This article explains how heat can be used to increase growth hormone, muscular hypertrophy, endurance, and otherwise aid performance. The article is authored by Rhonda Perciavalle Patrick, Ph.D.
For the most part, people don’t like to get hot.
The massive indoor climate control systems and pleasantly chilled water fountains found in most gyms speak to this fact. There are some exceptions — Bikram yoga, for example — but they’re few and far between.
But here’s the surprise: increasing your core temperature for short bursts is not only healthful, it can also dramatically improve performance.
This is true whether it’s done in conjunction with your existing workout or as an entirely separate activity. I’m going to explain how heat acclimation through sauna use (and likely any other non-aerobic activity that increases core body temperature) can promote physiological adaptations that result in increased endurance, easier acquisition of muscle mass, and a general increased capacity for stress tolerance. I will refer to this concept of deliberately acclimating yourself to heat, independent of working out, as “hyperthermic conditioning.”
I’m also going to explain the positive effects of heat acclimation on the brain, including the growth of new brain cells, improvement in focus, learning and memory, and ameliorating depression and anxiety. In addition, you’ll learn how modulation of core temperature might even be largely responsible for “runner’s high” via an interaction between the dynorphin/beta-endorphin opioid systems.
The Effects of Heat Acclimation on Endurance
If you’ve ever run long distances or exercised for endurance, it’s intuitive that increased body temperature will ultimately induce strain, attenuate your endurance performance, and accelerating exhaustion. What might not be as intuitive is this: acclimating yourself to heat independent of aerobic physical activity through sauna use induces adaptations that reduce the later strain of your primary aerobic activity.
Hyperthermic conditioning improves your performance during endurance training activities by causing adaptations, such as improved cardiovascular and thermoregulatory mechanisms (I will explain what these mean) that reduce the negative effects associated with elevations in core body temperature. This helps optimize your body for subsequent exposures to heat (from metabolic activities) during your next big race or even your next workout.
Just a few of the physiological adaptations that occur are:
- Improved cardiovascular mechanisms and lower heart rate. 1
- Lower core body temperature during workload (surprise!)
- Higher sweat rate and sweat sensitivity as a function of increased thermoregulatory control. 2
- Increased blood flow to skeletal muscle (known as muscle perfusion) and other tissues.2
- Reduced rate of glycogen depletion due to improved muscle perfusion. 3
- Increased red blood cell count (likely via erythropoietin). 4
- Increased efficiency of oxygen transport to muscles.4
Hyperthermic conditioning optimizes blood flow to the heart, skeletal muscles, skin, and other tissues because it increases plasma volume. This leads to endurance enhancements in your next workout or race, when your core body temperature is once again elevated.
Being heat acclimated enhances endurance by the following mechanisms…
- It increases plasma volume and blood flow to the heart (stroke volume).2, 5 This results in reduced cardiovascular strain and lowers the heart rate for the same given workload.2 These cardiovascular improvements have been shown to enhance endurance in both highly trained and untrained athletes.2,5, 6
- It increases blood flow to the skeletal muscles, keeping them fueled with glucose, esterified fatty acids, and oxygen while removing by-products of the metabolic process such as lactic acid. The increased delivery of nutrients to muscles reduces their dependence on glycogen stores. Endurance athletes often hit a “wall” (or “bonk”) when they have depleted their muscle glycogen stores. Hyperthermic conditioning has been shown to reduce muscle glycogen use by 40%-50% compared to before heat acclimation.3, 7 This is presumably due to the increased blood flow to the muscles.3 In addition, lactate accumulation in blood and muscle during exercise is reduced after heat acclimation.5
- It improves thermoregulatory control, which operates by activating the sympathetic nervous system and increasing the blood flow to the skin and, thus the sweat rate. This dissipates some of the core body heat. After acclimation, sweating occurs at a lower core temperature and the sweat rate is maintained for a longer period.2
So what sort of gains can you anticipate?
One study demonstrated that a 30-minute sauna session two times a week for three weeks POST-workout increased the time that it took for study participants to run until exhaustion by 32% compared to baseline.4
The 32% increase in running endurance found in this particular study was accompanied by a 7.1% increase in plasma volume and 3.5% increase in red blood cell (RBC) count.4 This increased red blood cell count accompanying these performance gains feed right back into those more general mechanisms we talked about earlier, the most obvious of which being: more red blood cells increase oxygen delivery to muscles. It is thought that heat acclimation boosts the RBC count through erythropoietin (EPO) because the body is trying to compensate for the corresponding rise in plasma volume.4
(Note from Tim: If “EPO” sounds familiar, it’s because it’s commonly injected by Tour de France competitors. More on that here.)
In other words, hyperthermic conditioning through sauna use doesn’t just make you better at dealing with heat; it makes you better, period. I do want to mention that while these gains were made with a small sample size (N=6) some of the later studies that I point out reinforce this conclusion.
