The effects of cold exposure and heat exposure

Introduction

Both heat exposure and cold exposure have both similar and different advantages, as well as disadvantages, in this, I discuss these, so you can decide if either or both of these therapies should be performed by yourself. I put information into different sections, and there is crossover amongst them, so I recommend reading all of them, so you don’t miss out on any valuable information.
 

Cold exposure is optimally performed by doing cold water immersion (such as in an ice bath) or by using whole-body cryotherapy. Cold showers and spending time outside in the cold probably won’t host the same benefits but they can host some. Cold water immersion should be at around 10-15°C/50-59°F and whole-body cryotherapy should be at around -178- -135°C/-289- -211°F. 

Heat exposure is optimally performed in a sauna at around 70°C/158°F at a humidity of about 10-20%, but it can also be performed in a hot bath or jacuzzi at around 40°C/104°F this probably doesn’t have as beneficial of an impact as a sauna. If you do choose the hot bath or jacuzzi option make sure to keep yourself emerged up to your shoulders for the most benefits. A hot shower is an option but it probably is nowhere near as beneficial as a sauna, bath or jacuzzi. Heat exposure can be performed and perhaps is optimally performed directly after exercise.

All the temperatures mentioned in this article are rounded to the nearest whole number.

Norepinephrine release from cold exposure

During cold exposure, norepinephrine is produced by the central nervous system and adrenal glands. Immersion of 14°C/57°F water increased norepinephrine by 530%, dopamine by 250% (which plays a vital role in memory, movement, motivation, mood and attention) and energy expenditure by 350% as compared to pre-immersion levels. It appears that there is a dose-dependent effect with more exposure to cold water the more norepinephrine produced, there are similar impacts with cryotherapy and even being outside in cold temperatures, however, norepinephrine levels drop over the hour following exposure, before returning to baseline. Heat exposure does not seem to stimulate norepinephrine release.

When acting as a hormone norepinephrine causes your blood vessels to constrict to conserve heat, but we can also create heat through thermogenesis, there are 2 types of thermogenesis; shivering thermogenesis which occurs when muscles contract and use up ATP for energy and therefore dissipate some energy as heat and nonshivering thermogenesis which occurs when norepinephrine acts on proteins in the electron transport chain, in the mitochondria, this disconnects the normal electrical process to create chemical energy and instead creates heat directly, but this only occurs in beige and brown fat, which have more mitochondria than white fat (more on this later). Norepinephrine may also decrease pain, due to its ability to decrease inflammation through vasoconstriction (the constriction and therefore narrowing of blood vessels) inhibiting blood flow to the inflamed area.

Norepinephrine can also function as a neurotransmitter. Animal studies have revealed that chronic cold exposure promotes norepinephrine release from the locus coeruleus region of the brain. In the brain, norepinephrine is involved in vigilance, focus, attention and mood. Low levels of norepinephrine are associated with decreased attention, cognitive ability, energy and mood, it can also lead to depression and this is why both ADHD and depression are sometimes treated with norepinephrine reuptake inhibitors, which block the reabsorption of norepinephrine, increasing norepinephrine levels, however, this comes with risks and norepinephrine increase through cold exposure usually comes with fewer risks.
 

Norepinephrine activates PGC-1α, a key regulator of genes involved in the energy metabolism of glucose and fatty acids, muscle fibre remodelling which is when damaged cellular components are replaced by new cellular components, the thermoregulatory function which aims to keep the core body temperature at 37°C/99°F and mitochondrial biogenesis which is when cells increase the number of mitochondria, which helps increase aerobic capacity and lower the risk of Alzheimer's, Parkinson's and type 2 diabetes.
 

Norepinephrine is also responsible for increasing your heart rate, perhaps as an adaption to cope with the cold and modifying immune function (more on this later), it also increases the activity of brown and beige fat, which is involved in nonshivering thermogenesis.

