Podcast Host: Dr. Andrew Huberman (Stanford Associate Professor - brain development, brain plasticity, and neural regeneration and repair fields).
Podcast Guest: Dr. Susanna Søberg (Ph.D. University of Copenhagen Denmark)
The featured individual in this particular episode is Susanna Søberg, who holds a Doctor of Philosophy degree. The individual obtained her doctorate from the University of Copenhagen in Denmark, where she conducted research on the impact of intentional exposure to cold and heat on human metabolism and other physiological factors. The potential benefits of cold exposure or sauna on various aspects of human health, such as metabolism, cardiovascular function, brain health, hormonal regulation, and inflammation reduction, are explored by Dr. Huberman and Dr. Søberg. The article by Dr. Søberg highlights the potential benefits of employing intentional cold protocols to enhance glucose metabolism and insulin sensitivity and stimulate the release of neurotransmitters such as dopamine and norepinephrine, which can lead to improvements in energy levels, mood, and cognitive function. Both methods are also compared in terms of their effectiveness, including cold showers and cold immersion, as well as traditional and infrared saunas, among other variables. This episode furnishes practical methodologies and solutions to frequently asked inquiries concerning using intentional cold and heat exposure to enhance one's well-being.
Dr. Susanna Soberg conducted her thesis research at the Centre for Inflammation and Metabolism and the Centre for Physical Activity Research at the University of Copenhagen, Denmark. Her research is centered on investigating the potential of deliberate cold exposure to improve human metabolism. The individual in question is recognized for being the primary author of a groundbreaking research paper that identified the minimum levels of intentional heat and cold exposure required to stimulate brown fat thermogenesis. This process involves elevating heat production and metabolic activity within the body, and the study also established practical guidelines that can be implemented beyond laboratory settings to enhance human health and metabolism. The research conducted by Dr. Soberg was recently published in the esteemed journal Cell Reports Metabolism in the year 2021. This study contributes to a significant and extensive body of research that has been conducted on the effects of cold and heat on various physiological aspects of the human body, including hormone health, metabolism, and alterations in neurotransmitters such as dopamine and epinephrine. The present discourse with Dr. Soberg pertains to the influence of intentional heat and cold exposure on metabolism. Additionally, it encompasses an examination of the impact of temperature on neurotransmitter production, specifically dopamine, epinephrine, and norepinephrine, which are categorized as catecholamines and have a profound effect on both mood and metabolism. Furthermore, Dr. Soberg provides comprehensive information on deliberate cold and deliberate heat exposure, addressing common queries such as the distinctions between cold shower, cold immersion up to the neck, and total body cold immersion. Additionally, the impact of alternating between heat and cold on metabolism, hormones, and neurotransmitter production is explored. The speaker will provide a comprehensive analysis of the deliberate cold and deliberate heat exposure protocols, with a focus on the underlying scientific principles. Specifically, she will examine the impact of cold receptors at the skin level versus cold reception and perception at the brain level, and how these factors affect the various systems of the brain and body, including mental health, physical health, and performance. This analysis is based on the speaker's extensive scientific research and academic training, as well as her practical experience with deliberate heat and cold exposure protocols.
Dr. Soberg is widely recognized as a preeminent authority on the subjects at hand. In addition, she has authored a highly regarded publication entitled "Winter Swimming," which comprehensively explores the multifaceted components of intentional exposure to both heat and cold. The book delves into topics such as the cold shock response, the potential hazards and safety considerations associated with cold water immersion, the physiological effects of both cold and heat on human health, and the comparative merits of various modalities such as sauna, ice, and cold water swimming .
Figure 1. Dr. Susanna Soberg is engaging in deliberate cold exposure.
1.The Soeberg Institute. Available from: https://www.soeberginstitute.com/.
In September 2023, Dr. Andrew Huberman is scheduled to conduct two live events. The first event is slated to take place in Toronto on September 12, while the second event is scheduled for September 28 in Chicago. Both events will feature a lecture and a question-and-answer session, and are titled "Brain-Body Contract." During these events, Dr. Huberman will delve into the tools and scientific concepts related to mental health, physical health, and performance .
2. Huberman, A. The Brain Body Contract. 2023; Available from: https://hubermanlab.com/tour/.
The act of immersing oneself in cold water is widely regarded as the most effective stressor. However, exposure to cold wind can also elicit a similar response by activating the sympathetic nervous system, leading to the release of neurotransmitters such as catecholamine within the body. Suppose an individual undergoes a cold plunge. If they experience high temperatures prior to immersing themselves in cold water, the experience may be less taxing. However, the temperature differential between their skin and the cold water may induce a shock response. Nevertheless, their core temperature may remain relatively warm, which could provide some relief. This phenomenon may explain why individuals who transition from a sauna to cold water may find the experience more manageable compared to those who were already cold.
Research in the field of physiology pertaining to extreme cold environments has primarily focused on analyzing the physiological responses and adaptations of the human body to extremely low temperatures. Various studies have identified hypermetabolic, insulative, and habituated responses as the primary adaptations to cold exposure. Research has been conducted to examine the specific physiological reactions of individuals who are subjected to exceedingly low temperatures. Undertaking further research in this domain is crucial to enhance our understanding of human responses to cold environments and to devise strategies for improving well-being and safety in such situations .
3. Castellani, J.W. and A.J. Young, Human physiological responses to cold exposure: Acute responses and acclimatization to prolonged exposure. Autonomic Neuroscience, 2016. 196: p. 63-74.
In the event of extremely low temperatures and concurrent personal sensations of coldness, it is advisable to prolong one's immersion in water. However, it is important to note that a degree of discomfort is to be expected, provided that such discomfort is limited to the realm of coldness. Exposure to low temperatures can induce a physiological response known as cold shock, which triggers the activation of the sympathetic nervous system and the release of catecholamines. During cold shock, an individual experiences hyperventilation, resulting in an accelerated breathing rate. This increase in breathing rate can be attributed to the activation of the grasping reflex. It is possible to undergo training in cold exposure, resulting in adaptation to the stimulus. Therefore, it is unnecessary to experience a hyperventilation response each time one immerses themselves in cold water. Enhancing one's resilience and adaptability is crucial for expediting the process. Initiating a task may pose a greater challenge in its nascent stages, however, it is imperative to undertake arduous endeavors. The perception of cold water and cold water immersion should not be construed as a comfortable experience, rather it should be regarded as a challenging one.
Individuals exhibit varying responses to cold temperatures, with some experiencing a sense of comfort while others may harbor a sense of aversion. Paradoxically, those who actively resist the cold may actually experience heightened sensitivity to its effects. Numerous soldiers have acclimated to cold environments, and those who are not intimidated by the cold tend to cope with it more effectively. Additionally, some individuals possess a more sensitive nervous system, and if they are sensitive to the cold, they will naturally attempt to avoid it. Consequently, they may experience heightened sensations of cold pain, which intensify if they continue to avoid exposure to cold environments .
Figure 2. Cold water shock and its impacts on health
4. Knechtle, B., et al., Cold Water Swimming-Benefits and Risks: A Narrative Review. Int J Environ Res Public Health, 2020. 17(23).