The Effects of Hyperthermic Conditioning on Muscle Hypertrophy (Growth)
Exercise can induce muscular hypertrophy. Heat induces muscular hypertrophy. Both of these together synergize to induce hyper-hypertrophy.
Here are a few of the basics of how muscle hypertrophy works: muscle hypertrophy involves both the increase in the size of muscle cells and, perhaps unsurprisingly, an accompanying increase in strength. Skeletal muscle cells do contain stem cells that are able to increase the number of muscle cells (TIM: called “hyperplasia”) but hypertrophy instead generally involves an increase in size rather than number.
So what determines whether your muscle cells are growing or shrinking (atrophying)?
A shift in the protein synthesis-to-degradation ratio…and an applied workload on the muscle tissue (of course). That’s it.
At any given time your muscles are performing a balancing act between NEW protein synthesis and degradation of existing proteins. The important thing is your net protein synthesis, and not strictly the amount of new protein synthesis occurring. Protein degradation occurs both during muscle use and disuse. This is where hyperthermic conditioning shines: heat acclimation reduces the amount of protein degradation occurring and as a result it increases net protein synthesis and, thus muscle hypertrophy. Hyperthermic conditioning is known to increase muscle hypertrophy by increasing net protein synthesis through three important mechanisms:
- Induction of heat shock proteins. 8 9
- Robust induction of growth hormone.1
- Improved insulin sensitivity. 10
Exercise induces both protein synthesis and degradation in skeletal muscles but, again, it is the net protein synthesis that causes the actual hypertrophy. When you exercise, you are increasing the workload on the skeletal muscle and, thus, the energetic needs of your muscle cells. The mitochondria found in each of these cells kick into gear in order to help meet this demand and start sucking in the oxygen found in your blood in order to produce new energy in the form of ATP. This process is called oxidative phosphorylation. A by-product of this process, however, is the generation of oxygen free radicals like superoxide and hydrogen peroxide, which is more generally referred to simply as “oxidative stress”.
Heat Stress Triggers Heat Shock Proteins That Prevent Protein Degradation
Oxidative stress is a major source of protein degradation.
For this reason, any means of preventing exercise-induced oxidative protein damage and/or repairing damaged proteins, while keeping the exercise induced protein synthesis, will ultimately cause a net increase of protein synthesis and therefore will be anabolic.
Heat shock proteins (or HSPs), as the name implies, are induced by heat and are a prime example of hormesis. Intermittent exposure to heat induces a hormetic response (a protective stress response), which promotes the expression of a gene called heat shock factor 1 and subsequently HSPs involved in stress resistance.
- HSPs can prevent damage by directly scavenging free radicals and also by supporting cellular antioxidant capacity through its effects on maintaining glutathione.8,9
- HSPs can repair misfolded, damaged proteins thereby ensuring proteins have their proper structure and function.8,9
Okay, let’s take a step back from the underlying mechanisms and look at the big picture of heat acclimation in the context of increasing muscle hypertrophy:
It has been shown that a 30-minute intermittent hyperthermic treatment at 41°C (105.8°F) in rats induced a robust expression of heat shock proteins (including HSP32, HSP25, and HSP72) in muscle and, importantly, this correlated with 30% more muscle regrowth than a control group during the seven days subsequent to a week of immobilization.8 This HSP induction from a 30-minute intermittent hyperthermic exposure can persist for up to 48 hours after heat shock.8,9 Heat acclimation actually causes a higher basal (such as when not exercising) expression of HSPs and a more robust induction upon elevation in core body temperature (such as during exercise). 11 12 13 This is a great example of how a person can theoretically use hyperthermic conditioning to increase their own heat shock proteins and thereby reap the rewards.