The release of norepinephrine does not seem to change with colder temperatures, so if you're already at the optimal temperature for the largest norepinephrine release, there is no need to make the exposure colder as you get more accustomed to the cold temperature if your goal is the largest norepinephrine release possible. Interestingly, going from a sauna to cold water causes norepinephrine to increase even more than cold exposure alone, but perform this with caution if at all.

Norepinephrine release in the brain activates the cold shock protein RBM3 (more on this in the next section).

RBM3 release from cold exposure

Synapse loss occurs with normal brain ageing and is accelerated with neurodegenerative diseases such as Alzheimer’s and Parkinson's, cold exposure increases RBM3 levels through norepinephrine (discussed in the previous section), which may be responsible for synaptic regeneration as backed by animal studies as RBM3 binds to RNA to increase protein synthesis and stimulates the regeneration of damaged neurons. RBM3 may also aid in preventing neuronal death. The levels of RBM3 increase and may stay elevated after repeated application of cold exposure.

A study of mice with Alzheimer's disease revealed that they lost their ability to increase levels of RBM3 and therefore lost their ability to reassemble synapses. Increasing levels of RBM3 in the mice promoted sustained synaptic protection, prevented behavioural deficits and neuronal loss as well as prolonged survival.

It is still not clear the degree to which RBM3 can be activated by cold exposure in humans and its potential to protect against neurodegeneration in humans.

Immune system adaptions from cold exposure

Cold exposure helps to boost the immune system by decreasing inflammation, inducing cell death, and increasing T cell activity, which can kill cancer cells, these remain elevated at 2 hours after the exposure. Cold exposure also helps the immune system by increasing the number of lymphocytes, which are responsible for fighting cancer, viruses and bacteria and also increasing myeloid cells, important for phagocytosis (the cellular process for ingesting and eliminating fairly large particles) and for inflammatory cytokines (even though cold exposure will usually lead to a net decrease in inflammation). Cold exposure over a longer period can increase CD25 lymphocytes, important for T cell proliferation, activation-induced cell death (the process by which cells undergo apoptosis in a controlled manner through the interaction of a death factor and its receptor) and activating regulatory T cells which act to suppress the immune response, thereby maintaining homeostasis and self-tolerance and activating effector T cells which help the immune response to perform its functions, It also increases over a longer period of time CD14 monocytes, important for the inflammatory response.

Cold exposure also boosts the innate immune system which has an important role in barrier defence to help stop an infection from occurring, enhance other immune cells and cause inflammation when immune cells detect an infection or tissue injury.

Microbiome adaptions from cold exposure

The microbiome is the variety of microorganisms residing within or on the human body. It has been revealed that cold exposure changes the microbiome in the gut of mice in order to support the activation of nonshivering thermogenesis by increasing the uptake of carbohydrates and lipoprotein-derived triglycerides, a lack of adaption to the gut microbiome causes impaired nonshivering thermogenesis and insulin sensitivity. It is not yet sure if the human body has similar adaptions, but this is a good indication.

Antioxidant enzymes activation from cold exposure

A byproduct of energy metabolism is reactive oxygen species which can promote muscle damage, fatigue, immune dysfunction, DNA damage and cellular senescence (when a cell has stopped dividing but is not dead), cold exposure activates antioxidant enzymes which may help deal with the damage caused by reactive oxygen species.

Mitochondrial biogenesis increase from cold exposure

Mitochondrial biogenesis (increase in mitochondria) is regulated by PGC-1α and norepinephrine, these are increased via cold exposure. Cold exposure also increases levels of MAPK and AMPK, which are proteins involved in cellular signalling of mitochondrial biogenesis, it also increases mitochondrial respiratory complex proteins, which convert energy from food into a useful form as ATP. Exercise, heat shock and ketones activate mitochondrial biogenesis.

There are 3 different types of fat; white fat stores excess lipids in the form of triglycerides and release fatty acids for energy; beige fat is inside white fat tissue stores, they can adopt storage or thermogenic properties based on their environmental conditions, this is the most predominant type of fat; brown fat has a purpose mostly for thermogenesis.