The practice of cold showers and cold immersion in water elicits distinct physiological responses due to the differential activation of cold receptors in the skin. Exposure to cold water results in more comprehensive coverage of the body, thereby bringing the cold molecules closer to the skin and leading to more robust activation of cold receptors. This, in turn, triggers a more rapid and heightened response from the autonomous nervous system compared to exposure to cold air while wearing a T-shirt. The sympathetic nervous system is also activated, leading to an increase in norepinephrine and the activation of BAT, a healthy tissue in the body. Upon activation, it enhances metabolic rate.
Based on empirical evidence, it can be inferred that the practice of taking cold showers has the potential to enhance metabolic processes. As per a research investigation, exposing the human body to cold-water immersion at a temperature of 14℃ led to a noteworthy elevation in metabolic rate, exhibiting a recorded surge of 350% . The augmentation of metabolic rate possesses the capability to assist in the regulation of body mass. In addition, a cold shower featuring a brisk water flow has the potential to provide a comprehensive boost of energy that endures for the duration of the day. While the exact mechanisms responsible for these effects are not yet fully understood, some scholars suggest that cold showers may have the potential to activate the sympathetic nervous system and increase the release of certain hormones such as adrenaline and noradrenaline, which could potentially enhance metabolic activity. Additional investigation is necessary to fully understand the benefits of cold showers on metabolic processes .
Research has been carried out on rodents and humans, demonstrating that the exposure of the skin to cold temperatures leads to the activation of (BAT). The aforementioned tissue functions as the principal effector in the human body to uphold thermoregulation, while the muscle tissue assumes the role of a secondary effector. The elevation of body temperature is facilitated by the activation of (BAT) and muscle tissue via shivering. Research has demonstrated that cold showers can stimulate (BAT) and may consequently result in a rise in metabolic rate. Nevertheless, additional investigation is required to ascertain the magnitude of brown adipose tissue stimulation that arises from exposure to cold showers .
The practice of winter swimming in Scandinavia involves brief submersions in extremely cold water, often accompanied by subsequent exposure to high temperatures in a sauna. The current study examined the effects of specific activity on the thermoregulatory response and thermogenic (BAT) in skilled male winter swimmers who engaged in this regimen on a regular basis, typically attending 2-3 sessions per week. The results indicate that individuals who participate in winter swimming demonstrate a diminished level of thermal comfort, as evidenced by a decline in core temperature and a lack of activation of (BAT), relative to the control cohort. However, the group of individuals who engaged in winter swimming exhibited greater improvements in cold-induced thermogenesis and supraclavicular skin temperature as compared to the control group when exposed to cold. Both groups demonstrated similar improvements in glucose uptake in brown adipose tissue and muscle activation. It is worth mentioning that a decrease in supraclavicular skin temperature was detected in every participant throughout the night-time phase. Nevertheless, it was observed that individuals who participated in winter swimming experienced a significant increase in body temperature during the time frame of 4:30-5:30 a.m. The aforementioned findings enhance our understanding of (BAT) in the adult human population and offer valuable insights into the distinctive physiological responses observed in individuals who participate in winter swimming .
Figure 3. Investigation of the impact of modified brown adipose tissue thermoregulation on cold-induced thermogenesis .
5. Srámek P, Simecková M, Janský L, Savlíková J, Vybíral S. Human physiological responses to immersion into water of different temperatures. Eur J Appl Physiol. 2000 Mar;81(5):436-42. doi: 10.1007/s004210050065. PMID: 10751106.
6. Laurence, E. Here’s Why Taking a Cold Shower Might Boost Your Metabolism. 2019; Available from: https://www.wellandgood.com/cold-shower-benefits-metabolism/.
7. Ouellet, V., et al., Brown adipose tissue oxidative metabolism contributes to energy expenditure during acute cold exposure in humans. J Clin Invest, 2012. 122(2): p. 545-52.
7. Søberg, S., et al., Altered brown fat thermoregulation and enhanced cold-induced thermogenesis in young, healthy, winter-swimming men. Cell Reports Medicine, 2021. 2(10): p. 100408.
The exposure of skin to cold, commonly referred to as cold shock, triggers the activation of brown adipose tissue. This process involves a pathway from the cold receptor located on the skin to the hypothalamus in the brain, which serves as the temperature-regulating center. The skin contains a significant number of cold receptors. TRPA1 and transient receptor potential melastatin 8 (TRPM8) play a crucial role in the perception of cool and cold sensations. TRPA1 and TRPM8 are two distinct ion channels that play a role in the perception of temperature sensation. Specifically, TRPA1 is responsible for detecting noxious cold stimuli, whereas TRPM8 is responsible for detecting cool and refreshing sensations, such as those associated with mint. When the body is rapidly exposed to cold stimuli, such as through immersion, there is a corresponding increase in neurotransmitter levels in the brain, including adrenaline, noradrenaline, and cortisol. The elevation of noradrenaline levels has the potential to promptly stimulate the activation of brown adipose tissue. The activation of brown adipose tissue is facilitated by potent activators, namely cold and noradrenaline. This activation is mediated by a direct pathway from the cold receptor to the BAT. The human body has a mechanism to regulate temperature and maintain homeostasis. This involves the development of a tissue that serves to keep the body warm in response to changes in the environment. This tissue is crucial in preventing hypothermia or hyperthermia, as it helps to maintain the body's optimal temperature balance. It appears that BAT is also stimulated by cutaneous warming. The human body possesses a mechanism akin to a temperature regulator. An additional mechanism involves the collaboration between muscle and BAT to regulate body temperature and mitigate the effects of cold water immersion .
Figure 4. Effect of Cold on Brown Adipose Tissue .
8. Park, B. and S.J. Kim, Cooling the Skin: Understanding a Specific Cutaneous Thermosensation. J Lifestyle Med, 2013. 3(2): p. 91-7.
9. Pinckard, K.M. and K.I. Stanford, The Heartwarming Effect of Brown Adipose Tissue. 2022. 102(1): p. 39-50.
The shivering physiological response is considered advantageous for the human body due to its ability to increase the metabolic rate, resulting in the consumption of calories. Thus, there is no need for individuals to have any concerns regarding shivering. The act of shivering is a biological reaction to exposure to cold temperatures, and it can serve the purpose of regulating body temperature, provided that it does not escalate to the point of hypothermia. The post-immersion or after-drop phenomenon is a regular occurrence in the human body, whereby the core body temperature continues to increase even after the individual has exited cold water. When an individual is exposed to cold water, the blood vessels undergo vasoconstriction as a means of regulating core temperature and safeguarding the warmth of essential organs. Following immersion in cold water, the process of vasodilation takes place, which facilitates the circulation of warm blood to the peripheral regions. This blood then undergoes cooling before being transported back to the core. The aforementioned procedure ultimately leads to a reduction in the central body temperature. There is no need for individuals to suppress shivering, as it can potentially function as a mode of physical conditioning for both the musculature and metabolic processes. This phenomenon has the potential to result in heightened insulin sensitivity. By considering shivering as a form of exercise training, albeit unconventional, it has the potential to elicit beneficial stress or homeostasis in cellular processes. Sustained exposure of muscle and brown adipose tissue cells to salubrious stressors, such as thermal fluctuations, can result in advantageous effects. Exposure to heat shock proteins and cold shock proteins has been reported to potentially augment cellular activation and foster general well-being. Exposure to these proteins results in enhanced cellular resilience, leading to improved capacity to endure subsequent stresses. The activation of multiple muscle groups during shivering can result in a substantial increase in metabolic rate and heat generation, reaching up to four to five times the standard rate. The main focus of Jacobs et al.'s research has been on the substrates that are employed by skeletal muscle to augment thermogenesis while shivering. Prior to roughly a decade ago, there was a scarcity of empirical evidence concerning this issue with regard to human subjects. As a result, a number of fundamental experiments were carried out with the aim of bridging this knowledge gap .