Heat Stress Triggers A Massive Release of Growth Hormone
Another way in which hyperthermic conditioning can be used to increase anabolism is through a massive induction of growth hormone. 14 15, 1 Many of the anabolic effects of growth hormone are primarily mediated by IGF-1, which is synthesized (mainly in the liver but also in skeletal muscle and other tissues) in response to growth hormone. There are two important mechanisms by which IGF-1 promotes the growth of skeletal muscle:
- It Increases protein synthesis via activation of the mTOR pathway. 16
- It decreases protein degradation via inhibition of the FOXO pathway.16
Mice that have been engineered to express high levels of IGF-1 in their muscle develop skeletal muscle hypertrophy, can combat age-related muscle atrophy, and retained the same regenerative capacity as young muscle. 17, 18 In humans, it has been shown that the major anabolic effects of growth hormone in skeletal muscle may be due to inhibition of muscle protein degradation (anti-catabolic), thereby increasing net protein synthesis.16 In fact, growth hormone administration to endurance athletes for four weeks has been shown to decrease muscle protein oxidation (a biomarker for oxidative stress) and degradation by 50%. 19
My point is good news. You don’t need to take exogenous growth hormone. Sauna use can cause a robust release in growth hormone, which varies according to time, temperature, and frequency.1,15
For example, two 20-minute sauna sessions at 80°C (176°F) separated by a 30-minute cooling period elevated growth hormone levels two-fold over baseline.1,15 Whereas, two 15-minute sauna sessions at 100°C (212°F) dry heat separated by a 30-minute cooling period resulted in a five-fold increase in growth hormone.1,15 However, what’s perhaps more amazing is that repeated exposure to whole-body, intermittent hyperthermia (hyperthermic conditioning) through sauna use has an even more profound effect on boosting growth hormone immediately afterward: two one-hour sauna sessions a day at 80°C (176°F) dry heat (okay, this is a bit extreme) for 7 days was shown to increase growth hormone by 16-fold on the third day.14 The growth hormone effects generally persist for a couple of hours post-sauna.1 It is also important to note that when hyperthermia and exercise are combined, they induce a synergistic increase in growth hormone. 20
Increased Insulin Sensitivity
Insulin is an endocrine hormone that primarily regulates glucose homeostasis, particularly by promoting the uptake of glucose into muscle and adipose tissue. In addition, insulin also plays a role in protein metabolism, albeit to a lesser degree than IGF-1. Insulin regulates protein metabolism in skeletal muscle by the two following mechanisms:
- It increases protein synthesis by stimulating the uptake of amino acids (particularly BCAAs) into skeletal muscle. 21
- It decreases protein degradation through inhibition of the proteasome, which is a protein complex inside cells that is largely responsible for the degradation of most cellular proteins. 22
In humans, there is more evidence indicating that the major anabolic effects of insulin on skeletal muscle are due to its inhibitory action on protein degradation.
For example, insulin infusion in healthy humans, which increased insulin to normal physiological postprandial (after a meal) levels, suppressed muscle protein breakdown without significant affecting muscle protein synthesis. 23, 21 In contrast, insulin deficiency (such as in type 1 diabetes mellitus) and insulin resistance (to a lesser extent) are both associated with increased skeletal muscle breakdown.22, 24
For this reason, hyperthermic conditioning may also lend itself to promoting muscle growth by improving insulin sensitivity and decreasing muscle protein catabolism. Intermittent hyperthermia has been demonstrated to reduce insulin resistance in an obese diabetic mouse model. Insulin resistant diabetic mice were subjected to 30 minutes of hyperthermic treatment, three times a week for twelve weeks. This resulted in a 31% decrease in insulin levels and a significant reduction in blood glucose levels, suggesting re-sensitization to insulin.10 The hyperthermic treatment specifically targeted the skeletal muscle by increasing the expression of a type of transporter known as GLUT 4, which is responsible for the transporting of glucose into skeletal muscle from the bloodstream. Decreased glucose uptake by skeletal muscle is one of the mechanisms that leads to insulin resistance.
(TIM: For more fun with GLUT 4 transporters, read the “Damage Control” chapter in The 4-Hour Body, which covers how to minimize (or eliminate) fat gain from cheat meals or cheat days.)
Relevance for Muscle Injury
Animal studies using rats have shown that a 30-minute and 60-minute hyperthermic treatment at 41°C (105.8°F) attenuates hindlimb muscle atrophy during disuse by 20% and 32%, respectively.9, 25 In order to return to a hypertrophic state after injury, muscle regrowth (“reloading”) must occur. Muscle reloading, while important for hypertrophy, induces oxidative stress particularly after periods of disuse, which slows the rate of muscle regrowth. A 30-minute hyperthermic treatment at 41°C (105.8°F) increased soleus muscle regrowth by 30% after reloading as compared to non-hyperthermic treatment in rats.8 The effects of whole body hyperthermia on preventing muscle atrophy and increasing muscle regrowth after immobilization were shown to occur as a consequence of elevated HSP levels.8,9,25
During injury, you may be immobilized but you don’t have to be very mobile to sit in the sauna a few times a week to boost your HSPs! This is a clear win in the injury and recovery department. Remember, hyperthermic conditioning (from sauna use) results in an elevation in HSP levels under normal conditions and leads to an even greater boost during exercise (or when core body temperature is elevated).11-13
Relevance for Rhabdomyolysis
Hyperthermic conditioning may also be able to protect against rhabdomyolysis (muscle breakdown due to severe muscle overuse) through the induction of HSP32 also known as heme oxygenase 1. 26, 27
Rhabdomyolysis releases myoglobin, a byproduct from broken down muscle tissue, into the bloodstream, which can cause kidney failure. (TIM: CrossFitters, watch your CPK levels after glute-ham ab work. If you can’t do long planks with your feet against a wall, don’t do hyper-extended ROM, ballistic ab work.)
