The risks of cold water immersion
The risks of cold water immersion
Lately I have noticed a trend in cold water swimming articles and their promise to cure dementia as reported by Runnersworld and BBC. It is also suggested to be a strategy to survive or ride out the Covid winter in Europe, according to the Guardian. Of course cold water immersion is one of the topics that the world renowned health expert Wim Hof has experimented with at length. As time progresses, the scientific community is increasingly interested as to whether the iceman is superhuman, or whether these benefits are available to everybody.
So one of the key questions is: how does the body keep itself heated?
One of the key benefits of cold exposure is the increased activation of brown adipose tissue (BAT). BAT is limitedly stored in scarce places in the body, mostly the neck area. It is different from white adipose tissue (WAT), which is responsible for whole body energy metabolism and expands with excessive energy intake. Brown fat is activated by cold and works to prevent shivering of muscles to warm up the body. Although BAT is especially present in infants and seems to decrease with age its oxidative capacity and activity may increase after repeated cold exposure. Eventually this leads to changes in lipid metabolism, and in this way activation of brown fat may lead to weight reduction.
That being said, there is also an important role that is played by subcutaneous fat (WAT), which mobilizes free fatty acids during cold immersion. Lipid metabolism plays a rather insignificant role there, with an exception for norepinephrine and cortisol. So with that being said, the fuel sources have been identified but their interaction and play are still not completely clear.
The proclaimed benefits on health are mainly related to strengthening of the immune system and increased vitality, but the prove for these claims are actually rather flimsy. The scientific proof for these claims based on randomized controlled trials are not in place, and any claims are typically at most causal, not associational. If someone that never gets out in nature or never engages in physical activity will start a practice of cold-water swimming, the effect of nature and exercise cannot simply be taken out of the equation. In certain adipose tissue in mice, cold exposure alone had no effect, whereas a combination of exercise and cold exposure did have effect on mitochondrial bio-genesis. So it seems that especially this combination may be more effective than either one in isolation.
For example, it is now well known that fish that thrive in cold ocean temperatures survive through increases in mitochondrial volume density. But other than that, there are some studies that found that repetitive cold showers for a minimum of 30 days results in a lower number of self-reported sick leave, but it is not clear exactly which factors attributed to that outcome. There seems to be no cognitive benefits to repeated cold stress either.
Cold water immersion for athletes
Also in sports, CWI is getting more and more popular and is applied as a way to enhance recovery for competition and training in pro athletes. The immediate physiological changes are mainly cardiovascular changes, like a reduction in heartrate and an increase in blood pressure as a result of vasoconstriction. In the attempt to maintain baseline core body temperature, oxygen consumption increases as well.
But how beneficial are these immediate effects and do they translate to better recovery? Many of the claims for better recovery are anecdotal rather than anything else. For example, metabolic variables such as enhancement of metabolic waste materials through hydrostatic pressure (enhanced return of fluids from muscles back into the blood) is not supported. Also, inflammatory markers such as creatine kinase and c-reactive protein seem not to be affected by cold water immersion. The endocrine system which uses hormones to send messages to organs and tissues in the body also does not seem to be affected, with very little changes found in testosterone and prolactin concentrations.
There have been studies that have indeed shown PERCEIVED improved recovery, as shown in below graph that investigated perceived recovery by jiu-jitsu athletes after cryotherapy sessions post training.
However, studies that aim to certify this claim tend to find the OPPOSITE result. CWI leads to a reduction of acute inflammation of the muscle, but an acute repression of inflammation may negatively affect the repair and regeneration process. Remember that inflammation is not always negative but has a key role in supercompensation and interfering with CWI practices may hamper this process. As you can see in the graph below, repeated sprint ability was tested through maximal voluntary contractions. And although very short term recovery had increased, at the sweet spot of 24h post-exercise, the voluntary force and motor unit recruitment had decreased compared to passive recovery.
There are also different studies that find negative effects for recovery in cyclists. And also sprint performance and strength training has been reported to be negatively affected by cold-water immersion. Evidence for that can be found in below graph, where Active Recovery (ACT) was compared with CWI for recovery. Strength, isometric torque and rate of force development all favored the active recovery group over the CWI group.
Risks of Cold Water Immersion
Finally I want to talk a little bit about the risks of CWI. There is a real need for proven beneficial protocols, most of the anecdotal feedback is positive but some studies report no effects or even detrimental effects. Many of the current protocols vary, and there is not enough understanding with regards to optimal duration, temperature, level of immersion and frequency. It should also depend on body size, body fat level, ethnicity, sex and age among other things.
What many people don’t know is that sea water freezes at −1.9°C while human tissue freezes at −0.55°C. Therefore, in theory, it is possible to get frostbite in the sea, although it is pretty rare. A more common risk is non-freezing cold injury which can be caused by long duration immersion in cold water or short immersion in very cold water. The consequences of non-freezing cold injury can vary from very heavy sweating, to cold sensitivity, to intractable pain.
When body temperatures plummet below 32.2 ◦C as a result of CWI, dysrhythmia, hypotension and decreased levels of consciousness may appear. This is even more risky in the early morning, as body temperatures are typically lower by 0.9 ◦C. Next to some of these risks, CWI also stimulates diuresis, an increased excretion of water from the body. Also electrolytes, especially sodium is excreted to a greater extend. Thus, one should carefully review their body fluid level when engaging in CWI and assure homeostasis of body fluids. Children and elderly should be especially careful in this regard.
Personal experience cold swimming
With this information I do not want to scare anyone to refrain from the cold, but it is always good to be aware of the risks. In order to show that the cold and especially cold-water swimming is not the be-all and end-all, I would like to share some personal experience. In 2019 I had a great trip to the Arctic Circle in Lofoten, where I swam on 6 consecutive days and did over 15km divided over 11 individual sessions. I had been wearing an Oura ring prior through which I was able to track resting heart rate, heart rate variability, readiness score and respiratory rate.
I had been engaging in cold shower practice prior and had plenty of long-distance swimming experience. However, the results are pretty shocking, as you can see from the data below:
1. Resting heart rate
As indicated by the arrow, my resting heart rate jumped up from ~40 bpm to close to 50 bpm. In the year after, it never returned to baseline.
2. Average Heart Rate Variability
My HRV also plummeted from around of 100ms to 70-80ms during swimming. The year after it never returned back to baseline.
3. Readiness score
Oura’s readiness score is calculated by combining multiple factors. This graph is perhaps the best indicator of the immediate stress that multi-day cold water swimming puts on the body. Consistent low scores for multiple days had not occurred anywhere else in the year.
4. Respiratory rate
Here you can see especially that it has been a great challenge on the lungs as well.
To repeat, I do not want to scare anyone with these figures, but it is fair to provide a footnote and to know that there is a shadow side to cold water immersion. So, proceed with caution and if you do it would not hurt to quantify your data so that we are not basing the benefits solely on anecdotal evidence.
What are your experiences with CWI? Let me know in the comments!