The increase in mitochondria can occur in muscle tissue or adipose tissue, as an adaptation to the cold by increasing respiration and therefore increasing body temperature. Mitochondrial biogenesis in muscles improves aerobic capacity and endurance. The increase in mitochondria inside fat leads to the darkening of fat under an electron microscope, this increases the quantity of brown fat, named after its colour under an electron microscope. It was once believed that brown fat was only present in newborns for protection, it has now been revealed that adults can also have active brown fat, typically following cold exposure. The mitochondria can undergo respiration quickly to heat you up and you can lose fat in this process, which may lower the risk of health issues and give people a more aesthetic physique for bodybuilding or just personal goals. A study of young men who exposed themselves to the cold for two hours per day for 20 days found that brown fat volume increased by 45% and cold-induced brown fat metabolism increased by more than 200%. However cold exposure won’t make a significant impact on weight loss, and it shouldn’t replace a good diet.
 

A study of young men revealed that cold exposure increased resting energy expenditure by 15%, only if there was detectable brown fat present, this increase was primarily due to the oxidation of plasma-derived fatty acids (70%) and glucose (30%), this is due to better glucose and insulin sensitivity from the browning of fat. So this reveals the benefits of cold exposure for weight loss. In humans, the quantity of brown fat typically drops with an increase in body fat, so to gain the full benefits of fat browning try to stay at a healthy body fat percentage for you, which is usually between 8-19% for men, and 21-32% for women.
 

Brown fat and Beige fat only in certain environments are responsible for nonshivering thermogenesis (as discussed earlier) as well as skeletal muscle. In people with type 2 diabetes, nonshivering thermogenesis may be an effective method for weight loss, in order to improve metabolic health, also the fact that cold exposure causes glucose uptake, particularly in brown fat, which exceeds the insulin-stimulated uptake rate in skeletal muscle and therefore increases insulin sensitivity which will help further with metabolic disorders. Even though brown fat volume and metabolic activity can increase in people with type 2 diabetes, they are still lower than in people with healthy bodies. A study of men with obesity revealed that brown fat volume leads to more fat metabolism and oxidation, for energy, which is aided by cold-induced lipolysis (the release of fatty acids from fat stores) and free fatty acid oxidation, to form ATP in the mitochondria of cells.

Heat/cold shock proteins

Heat shock proteins are activated by heat, and they stay active for about 48 hours. A study revealed that being in a sauna at 73°C/163°F for 30 minutes increased heat shock protein levels by 50% over baseline on average, a similar study with hot bath emersion from the waste down at 40°C/104°F for 60 minutes revealed a 40% increase in heat shock protein levels over baseline on average, suggesting that a hot bath can nearly host the same benefits as a sauna.

Proteins are often damaged, from simple things such as inhaling air pollution and the foods we eat that the body uses for energy, these create reactive byproducts that make proteins in cells misfold, they then stay in the cell for longer than desired and aggregate with other misfolded proteins and take up essential space in the cell, which may lead to neurodegenerative diseases and cardiovascular diseases. Heat shock proteins repair the damage to the proteins and make them fold back into their normal structure, so the protein doesn’t form aggregates. Heat shock proteins largely help in preventing muscle atrophy, as it helps prevent proteins from being degraded. Heat shock proteins can help protect against the formation of amyloid-beta plaques which helps prevent Alzheimer’s disease, as these plaques can accumulate and clump in the synapse of nerve cells.

In a study, a fly was exposed to heat for 70 minutes, and its lifespan increased by 15%, this is due to heat shock proteins. Interestingly, Some humans have a variation in the gene which creates heat shock proteins which makes them active all the time, which causes them to have exceptional longevity. Interestingly cold temperatures also activate heat shock proteins, but not as much as hot temperatures.

Cold shock proteins are induced by cellular stressors, such as cold exposure. The cold shock protein cold-inducible RNA binding protein promotes cell survival, activates antioxidant enzymes under conditions of mild hypothermia (a 35°C/95°F core body temperature) and also activates RBM3 which is beneficial to the body (as discussed earlier).