10. Jacobs, I. Muscle Metabolism and Shivering During Cold Stress. Available from: https://www.ncbi.nlm.nih.gov/books/NBK232868/#:~:text=Because%20so%20many%20muscle%20groups,rate%20and%20in%20heat%20production.
Individuals acclimatize to intentional exposure to cold, with each instance of cold exposure contributing to their acclimation. Repeated exposure to cold water leads to increased tolerance and comfort in cold environments. This adaptation occurs at a metabolic level, specifically through the activation of BAT. The mitochondria within brown adipose cells become more efficient at generating heat, resulting in improved thermogenesis. The capillaries present in the skin undergo an improvement in their constriction ability, thereby providing enhanced protection to the body against cold water. This adaptation enables the body to develop a mechanism that facilitates better tolerance to cold water exposure in subsequent instances. Consequently, the body undergoes changes that enable it to cope with cold water exposure more efficiently. Over time, the catecholamine surge will decrease, resulting in a reduction of the stress response. Due to the activation of muscles, there is a subsequent increase in metabolic rate, leading to improved insulin sensitivity. The findings of a study indicate that repetitive exposure to cold water while swimming can trigger metabolic changes, suggesting that adaptive metabolic mechanisms involving leptin and insulin may be at play. The aforementioned phenomenon was detected in physically fit females who were not classified as overweight and who participated in low-intensity physical activity in conjunction with exposure to cold temperatures .
11. Gibas-Dorna, M., et al., Variations in leptin and insulin levels within one swimming season in non-obese female cold water swimmers. Scandinavian Journal of Clinical and Laboratory Investigation, 2016. 76(6): p. 486-491.
The phenomenon of intentional exposure to cold temperatures has been the subject of scientific inquiry with regard to its potential impact on blood pressure regulation and general well-being. According to existing research, consistent and intentional exposure to cold temperatures has the potential to enhance the activity of the parasympathetic nervous system, thereby resulting in a range of health advantages. Several possible consequences may arise, including The reduction of blood pressure has been linked to cold exposure. The experience of cold exposure can result in the activation of the parasympathetic nervous system, which can lead to a reduction in heart rate, blood pressure, and respiratory rate. This physiological response can facilitate a feeling of relaxation and tranquility. Cold exposure has been suggested to have potential benefits on vascular health through the enhancement of blood flow and endothelial function. Exposure to low temperatures has the potential to trigger vasoconstriction, followed by vasodilation, which could potentially improve the functionality and integrity of blood vessels. Cold exposure has been demonstrated to elicit a response from the immune system, which may result in various advantages, including decreased inflammation, heightened immune reaction, and increased resilience against specific illnesses. Exposure to cold temperatures has been found to potentially enhance the generation of leukocytes and regulate the functioning of immune cells .
Figure 5. Cold exposure and stroke .
12. Komulainen, S., et al., Blood pressure responses to whole-body cold exposure: effect of carvedilol. Eur J Clin Pharmacol, 2000. 56(9-10): p. 637-42.
13. Chen, Z., et al., The underlying mechanisms of cold exposure-induced ischemic stroke. Science of The Total Environment, 2022. 834: p. 155514.
BAT, commonly referred to as brown fat, is a variety of adipose tissue that contributes to thermogenesis, the physiological mechanism of producing heat within the body. In contrast to white adipose tissue, which predominantly serves as an energy reservoir, BAT is specialized in thermogenesis through the process of calorie oxidation. The tissue in question exhibits a greater concentration of mitochondria and proteins that are rich in iron, thereby imparting a brown hue and facilitating thermogenesis. According to existing research, the activation of BAT and its prospective function in the regulation of body temperature may bear significance for metabolic well-being. The activation and recruitment of BAT cells can be stimulated by exposure to lower temperatures. Research has indicated that the activation of BAT cells may be facilitated by exposure to temperatures of approximately 66°F (19°C) for a minimum duration of 2 hours per day .
The potential involvement of brown adipose tissue in energy expenditure and weight regulation has garnered considerable interest. Research has indicated that individuals possessing elevated levels of brown adipose tissue exhibit a reduced body mass index (BMI) and potentially improved glucose and lipid metabolism. Notwithstanding, the precise mechanisms and impacts of brown adipose tissue on the overall metabolic processes and weight regulation are currently under investigation. Apart from its thermogenic function, brown adipose tissue has been discovered to have a regulatory function in maintaining body temperature when exposed to cold. The physiological response of thermogenesis is triggered in response to exposure of the body to cold conditions, thereby facilitating heat production and preservation of the body's core temperature. The significance of this phenomenon is particularly pronounced in neonates and young children, given their relatively greater abundance of BAT in comparison to mature individuals, which serves to facilitate thermoregulation.
It is noteworthy that although BAT has been extensively researched for its plausible metabolic advantages, the pragmatic ramifications of intentionally regulating or altering brown adipose tissue to manage blood pressure, in particular, remain inadequately established. Additional investigation is required to comprehensively comprehend the correlation among BAT, regulation of blood pressure, and holistic cardiovascular well-being. In general, the study of brown adipose tissue is a captivating field of research with potential ramifications for metabolic well-being and thermoregulation. Further research is needed to explore the precise impacts of intentional cold exposure on the regulation of blood pressure .
The research illustrates that there is a presence of active (BAT) in particular adipose depots among fully grown individuals. Nonetheless, acute cold exposure only stimulates less than 50% of the fat present in these depots, which suggests that their thermogenic potential has been undervalued. Additional investigation is required to enhance comprehension of the operational attributes of (BAT) and its ramifications for the management of obesity and metabolic disorders .
Figure 6. Mechanisms underlying insulin resistance in relation to white, beige, and brown adipocytes .
14. Marcin, A. Brown Fat: What You Should Know. 2022; Available from: https://www.healthline.com/health/brown-fat.
15. Brown Fat. Available from: https://my.clevelandclinic.org/health/body/24015-brown-fat.
16. Leitner, B.P., et al., Mapping of human brown adipose tissue in lean and obese young men. 2017. 114(32): p. 8649-8654.
17. Czech, M.P., Mechanisms of insulin resistance related to white, beige, and brown adipocytes. Molecular Metabolism, 2020. 34: p. 27-42.
The activation of BAT is triggered by temperature, particularly exposure to cold. This stressor is the most potent activator of brown fat due to its role as a temperature-regulating organ in the human body. Brown fat serves as the first responder to cold, while muscle tissue is comparatively slower to respond. As such, these two types of tissue play distinct roles in the body's response to temperature. Research has indicated that exposing oneself to cold water can significantly activate brown adipose tissue. Additionally, individuals who experience a change in skin temperature by venturing outdoors in T-shirts, engaging in outdoor work, or engaging in physical activity are likely to possess a greater amount of BAT. Activating BAT can lead to heightened metabolism and improved insulin sensitivity. The brown fats can be activated as soon as you just change the temperature in the skin, at the initial stage you don’t have to start with a cold shower or cold plunges if you are not ready for that yet but just exposing yourself to a wind has also shown to activate your brown fats. One may initiate the process of activating their BAT by sleeping in a cool environment with a temperature of 19 degrees Celsius, thereby promoting the growth of brown adipocytes. So you have more of it.
