The Science of Cold Exposure: Understanding its Impact on Human Health through Cold Water Swimming, Plunge Pools, and Cold Showers

Overview

Cold exposure has become ever-present in the health, fitness, and longevity community in recent years, with several high-profile celebrities, such as Chris Hemsworth, Kevin Hart, and Joe Rogan, endorsing the benefits of brief exposure (2-3 minutes) of cold water from a shower, plunge pool or a jump in the sea.

Interestingly, the practice of cold exposure is nothing new and has been dated back to 3500 BC, with the ancient Greeks using this method to treat various ailments for medicinal purposes. Since then, several research groups worldwide have produced ground-breaking research deciphering the effects of short-term cold exposure on human health, metabolic function, and as a recovery tool following exercise training.

Furthermore, many podcasters, bloggers and 'fitness experts' emphasize and potentially exaggerate the effects of cold water exposure benefits. Here we will provide a deep-dive on the advantages and disadvantages of cold water immersion on human physiology.

Background

Cold water immersion has been popularised in the last decade by the likes of Dutch extreme athlete Wim Hof and, more recently, a Ph.D. graduate from the University of Copenhagen, Dr. Susanna Soberg, Ph.D., The latter, Dr. Susanna Soberg, completed her Ph.D. in the world-leading lab of Professor Camilla Scheele, within the Center of Inflammation and Metabolism and the Center for Physical Activity Research, researching human metabolism and the effects of stressors on brown and white fat.

Since completing her Ph.D., Dr. Soberg has published several high-impact publications and a book titled "Winter Swimming: The Nordic Way Towards a Healthier and Happier Life." Her research has led to the development of the 'Soberg Principle', which is described as the minimal amount of cold exposure needed per week to see health and metabolic benefits.

In her research, she proposes that a minimum of only 11 minutes of cold exposure per week is enough to reap the health benefits, which can be enhanced by increased exposure and the addition of heat; however, we'll discuss the benefits of heat & sauna in an upcoming article next month.

As with anything, this 'trend' of exposing yourself to cold water for a specified duration has been overly stated on social media platforms and podcasts to improve immunity, fat loss, inflammation, anxiety, depression, and metabolism. In a world where the general public derive a lot of their science information from podcasts, Instagram, and ticktock, how true are some of these statements? Here we will separate fact from fiction in an attempt to give you the best understanding of the benefits or disadvantages of cold water exposure.

Molecular Response to Cold Exposure

As humans, we are consistently battling external stressors in an attempt to maintain homeostasis. Following a stimulus, our body responds via hundreds, if not thousands, of signals between cells and tissues to adapt to the stressor to handle the stimulus again. As humans in the 21st century, we are typically accustomed to wearing multiple layers of clothes, enjoying the comfort of our warm homes, and having heated seats while driving.

We have become so habituated to the warmth that exposure to the cold is incredibly stressful and can even lead to cold shock, and death in certain extreme circumstances when not done correctly [1]. Nevertheless, the basic biology behind the brief exposure is fascinating, and even a short [2-3] minutes out of your comfort zone and under a cold shower or in an ice bath can have on your metabolism [2–5] & mental health [6,7]. Here we will discuss the main molecular responses to cold exposure and how this translates into improved health.

Thermogenesis & Brown Tissue Activation

Immediately upon exposure to a cold environment, our bodies enter their 'fight or flight' response, commonly termed increased sympathetic nervous system activity. This response stimulus activates brown adipose tissue (BAT) [8], which is responsible for thermogenesis and producing heat in response to a cold environment and increasing energy expenditure [5]. Much research has been published on brown adipose tissue in mice, with little research in humans. This is primarily due to the technical difficulty in measuring its activity but also due to the limited depots of BAT in humans, which are only located in small amounts around the clavicles, upper back, and spine [9] (see PET CT scan below).

Thermogenesis & Brown Tissue Activation

"Brown fat is a type of fat that generates heat by burning calories. It contains a lot of mitochondria, which give it a brown colour and help produce energy. Its main role is to regulate body temperature and keep us warm. It is found in small amounts in adults, mostly in the neck and upper back. White fat is the most common type of fat in our bodies. Unlike brown fat, white fat doesn't produce much heat. Excess white fat can lead to obesity and health problems. It is located under the skin (subcutaneous fat) and around organs (visceral fat)."

