impact on the production of cortisol, norepinephrine and other hormones that play a role in the body’s immune response.

Or it may be that shivering provides some kind of (very) short daily physical activity that helps to enhance the immune response.

So stay tuned.  …. Oh, and don’t confuse hot-to-cold showering with whole-body cryotherapy, a “health” fad that exposes the body to extreme low temperatures (in a tank) for several minutes. Whole-body cryotherapy has been massively hyped with misleading claims, but, as the Food and Drug Administration made clear earlier this year, the so-called treatment is unproven, unregulated and unsafe.

FMI: PLOS One is an open-access journal, so you can read the Dutch study in full on the publication’s website.

UPDATE: In an earlier version of this post, I referred to the study as an observational one. That is incorrect, of course, for the study’s participants were randomized to each of the four groups. The study was not a “blinded” one, however, for the participants obviously knew when they were in the intervention group. That means that the study’s results — like the results of an observational study – cannot prove causation. That’s the point I was trying to make, and I did it rather sloppily.