Inflammation

It appears that cold exposure leads to a large decrease in inflammation for people with inflammatory conditions or those who have just performed exercise due to vasoconstriction of blood vessels reducing blood flow to the area, but there is also an increase in CD14 monocytes important for the inflammatory response and to boost the innate immune system which causes inflammation when immune cells detect an infection or tissue injury, but this inflammatory response from cold exposure is usually necessary in the case of healing tissues, so it has a more beneficial impact. C-reactive protein rises in response to inflammation, a study of runners revealed that those who underwent passive recovery (just rested without any therapies and without putting any extra strain on their body) had a  515% increase in C-reactive protein 24 hours after running, whilst there was only a 123% increase in C-reactive protein in those who used whole body cryotherapy, and their levels of C-reactive protein returned to baseline just 72 hours after running but remained elevated in those performing passive recovery.

Cold exposure can be particularly beneficial for those with arthritis as cold exposure decreases pro-inflammatory signalling molecules whilst increasing anti-inflammatory molecules leading to a decrease in inflammation and therefore pain, this is about as beneficial for recovery as a standard rehabilitation programme for arthritis.

It appears that heat exposure usually leads to a decrease in inflammation but to a lesser degree, In heat exposure, there is an increase in anti-inflammatory cytokines, a decrease in pro-inflammatory cytokines, activation of FOXO3 which stops senescent cells from occurring and producing pro-inflammatory molecules, an increase in the  IL-6 myokine, released from muscles, which can cause inflammation or cause a powerful anti-inflammatory response, NRF2 activation which recruits anti-inflammatory cells and antioxidant genes but there is also vasodilation (the dilation and therefore widening of blood vessels) increasing blood flow to the injured area and an increase in myeloid cells which increase inflammatory cytokines, but overall there will usually be a net decrease in inflammation and an increase in muscle repair for those who haven’t recently performed weight training.

The following information in this section may not be important to you but I speak on inflammation a lot in this article and I like my work to be understandable to people of many educational levels, so no questions will need to be answered once you have completed reading the article.

Tissue-resident immune cells (such as macrophages) encounter an inflammatory stimulus (such as a damaged cell), which binds to the receptor of the immune cell and creates a signalling cascade that activates inflammatory mediators (such as cytokines), these dilate blood vessels which is responsible for the redness of the skin, heating and swelling of the area. Interestingly macrophages, produce myokines, which are believed to promote muscle hypertrophy. The blood vessel dilation also enhances blood vessel permeability, so plasma fluid and immune cells can seep through and accumulate in the tissue. The plasma fluid contains various antimicrobial mediators, which kill pathogens (an organism causing disease to its host) as well as platelets and blood clotting factors which both help to stop bleeding. Once activated by inflammatory mediators endothelial cells of blood vessels become adhesive, slowing down neutrophils in the blood and forcing them through the blood vessel and into the tissue, neutrophils destroy bacteria, and they also may release highly reactive oxygen species, in what is known as an oxidative burst, which kills pathogens faster and more efficiently, this is also likely to promote muscle hypertrophy, through it's activation of the MAPK signalling pathway for protein synthesis, the neutrophils then die via apoptosis (the process of programmed cell death). Monocytes then enter the inflamed area and differentiate into macrophages which remove pathogens, injured cells and neutrophils via phagocytosis (the cellular process for ingesting and eliminating fairly large particles). Accumulation of fluid puts pressure on lymphatic capillaries, forcing open their one-way valve, therefore removing fluid and macrophages, this is known as lymphatic drainage, the debris passes through lymph nodes and is filtered out of the body. Once the site is clear immune cells stop producing pro-inflammatory chemicals and instead start producing anti-inflammatory mediators which actively drive the termination of inflammation, many of these mediators are lipids, some of which are synthesised from omega-3 fatty acids.