In Dr. Soberg's landmark 2021 publication from her Ph.D. thesis [5], she measured the effects of cooling via an unconventional method of cooling blankets. These are effectively water-cooled blankets (found here) that are capable of temperatures as low as 10°C.

In this study, she compared two groups of Danish men; one group were open water swimmers, that were accustomed to cold exposure, and a control group of men with no prior experience of cold exposure [5]. Following a rather long ~3hrs of cold cooling, the control group observed a ~90% increase in BAT activation.

Remarkably, the open-water swimmers observed a ~3000% increase in BAT activation [5]. The increased activation of BAT subsequently led to an extra ~425 calories and ~930 calories in the control and open water swimmers, respectively.

One of the primary mechanisms of the increased energy expenditure may be due to the increased shivering during and after cold exposure as a mile ‘exercise mimetic’ due to muscle activity & contraction [10].

"The involuntary contraction of skeletal muscle during shivering leads to heat production, energy utilization, and secretion of small molecule growth factors FGF21 & Irisin [11], thereby increasing energy expenditure."

In her 2021 publication, Dr. Solberg indirectly measured 'muscle activity' with EMG electrodes placed on the pectoralis and quadriceps in an attempt to gain insights into cold exposure's effect on muscle activity [5].

As a result, cold exposure had a significant increase in muscle activity. Similar studies using the cold water cooling blankets by the Diabetes, Endocrinology, and Obesity Branch at the National Institute of Diabetes and Digestive and Kidney Diseases, have shown a 96% increase in muscle activity, and a ~50% increase in energy expenditure over a 24 hours [4].

Overall, the increased energy expenditure shows incredibly promising data that cold exposure could potentially help with weight loss when practiced over weeks to months. However, a large limitation of the above studies is the use of cold water cooling blankets, which people do not use, nor have access to.

Instead, the use of cold plunge pools, ice baths, and cold showers is preferred. Although they likely have similar responses, further research is needed on more widely available cold exposure methods to see if there are similarities between methods.

Metabolic Health Benefits of Cold Exposure

Insulin Sensitivity & Glycaemic Control

The increase in BAT activation as a result of cold exposure increases the amount of glucose utilised by BAT in an attempt to increase energy expenditure and heat production to maintain homeostasis [8]. As a result, whole-body insulin sensitivity is improved [2]. Researchers from German Center for Diabetes Research in Neuherberg, Germany, showed a 20% increase in insulin sensitivity following a single session of cold exposure, with a concurrent rise in circulating fatty acids [3]. Similar to the above research, the authors used a wearable water-perfused suit for 100 minutes at a temperature of ~18°C [3].

Another method of cold exposure researchers use is living in a cold environment. Notably, Professor Patrick Schrauwen from Maastricht University has been at the forefront of this research. With access to a free-living research facility at Maastricht Medical Centre, Patrick and his research team can set the room's temperature to a moderate 14°C to determine how exposure to a mild cold temperature for 10 days alters human metabolism.

In a landmark study in Nature Medicine, Professor Schrauwen and his team showed that as little as 10 days of living in the cold room wearing only shorts and a t-shirt (to maximise cold exposure) resulted in a 43% increase in insulin sensitivity and ~57% increase in brown adipose tissue activation in adults with type II diabetes [2].

The paper's authors also showed that the improvements in insulin sensitivity may be driven by an increase in skeletal muscle GLUT4 expression [2].

"GLUT4 is a protein that helps cells take up glucose from the blood. It does this in response to a hormone called insulin. When insulin levels rise, GLUT4 moves from inside the cell to its surface, creating channels that allow glucose to enter. This process helps regulate blood glucose levels and provides cells with the energy they need to function properly."

Cold Exposure for Mental Health and Depression

Compared to metabolic health research, the data to support the cognitive effects of cold water exposure needs to be more defined. However, the available evidence suggests improvements in mental health and depression.

Notably, a study in the UK at the University of Chichester saw drastic improvements in depressive symptoms following cold exposure by 20 minutes in the sea at 13°C [6]. Before and after the cold water exposure, volunteers completed a questionnaire to determine mood states.

What they observed was a 75% reduction in depressive symptoms, ~60% reduction in anger, ~50% reduction in confusion, and ~34% reduction in fatigue [6]. With the little data on cold exposure, university institutes and mental health foundations should work on discovering the potentially powerful effects of short-term cold showers & cold plunges on mental health outcomes as a non-pharmacological alternative to treat depression, anxiety and other mood disorders.