There is a study that shows that a group of subjects slept in a room at 24 degrees and then the researcher did a CT scan of these individuals to see how much brown fats they have from the beginning, what we called a baseline. Subsequent to a month of sleeping at a temperature of 19 degrees, the participants repeated the aforementioned activity. It was observed that there was an augmentation in insulin sensitivity. However, in the following month, the temperature was elevated to 24 degrees, resulting in a slight decline in insulin sensitivity. At a temperature of 27 degrees, which can be considered relatively warm, there was a decreased activation of brown adipose tissue and insulin sensitivity was observed. So it seems that you can expose yourself and the brown fats will rapidly respond to this because it’s so sensitive to noradrenaline, so if you keep exposing yourself to a little bit of cool, you also get a little bit adapted to it. Because brown fats have grown more mitochondria in these cells, that’s going to activate the cells and that’s going to take up glucose and fatty acid from the bloodstream to keep the thermogenesis up. And that’s going to clear up some sugar. BAT has the potential to reduce the amount of white adipose tissue, which is considered unhealthy. Excessive accumulation of white adipose tissue is undesirable. White fats are found in our thighs and also around our inner organs that’s where it's located. So if we can activation of brown fats just by going out in the cold or sleeping in a cold room. Maintaining a precise hemostatic equilibrium with respect to temperature was a complex process necessary for the preservation of human life.
An experiment was conducted by Dr. Susanna Søberg wherein she subjected herself to a cold water immersion of 4 degrees Celsius for a duration of 4 minutes. The objective of the experiment was to measure the activation of brown fats through the use of an infrared tomography camera, while also monitoring her blood pressure and heart rate. The activation of BAT is not readily observable due to its central location around the central nervous system and proximity to the skin surface beneath the clavicular bone. As a result, specialized and costly imaging equipment is typically required for measurement. In her study, Dr. Soberg also identified the depth at which BAT is situated beneath the epidermis .
Figure 7. Blood sugar, fat, and amino acids heat brown fat in cold weather. Inactive brown fat. Bottom: Cold activates brown fat, turning shoulders and neck orange .
18. How brown fat improves metabolism. 2019; Available from: https://www.nih.gov/news-events/nih-research-matters/how-brown-fat-improves-metabolism#:~:text=Brown%20fat%20breaks%20down%20blood,fat%20builds%20up%20in%20obesity.
According to a particular concept, it is advisable to don lighter clothing prior to the onset of winter and heavier attire in anticipation of summer. In Scandinavia, it is a common practice to put infants outdoors to sleep in cold temperatures. This is believed to enhance their resistance to cold weather and boost their immune system. This practice has been observed in Denmark as well. In Scandinavia, it is customary to expose infants to cold temperatures by placing them outside. Dr. Soberg, in a podcast, revealed that she also followed this practice with her two children. She further mentioned that her sons were habituated to sleeping outdoors during the winter months, starting from the ages of three, four, and five years. According to her statement, the consumption of the said product is beneficial for the enhancement of their immune system and development of resistance to colds, ultimately resulting in a reduction of the frequency of colds.
The prevalence of the ACTN3 gene is limited to a minority of individuals, with a mere 18% occurrence rate in the European population. The Scandinavian population exhibits a significantly lower proportion in relation to the overall European population. The gene that governs the expression of the ACTN3 protein, which is produced in fast-twitch muscle fibers, exerts regulatory control over it. The absence of the α-actinin-3 (XX) gene in an individual is associated with improved regulation of core body temperature in response to cold water exposure. This phenomenon is thought to be a result of changes in skeletal muscle thermogenesis .
19. Wyckelsma, V.L., et al., Loss of α-actinin-3 during human evolution provides superior cold resilience and muscle heat generation. Am J Hum Genet, 2021. 108(3): p. 446-457.
At first, Dr. Susanna Soberg's investigation into the effects of winter swimming may have been perceived as insignificant due to the initial skepticism surrounding the concept of how this activity could potentially activate BAT. The researcher initiated an investigation into the literature concerning the stimulation of BAT via exposure to cold water. However, no pertinent information was found apart from the mere act of immersing one's hand in pre-existing cold water. The investigators have reached a deduction that the stimulation of BAT exhibits a high degree of efficacy when subjected to cold water, yet it is significantly different from exposure to cold air. The finding was described as a novel by Dr. Soberg. Consequently, she opted to commence a conceptual inquiry at its outset, as it is hypothesized that winter swimming elicits stimulation of BAT. Nonetheless, she lacked knowledge of the factual occurrences that took place. The individuals in question entertained the idea that cold water and winter swimming may have the ability to stimulate metabolism. The person deliberately opted to explore this specific methodology. Although the sample size was limited, the study was able to effectively differentiate between the groups. The power analysis for the present investigation was performed on the positron emission tomography-computed tomography imaging of BAT. Consequently, there was a need to decrease the number of participants in order to enable a more comprehensive investigation of diverse mechanisms. The proof of concepts encompassed a diminution in magnitude whilst investigating diverse mechanisms. Furthermore, the researcher performed an analysis of the white adipose tissue to ascertain any noticeable differences between the groups before and after the intervention. The initiation of this project was initiated through a field study carried out by Dr. Soberg in its inaugural year.
The participant reported that they refrained from participating in winter swimming at the outset of this endeavor. Consequently, she commenced pondering the concept that if this practice is truly advantageous in theory, she should undertake an experiment with it. Dr. Soberg endeavored to engage in the aforementioned endeavor, yet it was not perceived as comical. The individual in question characterized the experience as an unpleasant one, akin to the sensation of muscular soreness that arises after a prolonged period of running following a hiatus, leading to discomfort on the subsequent day of physical activity. The sensation of cold exposure elicited discomfort, nevertheless, individuals tend to experience a heightened sense of well-being subsequently .
20. Søberg, S., et al., Altered brown fat thermoregulation and enhanced cold-induced thermogenesis in young, healthy, winter-swimming men. Cell Reports Medicine, 2021. 2(10).
Dr. Susanna Soberg selected male and female participants for her research and observed that there are variations in brown adipose tissue levels based on gender. Specifically, her findings indicated that females exhibit higher levels of BAT compared to males. According to research, women tend to have lower body temperatures than men, which can be measured through their hands, ears, and feet. Men possess larger hearts than women, allowing them to circulate a greater volume of blood throughout their bodies. There exists a discrepancy in their comfortable states between genders, whereby males tend to experience greater comfort at a temperature of 22 degrees Celsius, while females tend to experience optimal therm comfort at a temperature of 24 degrees Celsius.
Studies carried out on rodents have indicated that sex hormones can regulate the activity of BAT in a sex-specific manner, through both direct and indirect means. Estrogens have been found to stimulate the activity of BAT, thereby contributing to the beneficial metabolic outcomes observed in females. Based on the current body of evidence, it appears that androgens have a suppressive effect on BAT function. However, the specific effects of progesterone on BAT are not yet fully understood and require additional research. The interplay between sex hormones and glucocorticoids (GCs) plays a role in the regulatory mechanisms that govern the sexually dimorphic function of BAT. The aforementioned dialogue exchange serves as an illustration of how the perturbation of sexual hormones and GC levels, among other potential factors, can alter the influence of each of these hormones on BAT. Further investigation into the mechanisms that regulate the intracellular accessibility and responsiveness of hormones is imperative, as these variables could potentially contribute to the sex-specific sensitivity of BAT to glucocorticoids and sex hormones. The results of empirical studies conducted on human subjects demonstrate comparable outcomes. Nonetheless, further research is required to validate these findings. The analysis of BAT functionality within the framework of pathological conditions may promote a more thorough understanding .