FOXO3 activation in heat exposure

Heat exposure activates the FOXO3 gene which is a master regulator of many other genes, as it can activate or deactivate them, many of these genes are involved in stress resistance. We are exposed to things regularly that cause damage to our body, such as air pollution which we inhale, this damages the proteins in the body, which also damages our DNA which may lead to mutations and cause cancer. The damage of the DNA and proteins damages the cell and when the cell accumulates enough damage as we age, the cell becomes senescent and stays in the tissue or organ and secretes pro-inflammatory molecules and cytokines which can significantly decrease life span. FOXO3 is a protein encoded by the FOXO3 gene, this protein activates antioxidant genes forming antioxidants that prevent damage from occurring to proteins, DNA and cells of the tissue, FOXO3 activates genes, that make senescent and damaged cells undergo autophagy and clear it away, FOXO3 activates genes involved in cell repair, which occurs before a mutation forms, additionally, FOXO3 activates genes involved in cell death, so if a cell gets a mutation it will die, FOXO3 also activates cells involved in stem cell function, stem cells are important to replace any tissue as they can form any cell type in our tissues, this includes making more stem cells, this means organs can maintain there function properly.

The FOXO3A gene is associated with human longevity, some people have a variation of this gene which means it is active all the time, so this person gets more of the benefits previously discussed in the last paragraph, these people are 270% more likely to live to 100.

Dynorphin release from heat exposure

Heat stress releases dynorphin from a presynaptic neurone and it gives you an uncomfortable and dysphoric feeling, once it has bound to the kappa-opioid receptor, it also cools you down to help deal with the heat stress. Because the kappa-opioid receptors are becoming filled by dynorphin endorphins are forced to bind to the μ-opioid receptor, which makes it more sensitive and increases in number, so endorphins (which are responsible for happy, positive feelings) effects are intensified and occur more often. These effects can occur up to about 4 days after a single exposure. Therefore heat exposure may be an option to cure depression and anxiety. Interestingly endorphins also help to cool your body down, when the core body temperature rises, so endorphins also rise in the sauna as well as during exercise, but these may sometimes be overwhelmed by dynorphin, and when this happens just remember the better feelings you will have after.

BDNF increase from heat exposure

Both exercise and heat exposure increases levels of BDNF, which plays a role in neurogenesis (growing new neurons), this helps prevent brain ageing. BDNF also plays a role in neuroplasticity, so connections in the brain can change, which is useful to cope with a changing environment, this ability is usually hindered in older people and a lack of this adaptation may cause depression.

The benefits of sweating in heat exposure

Sweating during heat exposure, or just sweating in general, helps remove heavy metals and other toxins from the bloodstream, which otherwise can have the potential to damage various organs in the body. Toxins can be excreted from the body in urine but some not sufficiently, such as cadmium which strongly relies on sweat to be removed from the bloodstream, cadmium can enter the blood through foods we consume such as chocolate and cause cancer and damage to the cardiovascular system, respiratory system amongst various other bodily systems. Aluminium is another heavy metal which heavily relies on sweat for it to be removed from the bloodstream, aluminium can enter the blood through foods as well as the water we consume and the air we inhale and it can cause a variety of issues many involving the brain.
 

NRF2 activation from heat exposure

Heat exposure activates NRF2 which is a major regulator of anti-oxidant genes, forming antioxidants which work to detoxify things in the body which can otherwise cause harm, NRF2 also recruits anti-inflammatory cells, this and antioxidants help to reduce inflammation.

Impacts on endurance-based athletes

Immersion in cold water improves muscle soreness due to its ability to improve muscle recovery for those who haven’t recently performed weight training, this is backed by studies proving a decrease in markers of muscle damage in the bloodstream following cold exposure. It appears that the optimal temperature to decrease muscle soreness is 11-15°C/51-59°F, whilst 5-10°C/41-50°F appears to be 50% worse for improvements in muscle soreness, suggesting that colder temperatures may not necessarily be better for a decrease in muscular soreness.