Alternatively, individuals without mental health conditions may be able to use cold water exposure in an attempt to improve mental and psychological resilience to prevent future episodes of poor mental health [7].

Can the Cold Exposure Increase Lifespan & Longevity?

The effects of low temperature on longevity were discovered over a century ago, in 1916, by Professor Jacques Loeb from Rockefeller Institute for Medical Research, New York [12]; since then, the effects of low temperatures on lifespan & healthspan have remained relatively understood to this day.

Pre-clinical data in mice show that exposure to an environment of as little as 0.5°C higher than body temperate shortens lifespan [13], but living in an environment of 0.5°C cooler than body temperature increases lifespan, and by as much as 20% [14]. However, the cages of the mice are maintained at a set temperature 24/7 their entire lives and do not necessarily easily translate to the daily oscillations of cold and warm temperatures, as well as seasonal variability (i.e., summer/winter) that humans are exposed to.

A recent publication from the University of Cologne, Germany, in the prestigious journal Nature recently showed that cold temperatures (15°C), increased lifespan, reduced protein aggregation, and mitigated neurodegeneration. The mechanism behind the findings suggests cold exposure improves proteasome activity, and in particular, the activation of PA28γ/PSME3 to remove damaged, dysfunctional or aggregated proteins from a cell. Notably, the accumulation of aggregates or 'protein clumps' are highly prevalent in neurodegenerative diseases.

The activation of this proteasome activator via cold exposure may be a key regulator of longevity. However, future work in mice and human biopsy samples is obviously needed to confirm its role in cold adaptation.

A recent hypothesis has been thrown into the mix regarding the 'Live Cold, Die Old' [13], which was put forward by Professor John Speakman and his group at the Shenzhen Institutes of Advanced Technology in Nature Metabolism, where they state the importance of body temperature.

What they describe is the phenomenon that animals with a high metabolic rate tend to die earlier than those with slower metabolic rates [13]. Therefore, animals with a slower metabolic rate, display lower body temperatures, and it may be body temperature that is important for longevity [13]. So, methods to reduce an individual body temperature, either acutely via a cold shower, ice bath or cold plunge, or chronically by living in a cold environment, may actually enhance human lifespan and longevity.

The Negative Effects of Cold Exposure

Blunted Muscle Hypertrophy - Will Ice Baths Kill Your Gains?

Contrary to the aforementioned benefits, cold exposure may negatively affect muscle mass gains. This has been supported by several high-impact and well-controlled clinical trials worldwide. Most notably, Dr. Cas Fuchs from Maastricht University in the Netherlands showed a blunting of ~20% in muscle protein synthesis rates (i.e., the building of new muscle) following resistance training.

In this study, Cas had groups of volunteers place their legs in either a 30°C or 8°C for 20 minutes following exercise after a single session, then as part of a 2-week training intervention. This is important because each volunteer was subjected to neutral and cold water. The thermoneutral leg served as the control, making this an incredibly powered research investigation, as this limits participant-to-participant variation.

As part of the study, participants were required to maintain cold-water immersion following a further six training sessions. At the end of the 2-weeks, long-term muscle protein synthesis rates were ~26% lower in the leg that underwent cold water immersion.

So, what happens if you continue cold water exposure as part of a longer-duration exercise training program?

One of the first long-term studies of exercise training and cold water immersion was published in 2015 by Dr. Llion Roberts, Ph.D, observing the effects of 10 minutes of cold exposure at 10°C during a 12-week training intervention [15]. Overall, both the thermoneutral and cold water groups gained muscle mass and strength. However, the gains in both measures were severely blunted in the cold water group [15]. This was followed up by a research group at Victoria University, Melbourne, led by Professor David Bishop, a world leader in exercise physiology research.

The project was run by a PhD student at the time, Dr Jackson Fyfe, PhD, where he determined the effects of 15 minutes of 10°C cold water on muscle hypertrophy & strength [16]. Similar to the previous studies, Jackson's observed a blunting in muscle hypertrophy, but interestingly no differences in muscle strength improvement [16].

The evidence is clear! If your training goal is to gain muscle mass and strength, jumping in a cold pool, immediately post-training may not be a good idea. Now, I know what you're thinking, 20 minutes of cold exposure is very extreme and not recommended, especially considering cold water immersion experts recommend 2-5 minutes. Therefore, if you take a 2-5 minute cold shower or dip in a plunge pool, these shorter durations are less likely to impact muscle building.