Figure 8. illustrates the interplay between sex hormones and glucocorticoids in the regulation of BAT .
21. Kaikaew, K., A. Grefhorst, and J.A. Visser, Sex Differences in Brown Adipose Tissue Function: Sex Hormones, Glucocorticoids, and Their Crosstalk. 2021. 12.
The mammalian dive reflex, also known as the diving bradycardia or diving response, is a complex physiological reaction that is triggered in mammals, including humans, upon immersion in water. This response serves as a protective mechanism. The dive reflex was initially delineated by Edmund Goodwyn in 1786. Nevertheless, it was not until the publication of Paul Bert's work in 1870 that the physiological adaptations associated with this phenomenon were acknowledged. During aquatic immersion, the dive reflex is thought to facilitate the preservation of oxygen reserves in mammals by triggering a number of distinct physiological alterations. The act of submerging in the water while holding one's breath results in the wetness of the face and nose, leading to the occurrence of bradycardia, apnea, and increased peripheral vascular resistance. These three primary physiological changes are commonly known as the diving reflex. It is hypothesized that the escalation in peripheral resistance results in the redistribution of blood towards the crucial organs, while concurrently restricting oxygen utilization by non-essential muscular groups. Apart from vascular resistance, bradycardia is instigated as a means to reduce cardiac workload and curtail superfluous oxygen consumption. In general, the dive reflex is an inherent physiological response that involves multiple systems and is found in all vertebrates. Its primary purpose is to conserve oxygen reserves during periods of water submersion .
Submerging oneself in cold water can activate the diving reflex, resulting in a reduction in oxygen consumption and a deceleration of heart rate. This phenomenon occurs as a result of the body's effort to conserve oxygen, thereby delaying the onset of hypothermia. The human body possesses a vital survival mechanism that holds significant importance for its sustenance. Upon activation, there will be a brief window of approximately one to two minutes during which both the sympathetic and parasympathetic nervous systems will be fully activated. This activation will result in the release of serotonin in the brain, which is known to promote mental balance .
Figure 9. Diving response .
22. Freeman., D.G.A.M. Physiology, Diving Reflex. 2022; Available from: https://www.ncbi.nlm.nih.gov/books/NBK538245/.
23. Panneton, W.M. and Q. Gan, The Mammalian Diving Response: Inroads to Its Neural Control. 2020. 14.
24. Lemaître, F., The Trigeminocardiac Reflex: A Comparison with the Diving Reflex in Humans. 2015. p. 194-203.
The study participants in Dr. Susana Soberg's project engage in a cold water immersion activity, wherein they submerge themselves up to the neck in cold water for a duration of one to two minutes. This activity is performed two to three times per week, with each session consisting of three rounds of three dips and two sauna sessions. Notably, the participants begin and end each session with cold water immersion. The sauna is typically maintained at a temperature of 80 degrees Celsius, and individuals tend to remain inside for a duration of 10 to 15 minutes, contingent upon their frequency of attendance, which may be either twice or thrice weekly. According to the "Winter Swimming Book," the temperature of cold water in Denmark ranges from 12 degrees Celsius between October and April, with temperatures dropping to two to four degrees during the coldest months. All the individuals involved in this practice of alternating between cold saunas throughout the day. Exposure to cold temperatures can stimulate the sympathetic nervous system, resulting in an elevation of the body's stress response. This physiological effect may pose a challenge for individuals who struggle with falling asleep, even if they are experiencing significant fatigue .
25. How to use the sauna for recovery. Available from: https://trainwithmorpheus.com/how-to-use-the-sauna/#:~:text=Preheat%20the%20sauna%20to%20the,down%20for%202%2D3%20minutes.
The study revealed that nonshivering thermogenesis (NST) associated with BAT was observable during the morning hours, but not during the evening. This finding suggests that the morning hours may witness the higher activity of human BAT, which could potentially be linked to the correlation between skipping breakfast and obesity as well as other metabolic disorders .
In her research project, Dr. Susanna Soberg utilized both temperature measurement and PET-MRI imaging to assess brown fat activity. While PET-CT or PET-MRI scanning is prohibitively expensive for the general population, Dr. Soberg employed this method to obtain a continuous measure of brown fat activity in humans. In addition, she conducted research on the circadian rhythm. PET scanning was utilized to observe cold activation over a period of several hours. The objective was to determine whether activation occurred and whether brown fat was visible in the subject during both thermo-neutral and cold exposure conditions. Additionally, the study aimed to investigate the activation of brown fat in each group and assess the level of comfort experienced by participants at different temperature ranges throughout the study period. The study sought to determine whether brown fat activation occurred in participants who were completely thermally comfortable, as well as those exposed to cold conditions. Dr. Soberg devised a scale in her study, which required participants to rate their level of comfort on a scale of one to ten. The scale ranged from very cold (one) to extremely hot (ten). Additionally, the researcher conducted electromyography measurements of muscle activity to determine if individuals experienced shivering during the cooling period. It is worth noting that individuals may sometimes exhibit shivering before they are consciously aware of it. If an individual acclimates to cold water, they may experience reduced shivering intensity and frequency due to the increased density of mitochondria in muscle cells. This adaptation allows for efficient thermogenesis even with minimal shivering. In contrast, individuals who are new to cold water exposure must rely on the creation of mitochondria to generate heat and maintain warmth.
As per the findings of Dr. Soberg, winter swimmers exhibited reduced shivering or less intense shivering when they expressed feeling cold, and their perception of cold was relatively consistent across groups. Notably, muscle activation patterns differed between the control groups. According to Dr. Soberg's research, individuals who swim during the winter season exhibit enhanced insulin sensitivity and reduced insulin production throughout the experimental period. In addition to measuring brown fats on each cooling day, the study also involved cooling procedures. Additionally, the winter swimmer exhibited reduced insulin production and underwent a glucose tolerance test to assess their diabetic status. Notably, the winter swimmer demonstrated a more rapid glucose clearance in the bloodstream.
Figure 10. The regulation of brown adipose tissue through circadian rhythms .
26. Matsushita, M., et al., Diurnal variations of brown fat thermogenesis and fat oxidation in humans. International Journal of Obesity, 2021. 45(11): p. 2499-2505.
27. Straat, M.E., et al., Circadian control of brown adipose tissue. Biochimica et Biophysica Acta (BBA) - Molecular and Cell Biology of Lipids, 2021. 1866(8): p. 158961.