Cold exposure helps reduce the inflammatory response following exercise, which helps to reduce deterioration in performance due to restricted movement, pain and damage caused to healthy tissue (such as muscle) by inflammation, which causes a higher risk of injuries due to soreness and changes in muscular function, the decrease in inflammation will help improve endurance performance such as running and cycling. The increase in IL-6 from cold exposure helps muscle recovery, this may be enough of a benefit to sports players towards muscular repair to warrant doing shortly after a training session if you can’t perform it later. It has been proven that cold exposure has positive impacts on performance when performed over a long period of time, but I wouldn’t recommend performing cold exposure within 4 hours after weight training when the inflammatory response is vital for muscular repair. Cold water immersion can reduce decrements in neuromuscular performance 24 hours following team sports, but still more studies must be performed to find data beyond 48 hours to see whether the impacts of cold exposure can improve performance.

Cold exposure induces mitochondrial biogenesis (as discussed earlier) causing an increase in mitochondria in muscles increasing aerobic capacity and endurance, as well as browning of fat. Performing cold exposure immediately after performing cardiovascular exercise has been shown to cause endurance enhancements. It also reduces inflammation which helps improve your performance, as inflammation can restrict movement and cause pain, as it pushes on nerves and decreases neural drive, due to anti-inflammatory chemicals decreasing acidity surrounding the nerves. Inflammation can also deal damage to healthy tissue, increase the risk of cardiovascular disease, through plaque build-up which irritates blood vessels; cause asthma, through swelling of the airways and increase in mucus; cause type 2 diabetes, as inflammation increases TNF-α which helps to decrease inflammation but also increases expression of proteins that supply the insulin signalling pathway desensitising it, TNF-α also increases the risk of cancer, due to it causing damage to cell DNA. A study revealed that people who went outside for 6 hours at 16°C/61°F temperatures had a 37% increase in the browning of adipose tissue on average.

Cold exposure also reduces total cholesterol including LDL which can otherwise lead to a stroke or heart failure, as it builds up on the walls of arteries.

Heat exposure mimics low-intensity cardiovascular exercise, due to the rise in heart rate usually to 100-150bpm, plasma volume increase, elevated core body temperature, increase in sweat and blood flow to the heart which lowers cardiovascular strain (the deformation of the cardiac wall or chamber from a relaxed to a contracted condition) so your heart gets to do less work each beat it is pumping blood.

Heat exposure will help you become adapted to heat by sweating at a lower core body temperature, which will also help you perform better in endurance exercises and performances such as a marathon.

Heat exposure increases heart rate variability (the amount of time between when your heartbeats fluctuate slightly) through its effect on the autonomic nervous system, which causes an increase in parasympathetic activity (which is responsible for stimulation of activities that occur when the body is at rest) and lowers sympathetic activity (which is responsible for the rapid involuntary response to dangerous or stressful situations), this means the heart is more likely to better deal with stress such as a heart attack.

It is also worth noting that heat exposure improves endothelial cell function (which regulates exchanges between the bloodstream and the surrounding tissues) and improves left ventricular function (which sends oxygenated blood to tissues of the body).

Even though heat exposure mimics low-intensity cardiovascular exercise it shouldn’t replace exercise, doing both exercise and performing heat exposure will host the most benefits to cardiorespiratory health, but exercise is better for cardiorespiratory health than either alone. Obviously doing heat exposure and no exercise will increase cardiorespiratory health more than doing neither of them.

Impacts on strength and hypertrophy

Pro-inflammatory molecules and reactive oxygen species increase directly after exercise, anti-inflammatory molecules peak 1 hour after exercise, this helps create adaptions to exercise, but excessive exercise-induced inflammation can promote muscle damage, fatigue and immune dysfunction. Inflammation levels can vary in different people, performing different training regimes, styles and techniques. Cold exposure can decrease inflammation which is very important to activate different anti-inflammatory pathways and immune cells which produce IGF-1 necessary for healing the damaged muscle after exercise, so muscle repair is hindered.