(18): 01:31:14 Tool: Minimum Threshold for Cold & Heat; Sauna & Cardiovascular Health
Dr. Susanna Soberg has identified a minimum threshold that is crucial for achieving favorable outcomes, including decreased blood sugar levels, improved clearance of blood sugar, reduced insulin levels, and enhanced distribution and density of brown fats. To attain these benefits through cold and heat exposure, individuals are required to spend a total of 11 minutes per week, distributed across two to three visits to the water and sauna. This translates to spending one to two minutes in cold water and 10 to 15 minutes in the sauna during each session. According to a recent study, spending up to 30 minutes in a sauna can potentially reduce the risk of cardiovascular disease. However, exceeding this threshold does not provide any additional health benefits. Hormetic stress refers to the phenomenon where cells are subjected to highly potent stressful conditions, resulting in an upregulation of heat shock proteins that facilitate cellular repair . However, excessive exposure to such stressors can negate the beneficial effects on the cardiovascular system, indicating that minimal exposure to heat or cold is sufficient to elicit these health benefits. It is noteworthy that engaging in extreme activities such as swimming in cold water for extended periods of time is not a requirement. Individuals may immerse themselves in cold water for a duration of one to two minutes per session, with a recommended maximum of 11 minutes per week. In regards to sauna usage, a total of 57 minutes over a two-week period is advised. Dividing this time over two to three days and two sessions per day would correspond to a low threshold of 10 to 15 minutes per session, which is considered beneficial .
Figure 11. The utilization of the sauna as a habitual practice for the enhancement of cardiovascular health .
28. Bujarrabal, A. and B. Schumacher, Hormesis running hot and cold. Cell Cycle, 2016. 15(24): p. 3335-3336.
29. Meyer, P., et al., Exposure to extreme cold lowers the ischemic threshold in coronary artery disease patients. Can J Cardiol, 2010. 26(2): p. e50-3.
30. Patrick, R.P. and T.L. Johnson, Sauna use as a lifestyle practice to extend healthspan. Experimental Gerontology, 2021. 154: p. 111509.
To ensure the preservation of stimulus perception during cold adaptation and to conduct shorter sessions, it is imperative to carefully deliberate on the timing and duration of cold exposure. It is possible that abbreviated periods of cold exposure may not elicit notable alterations in thermoreceptor function or sensory adaptation. Thus, it may be imperative to extend the duration of cold exposure in order to discern significant impacts on sensory perception. Additional investigation is required to examine the precise parameters and circumstances that enhance stimulus retention while undergoing cold adaptation in abbreviated sessions.
According to existing research, it appears that alterations in thermoreceptor activity as a result of acclimation to cold temperatures may necessitate extended periods of exposure. The notion of sensory adaptation offers a more comprehensive comprehension of the possible mechanisms implicated in sustaining stimulus perception while undergoing cold adaptation. Further investigation is required to specifically examine the preservation of stimulus perception during cold adaptation and shorter sessions. Additional research in this field will enhance our comprehension of sensory reactions in frigid settings and facilitate the refinement of approaches aimed at preserving sensory acuity across various cold adaptation scenarios .
31. Scheuren, P.S., et al., Cold evoked potentials elicited by rapid cooling of the skin in young and elderly healthy individuals. Scientific Reports, 2022. 12(1): p. 4137.
Dr. Susanna Soberg conducted a study on participants who underwent deliberate cold exposure. The study revealed that the participants experienced enhanced sleep quality, efficient awakening, and comfort in cold temperatures. These findings were consistent among both winter and summer swimmers. Additionally, the participants displayed reduced social anxiety and were less apprehensive about exposing their skin.
Dr. Soberg has allocated a portion of her book to the topic of naked winter swimming or naked cold water exposure. This practice, which involves swimming in freezing waters without wearing a bathing suit, is gaining popularity in certain regions. It is an experience that some individuals choose to embrace. Engaging in this activity is perceived as a daring and audacious endeavor that pushes individuals beyond their customary boundaries and facilitates a connection with the natural world and their personal fortitude. Nonetheless, participation in winter swimming in the nude should be undertaken with care, as it is imperative to take appropriate safety precautions and make necessary preparations to minimize potential hazards linked to cold water exposure.
Although naked cold water swimming has been linked to potential advantages such as heightened BAT, enhanced mental well-being, and cardiovascular responses, further investigation is necessary to comprehensively comprehend its particular health benefits and individual variances. As with any physical endeavor, it is crucial to take into account individual health conditions, seek advice from a medical expert if necessary, and adhere to safety protocols for swimming in cold water settings .
Figure 12. The impact of the depth of cold water immersion on sleep .
Dr. Susanna Soberg did not mention that one of the participants in the winter swimming activity lacked brown adipose tissue, as per the user's statement. Additionally, prior research has indicated that certain individuals may not possess BAT. As per Dr. Soberg's account, in the cold experiment, the participant was subjected to a cooling procedure for a duration of two hours prior to undergoing PET-CT scanning. The individual in question exhibited a lack of ability to regulate their shivering response, similar to other individuals who engage in winter swimming. This resulted in a heightened susceptibility to cold temperatures, as evidenced by their tendency to shiver when temperatures were lowered. The protocols implemented were standardized across all participants, who were of equivalent size and gender. The scanning and experimentation procedures are deemed appropriate, accurate, and valid. The individual in question exhibited a lack of brown fat, resulting in early-onset shivering and poor temperature regulation. Additionally, their blood samples closely resembled those of the control group, with slightly elevated insulin levels compared to other swimmers. Furthermore, their blood glucose clearance was comparatively slower than that of other winter swimmers. The findings suggest that the activation of BAT is more effective in individuals who engage in winter swimming, despite the absence of the participant in that particular cohort. This did not compromise the integrity of the study.
The mechanism underlying the stimulation of heat production by fat cells in response to muscle tissue activity, a phenomenon known as muscle-fat crosstalk, has garnered significant attention from researchers due to the current lack of clarity surrounding this process. The article highlights the role of the protein irisin in this specific interaction. Irisin has the ability to induce white adipocytes, which are responsible for energy storage, to acquire the characteristics of brown adipocytes, resulting in increased thermogenesis and caloric consumption. The exploration of the activation and functionality of BAT is a topic of keen interest in the pursuit of potential treatments for metabolic disorders and obesity. Scholars strive to devise innovative methodologies for weight regulation and enhancement of metabolic well-being by investigating strategies to produce supplementary BAT or enhance its efficacy. Studies have indicated that the activation of brown adipose tissue can contribute to the maintenance of glucose homeostasis and thermoregulation in the human body. The publication by the National Institutes of Health underscores the observation that shivering triggers the activation of BAT cells, resulting in thermogenesis and energy dissipation. The process of converting white adipocytes, which are responsible for energy storage, into brown adipocytes, which are responsible for energy expenditure, is facilitated by the protein irisin, which mediates muscle-fat crosstalk. Future research endeavors in this field have the potential to yield substantial implications for understanding and developing interventions for obesity and metabolic disorders .
34. Shivering Triggers Brown Fat to Produce Heat and Burn Calories. 2014; Available from: https://www.nih.gov/news-events/nih-research-matters/shivering-triggers-brown-fat-produce-heat-burn-calories.
Intentional exposure to cold temperatures has been suggested to have potential benefits for reducing inflammation. Dr. Soberg conducted a study examining various anti-inflammatory markers, including IL-6 and IL-10. The results indicated an increase in both IL-6 and IL-10. Exposure to cold and heat has been found to reduce inflammation in the body, thereby potentially mitigating the onset of lifestyle diseases such as type 2 diabetes, as well as certain mental illnesses such as depression, anxiety, and Alzheimer's disease. The literature indicates that there is a positive correlation between inflammation and the likelihood of developing Alzheimer's disease, as well as experiencing symptoms of depression and anxiety. Reducing inflammation in the body has the potential to mitigate the prevalence of contemporary lifestyle ailments. Exposure to extreme temperatures, whether cold or hot, can induce a natural response in the body that resets its homeostatic balance. This response allows the body to repair itself and restore equilibrium.