A study on young elite athletes found that their anabolic hormones rise immediately after exercise but after ice pack application for two 15-minute sessions with a 15-minute break in between there was a decrease in anabolic hormones and an increase in the catabolic hormone IGFBP-1 as opposed to recovery without any therapy. This is why you shouldn't perform cold exposure preferably within 4 hours after weight training.

Cold water immersion blunts muscle hypertrophy and strength gains when performed after exercise as backed by various studies. It has also been shown that cold water immersion after exercise caused no increase in fast-twitch muscle fibres. Active satellite cells and mTOR signalling pathway in people who performed cold exposure decreased, further reducing muscular adaptions. There is suppression of ribosomal biogenesis when performing cold exposure (ribosomes are essential for protein synthesis), this interferes with regenerative processes and blunts muscle performance. When performing cold exposure cell membranes lose their fluidity inhibiting atoms and molecules entering the cell, potentially damaging the cell, and nucleic acid and proteins are destabilized so muscle protein synthesis is inhibited further. Further suggesting that cold exposure should be performed cautiously in those looking for hypertrophy and/or strength gains if at all, but some studies have also shown no significant effects on muscle adaptions when performing cold exposure, one study of healthy young men who underwent whole body cryotherapy one hour after plyometric exercise measured an improved performance 72 hours after the exposure. However this study only involved one training session, so additional studies may be needed to validate this study.
 

Heat exposure on the other hand is nearly guaranteed to cause an increase in muscular adaptions. Heat exposure increases nutrient delivery to muscles (due to vasodilation of blood vessels), which is necessary for muscular adaptions particularly after exercise. Heat exposure will also elevate levels of growth hormone, a study found that when people got into a sauna from 80-100°C/176-212°F for 5-30 minutes, then got out for 5-10 minutes, then went back in, and repeated this process for 2 hours, found a 1000% increase in growth hormone on average, however, a single exposure at a lower temperature, which is probably more viable and safe for most people, can expect a 200-300% increase in growth hormone, but if you perform heat exposure multiple times it can increase growth hormone by 1600%. Growth hormone is important for building muscle by stimulating collagen synthesis. Heat shock proteins are activated by heat exposure, and they significantly help prevent muscle atrophy, as they prevent proteins from being degraded.

Health risks

Cold exposure can be dangerous when not performed carefully. The most common risk is hypothermia which occurs when someone's core body temperature drops below 35°C/95°F. Symptoms of hypothermia are rapid breathing, shivering, pale skin, confusion and drowsiness. Hypothermia can lead to drowning in large bodies of water. After drop is another risk, this is when cold blood in the extremities returns to the central circulation once exiting the cold, this can cause hypothermia and therefore death as a worst-case scenario. Frostbite can also occur which is freezing of the skin, usually, in the peripheral tissue (the fingers, toes, nose, ears, cheeks and chin), this usually occurs in temperatures less than -12°C/10°F after over 30 minutes, this is considerably longer than the time spent doing whole-body cryotherapy, plus users usually wear socks, gloves and a hat reducing the risk of frostbite from cryotherapy.

People with hypothyroidism and people who have recently consumed alcohol should not perform cold exposure, as this leads to a poor ability to cope with the extreme environment and causes drastic changes in blood pressure which can damage the blood vessels of various organs in the body and can therefore lead to health problems. The rise in blood pressure can also lead to a stroke or cardiac arrest, plus many people simply fail to breathe correctly with relaxed deep diaphragmatic breaths which can lead to dizziness and possibly fainting.

Be cautious when alternating between hot and cold exposure as this can cause dramatic changes in blood pressure and lead to health problems. Also, it is not yet clear if switching between hot and cold therapies hosts any major benefits, so there's no real need to do it, as far as we know.
 