A meta-analysis was conducted to examine the efficacy of different cold and heat modalities, along with passive recovery, for alleviating pain resulting from delayed onset muscle soreness (DOMS). The results of the study indicate that hot packs may have greater efficacy and stability in mitigating pain compared to alternative interventions within a 48-hour timeframe. These findings suggest that hot packs may hold promise for clinical implementation. Cryotherapy (CRYO) was deemed as the primary option for pain management that extends beyond 48 hours, whereas pulsed cryo-compression (PCM) was regarded as a less favorable alternative. Both PCM and CRYO have demonstrated efficacy in alleviating symptoms of delayed onset muscle soreness (DOMS) in both professional athletes and non-athletic individuals. However, additional research is necessary to fully elucidate the potential side effects and adverse reactions associated with these treatments. Cold water therapy (CWT) could potentially serve as a straightforward and secure substitute in comparison to cryotherapy (CRYO) and phase change material (PCM). Further comprehensive investigation and rigorous randomized controlled trials are necessary to explore the precise impact of hot packs in mitigating pain associated with delayed onset muscle soreness (DOMS) and to establish definitive conclusions .
Figure 13. Ice or heat? How to treat your injury .
35. Wang, Y., et al., Effect of cold and heat therapies on pain relief in patients with delayed onset muscle soreness: A network meta-analysis. J Rehabil Med, 2022. 54: p. jrm00258.
36. Ice or heat? How to treat your injury. Available from: https://complete-physio.co.uk/ice-or-heat-how-to-treat-your-injury/.
Dr. Susanna Søeberg formulated the Søeberg Principle, which posits that to optimize the metabolic advantages of cold exposure, individuals should permit their bodies to naturally reheat after exposure. Dr. Søeberg, a researcher who deliberately subjects herself to cold conditions, proposed this principle. As an alternative, it is possible to conclude with the expression "Conclude with a dispassionate statement." In addition, allowing for the physiological reaction of shivering has the potential to enhance the metabolic increase caused by being exposed to low temperatures. The physiological response of shivering stimulates the release of succinate from skeletal muscles, which subsequently initiates the activation of thermogenesis in brown adipose tissue. Exposure to cold temperatures elicits a physiological response in which the body initiates endogenous heat production, resulting in the activation of BAT and muscle thermogenesis. This process also sustains the activation of neurotransmitters.
It is recommended to implement the following protocol in order to improve the shivering response, either simultaneously or immediately after exposure to cold stimuli: It is not recommended to assume a huddled or crossed-arm posture when subjected to cold temperatures or following exposure to a cold environment. Moreover, it is advisable to abstain from utilizing a towel for the purpose of drying one's body. Facilitate the inherent physiological mechanisms of thermoregulation and desiccation within your organism. Undoubtedly, this poses a formidable predicament. Apart from deliberate cold exposure in warm weather, it is widely recognized that individuals tend to favor taking a warm shower and using a towel to dry themselves after being exposed to cold temperatures. Nonetheless, this practice has the potential to limit the metabolic effects .
37. Huberman, A. Shivering and The Søeberg Principle. 2023; Available from: https://hubermanlab.com/the-science-and-use-of-cold-exposure-for-health-and-performance/#:~:text=Shivering%20and%20The%20S%C3%B8eberg%20Principle,enhance%20metabolic%20increases%20from%20cold.
The impact of cold exposure on the human body can vary depending on the individual's nutritional state, specifically whether they are in a fed or fasted state. Exposure to cold can lead to an increase in energy expenditure, whole-body glucose utilization, and fatty acid utilization in individuals who are in good health. According to a study, frequent exposure to cold temperatures can lead to a reduction in fasting glucose and insulin levels, as well as an improvement in the handling of dietary fatty acids, even among individuals who are considered healthy .
The present study examined the metabolic reactions to cold exposure in the context of fasting. Specifically, alterations in 24-hour energy expenditure (EE) and sleeping metabolic rate (SLEEP) were assessed in participants undergoing a 24-hour fast at both thermoneutral (24°C) and mildly cold (19°C) temperatures. The findings indicate that the metabolic response to mild cold exposure may bear resemblance to the metabolic characteristics observed during instances of acute fasting .
Furthermore, with regards to the correlation between fasting and cold exposure, a separate investigation carried out on cold-acclimated toads revealed that fasting under cold conditions resulted in a modification of the metabolic response, exhibiting a tendency towards reduced state 4 respiration in comparison to the fed state . In general, the impact of cold exposure on individuals, regardless of their feeding status, can have an impact on various physiological processes such as energy expenditure, glucose utilization, fatty acid metabolism, and metabolic responses. Additional investigation is required to comprehensively comprehend the distinct mechanisms and physiological adaptations linked with exposure to cold temperatures in both fed and fasted conditions.
Figure 14. The practice of fasting has been found to improve the ability of fish to withstand cold temperatures .
38. Ivanova, Y.M. and D.P. Blondin, Examining the benefits of cold exposure as a therapeutic strategy for obesity and type 2 diabetes. J Appl Physiol (1985), 2021. 130(5): p. 1448-1459.
39. Hollstein, T., et al., Metabolic Responses to 24-Hour Fasting and Mild Cold Exposure in Overweight Individuals Are Correlated and Accompanied by Changes in FGF21 Concentration. Diabetes, 2020. 69(7): p. 1382-1388.
40. Trzcionka, M., et al., The effects of fasting and cold exposure on metabolic rate and mitochondrial proton leak in liver and skeletal muscle of an amphibian, the cane toad Bufo marinus. Journal of Experimental Biology, 2008. 211(12): p. 1911-1918.
41. Lu, D., et al., Fasting enhances cold resistance in fish through stimulating lipid catabolism and autophagy. The Journal of Physiology, 2019. 597.
Raynaud's syndrome, which is also referred to as Raynaud's phenomenon, is a pathological state that is distinguished by the contraction of minor arteries, leading to occurrences of diminished blood circulation to the end arterioles. The condition was designated in honor of Auguste Gabriel Maurice Raynaud, a medical practitioner who initially documented it in his doctoral dissertation in 1862. According to the literature, Raynaud's syndrome predominantly impacts the digits of the upper extremities, with a lesser incidence of involvement in the lower extremities. Occasionally, there may be instances where the nose, ears, or lips are also impacted. The aforementioned ailment is frequently instigated by exposure to low temperatures or psychological strain, leading to specific regions of the body experiencing sensations of numbness and coldness. In the event of vasospasm, constriction occurs in the smaller arteries that are responsible for delivering blood to the skin, resulting in restricted blood circulation to the impacted regions .