Heat exposure causes low blood pressure which can lead to lightheadedness, dizziness and potentially fainting. Also, the rise in heart rate from heat exposure can lead to a stroke or cardiac arrest, your blood pressure and heart rate can drop below your normal resting heart rate after heat exposure as blood pools in your extremities, your blood vessels relax and their resistance is lowered and the parasympathetic nervous system activity increases, this can cause problems delivering chemicals to tissues so relax and wait until blood pressure and resting heart rate returns to normal following the exposure, after multiple heat exposures people usually experience a lower resting blood pressure and resting heart rate.

During heat exposure, you also sweat profusely and lose electrolytes mostly sodium but also others such as magnesium and potassium, which can lead to muscle weakness, spasms, cramps, respiratory problems and even paralysis, so you should stay hydrated and consume electrolytes in foods you eat, such as kale, lemon, and cucumber, particularly after the exposure.

Conclusion

It is clear that both cold exposure and heat exposure have benefits as well as setbacks and health risks, but people should be able to experience increased longevity, endurance, better mood, decreased inflammation, lower risk of brain diseases, inhibited brain ageing, and lower risk of cancer with both therapies. Cold exposure in particular will also increase cognitive abilities, increase focus, increase fat loss, boost the immune system, improve muscle soreness, improve performance in sports and increase muscle recovery in those who haven’t recently performed weight training. Heat exposure in particular will also repair tissues, inhibit harm to various organs of the body, increase cardiorespiratory health, make you better at dealing with stress on the body, increase muscular adaptions and prevent muscle atrophy.

You can get similar effects with both cold water immersion and whole-body cryotherapy. Cold showers and spending time outside in the cold probably won’t host the same benefits, as cold showers fail to emerge the whole body and being outside in the cold is probably still significantly warmer than the temperatures experienced in cryotherapy. It appears that there is a dose-dependent effect so the longer you spend performing cold exposure the more benefits you will experience, but colder temperatures may be less effective so the optimal temperature in cold water immersion should be at around 10-15°C/50-59°F for about 15 minutes with whole-body cryotherapy being at around -178- -135°C/-289- -211°F for only about 2 minutes. Cold exposure should not be performed within 4 hours of weight training for people looking for optimal muscular adaptions.

Heat exposure is optimally performed in a sauna at around 70°C/158°F at a humidity of about 10-20%, but it can also be performed in a hot bath or jacuzzi at around 40°C/104°F this probably doesn’t have as beneficial of an impact as a sauna. If you do choose the hot bath or jacuzzi option make sure to keep yourself emerged up to your shoulders for the most benefits. A hot shower is an option but it probably is nowhere near as beneficial as a sauna, bath or jacuzzi. Heat exposure can be performed and perhaps is optimally performed directly after exercise.

Heat exposure can be damaging to children and not host as many benefits to them as it does in adults, also pregnant women should not perform heat exposure as this can cause fetal abnormalities. A study which followed 2,000 middle-aged men for 20 years revealed that people who used the sauna only 2-3 times for over 19 minutes per week were 27% less likely to die from cardiovascular-related deaths compared to men who only used the sauna once per week, however, those who used the sauna 4-7 times for over 19 minutes per week were 50% less likely to die from cardiovascular-related deaths compared to men who used the sauna only once per week, another study of men who used the sauna 4 times per week revealed that they were 40% less likely to die from non-accidental diseases compared to those who didn’t use the sauna at all, I would therefore recommend performing heat exposure for 20 minutes, 4 times per week if it is safe for you.

It is currently unclear if the benefits of cold and heat exposure apply to older people as much as it does to young and middle-aged people, it is also important to have someone with you when performing either exposure, make sure to stop the exposure when you are experiencing symptoms of any health risks discussed in the previous section, do not rush to expose yourself to extreme temperatures immediately, instead, you should take your time to gradually build up to these optimal temperatures and time of exposures, in order to not increase the risk of any health issues and always ask your GP before performing either therapy if it is safe for you, you generally shouldn’t perform either exposure if you have low or high blood pressure, have recently had a heart attack or any cardiac diseases, are a child or have any serious problems affecting your respiratory system.