The intentional exposure to cold temperatures may not yield advantageous outcomes for individuals diagnosed with Raynaud's phenomenon. Raynaud's syndrome is typified by an amplified vascular reaction to cold temperature or stress, whereby the blood vessels in the impacted regions constrict excessively, resulting in diminished blood flow and indications such as numbness, pain, and alterations in coloration in the digits. Although deliberate cold exposure training is occasionally recommended as a means of enhancing cold tolerance, it is crucial to exercise prudence when considering this approach for individuals with Raynaud's syndrome. The condition known as Raynaud's syndrome is characterized by an anomalous reaction of the blood vessels to cold stimuli. Deliberately exposing the affected regions to cold temperatures may exacerbate symptoms and potentially incite an episode of vasospasm. Individuals diagnosed with Raynaud's syndrome are typically advised to steer clear of cold temperatures and adopt strategies to maintain warmth in their extremities, such as donning gloves, socks, and multiple layers of clothing, particularly in regions with colder climates. The maintenance of optimal circulation and stress management can yield advantageous outcomes. Seeking consultation with a healthcare professional is crucial in obtaining personalized advice and guidance on the management of Raynaud's syndrome. This includes determining the suitability of deliberate cold exposure or other interventions for individual cases  .
42. Raynaud syndrome. Available from: https://en.wikipedia.org/wiki/Raynaud_syndrome.
43. Raynaud's disease. Available from: https://www.mayoclinic.org/diseases-conditions/raynauds-disease/symptoms-causes/syc-20363571.
44. Delp, H.L. and R.A. Newton, Effects of brief cold exposure on finger dexterity and sensibility in subjects with Raynaud's phenomenon. Phys Ther, 1986. 66(4): p. 503-7.
A cold stimulus headache is a form of headache that can be triggered by exposure to low temperatures. Cold stimulus headache is characterized by the onset of a headache following the ingestion of a cold substance or exposure of the head to a cold environment without protection.
Research has indicated that immersion in cold water up to the neck at a temperature of zero degrees Celsius results in a reduction of cerebral blood flow by approximately 30 to 40 percent. The activation of the parasympathetic nervous system results in a reduction of cerebral blood flow. This physiological response is essential for maintaining adequate blood flow to the heart and ensuring survival. One potential strategy to increase cerebral blood flow is to keep the heart above water level. An additional research study has demonstrated that immersing oneself in water up to the neck results in a heat loss of 11 from the body's core. However, submerging one's head in water increases heat loss by 36 percent. This heightened heat loss rate from the core leads to a greater after-drop, which brings one closer to the onset of hypothermia compared to solely immersing up to the neck. It is recommended to cover the head during intentional cold exposure in order to minimize heat loss .
Tanik, N., H. Saçmaci, and T. Aktürk, The relationship between exposure to hot/cold weather and the clinical features of headaches in patients with migraine and tension-type headaches. Neurol Res, 2020. 42(3): p. 239-243.
Children are smaller in body size and in many features than adults so we cannot apply all the protocols on them. There is a study where they compare heat loss in children boys who are 12 years old and compared to adult men and studied heat loss of core temperature and exposing them to one or two minutes of cold exposure immersion up to the neck, they found that the boys in this study could actually defend the core temperature in the same way as an adult could but they have to use their muscle too faster it means that they couldn’t stay for long and they used more energy to defend their core temperature compared to the adult for one minute it seems that they could actually, but they could be colder when they come out because they are smaller in their mass.
The intentional exposure to cold temperatures poses an inherent risk of hypothermia, a medical condition that is characterized by an abnormally low body temperature. Hypothermia is a physiological condition characterized by a reduction in the core body temperature below the normal range, resulting from a rate of heat loss that exceeds the rate of heat production. Although deliberate cold exposure may offer potential advantages when executed in a secure and regulated environment, it is crucial to acknowledge the correlated hazards and adopt essential safety measures. Intentional exposure to cold, such as engaging in cold water swimming or participating in outdoor winter activities, has been found to elevate the likelihood of hypothermia. According to scholarly sources, risk factors associated with hypothermia encompass conditions such as exhaustion, advanced age, and medical ailments that impede the body's capacity to regulate temperature. Hypothermia is a critical medical condition that, if not addressed, can result in cardiac arrest and fatality. It is imperative to undertake essential measures, such as donning suitable attire and progressively acclimatizing oneself to low temperatures, while deliberately exposing oneself to cold conditions, in order to mitigate the likelihood of hypothermia .
Figure 15. The adverse effects of hypothermia on human health .
46. Hypothermia. Available from: https://www.mayoclinic.org/diseases-conditions/hypothermia/symptoms-causes/syc-20352682.
47. Kraft, S. Everything you need to know about hypothermia. 2023; Available from: https://www.medicalnewstoday.com/articles/182197.
There exists a discernible temperature differential between males and females, whereby the latter is more susceptible to cold exposure due to their heightened metabolic activity in BAT, as well as their greater abundance of said tissue. It has been observed that women engage in cold exposure through an 11-minute protocol. However, it is suggested that a distinct protocol for deliberate cold exposure in women may be necessary.
When discussing the topic of ice swimming and the distance one can endure in cold water without experiencing hypothermia, gender differences may arise. However, brief exposure to cold temperatures, also known as micro-stressing, can induce hermetic stress in the body. This effect is observed regardless of gender, as the duration of exposure is relatively short .
48. Kaikaew, K., et al., Sex difference in cold perception and shivering onset upon gradual cold exposure. Journal of Thermal Biology, 2018. 77: p. 137-144.
Brief exposure to cold followed by re-entry can enhance physical resilience by prompting the body to adapt to fluctuating temperatures. This practice can be achieved by alternating between cold water immersion and sauna use, or by adjusting the temperature of a cold plunge. Denmark experiences four distinct seasons, each characterized by varying temperatures. Exposure to a cold plunge followed by a change in temperature induces hermetic stress, prompting the body to adapt to the perceived toxicity of the temperature shift.
The circadian rhythm pertains to the endogenous biological clock of the body that governs diverse physiological functions, such as metabolic processes and sleep-wake patterns. Intentional exposure to cold stimuli has the potential to impact the circadian rhythm and thermoregulation mechanisms within the human body. Research has demonstrated that prolonged exposure to cold temperatures can result in a modification of liver metabolism and harmonization of transcriptional patterns of genes linked to metabolism in mice . The aforementioned observation implies that extended periods of exposure to low temperatures may have an impact on the hepatic metabolism's circadian rhythm. Cold exposure primarily impacts the latter portion of the sleep cycle, during which the preponderance of rapid eye movement (REM) sleep occurs. The impact of cold exposure on sleep patterns in semi-nude individuals is primarily observed in the REM sleep stage, as the thermoregulatory response is suppressed. Conversely, the slow wave sleep (SWS) stage is less impacted by cold exposure. This finding is supported by previous research. Conversely, exposure to heat has the potential to augment alertness and diminish both slow-wave sleep (SWS) and rapid eye movement (REM) sleep, particularly in instances where bedding and clothing are utilized. The results of this study indicate that intentional exposure to cold temperatures may have an effect on the body's natural sleep-wake cycle and the various stages of sleep, with a particular emphasis on the impact of cold exposure on the rapid eye movement (REM) phase of sleep. It is noteworthy that the aforementioned studies were centered on particular circumstances and animal models. Additional investigation is required to comprehensively comprehend the exact mechanisms and impacts of intentional exposure to cold on the circadian rhythm and temperature regulation in the human body .
49. Zhang, Z., et al., Chronic Cold Exposure Leads to Daytime Preference in the Circadian Expression of Hepatic Metabolic Genes. 2022. 13.
50. Okamoto-Mizuno, K. and K. Mizuno, Effects of thermal environment on sleep and circadian rhythm. J Physiol Anthropol, 2012. 31(1): p. 14.