Since myoglobin is a heme-containing protein, HSP32 (heme oxygenase 1) can rapidly degrade myoglobin before it has toxic effects on the kidney.26,27 In fact, induction of HSP32 in rats has been shown to protect against rhabdomyolysis in rats.26 This function of HSP32 is very different than the classical role of HSPs in preventing protein degradation. Again, heat acclimation causes a higher basal expression of HSPs and a more robust expression upon heat stress.11-13 The more heat acclimated your body is (the pre-conditioning is the key here), the higher your HSP32 expression will be during physical activity and this will protect your kidneys from the toxic myoglobin breakdown product.
That’s a sweet deal.
In flies and worms, a brief exposure to heat treatment has been shown to increase their lifespan by up to 15% and it’s been shown that this effect is specifically mediated by HSPs. 28, 29, 30
While studying the effects of something like hyperthermic conditioning on longevity is inherently hard in humans (obviously), there have been some preliminary positive associations with variations in the HSP70 gene associated with increased expression and longevity. 31
Effects of Heat Stress on The Brain
One of the ways that the brain responds to injury on the cellular level is increased HSP production.
This includes ischemic injury (i.e. stroke), traumatic injury, and excitotoxicity (epileptic). 32 What complicates things, however, in the context of “hyperthermic conditioning” (or deliberate heat acclimation) is that while on the one hand hyperthermia has been shown to reduce the frequency of seizures and the damage they cause post-conditioning, hyperthermia can actually increase the damage caused by seizures if they occur during a period of heat stress. In other words, the stress and its damaging effects are additive. 33 34
That (and it’s implicit warning) being said, sauna-induced hyperthermia has been shown to induce a robust activation of the sympathetic nervous system and the hypothalamic-pituitary-adrenal (HPA) axis.
One study demonstrated that men that stayed in the sauna that was heated to 80°C (176°F) until subjective exhaustion increased norepinephrine by 310%, had a 10-fold increase in prolactin, and actually modestly decreased cortisol.1,15 Similarly, in another study, women that spent 20-minute sessions in a dry sauna twice a week had a 86% increase in norepinephrine and a 510% increase in prolactin after the session. 35
Norepinephrine helps with focus and attention while prolactin promotes myelin growth, which makes your brain function faster, which is key in repairing nerve cell damage. 3637
In addition to increasing norepinephrine, heat acclimation has actually been shown to increase biological capacity to store norepinephrine for later release. 38 In light of the fact that the norepinephrine response to exercise has been demonstrated to be blunted in children with ADHD and that norepinephrine reuptake inhibitors (NRI) are frequently prescribed to treat ADHD (among other things), use of heat stress and subsequent acclimation should be tested for it’s effectiveness as an interesting alternative therapeutic approach. 39
Heat stress has been shown to increase the expression of brain-derived neurotrophic factor (BDNF) more than exercise alone when used in conjunction with exercise. 40
This is important because BDNF increases the growth of new brain cells as well as the survival of existing neurons. An increase in neurogenesis is thought to be responsible for enhancing learning. 41 BDNF’s role in the brain is also to modulate neuronal plasticity and long-term memory, while also having been shown to ameliorate anxiety and depression from early-life stressful events. 42 In addition to the function BDNF plays in the brain when it’s released as a consequence of exercise, BDNF is also secreted by muscle where it plays a role in muscle repair and the growth of new muscle cells. 43
While BDNF has specifically been shown to play some role in relieving depression from stressful early-life events, whole-body hyperthermia has also been demonstrated to improve depression in cancer patients. 44 In this particular study, however, it was speculated that beta-endorphin (which is also induced by hyperthermia), not BDNF, may have been the agent responsible for this effect. As an aside, one of the reasons whole-body hyperthermia is sometimes used with cancer patients is because it can enhance the effects of chemotherapeutic agents. 45
The Runner’s High and The Role of Dynorphin
Ever wonder what is responsible for the “runner’s high” or post-exercise highs, in general? You’ve probably heard that it’s due to endorphins, but that’s not the whole story.
Beta-endorphins are endogenous (natural) opioids that are a part of the body’s natural painkiller system, known as the mu opioid system, which block pain messages from spreading from the body to the brain in a process called antinociception. What is lesser known is that the body also produces a peptide known as dynorphin (a “kappa opioid”), which is generally responsible for the sensation of dysphoria. The discomfort experienced during intense exercise, exposure to extreme heat (such as in a sauna), or eating spicy food (capsaicin) is due to the release of dynorphin. The release of dynorphin causes an upregulation and sensitization of mu opioid receptors, which interact with beta-endorphin. 46 This process is what underlies the “runner’s high” and is directly precipitated by the discomfort of physical exercise. Translation: the greater the discomfort experienced during your workout or sauna, the better the endorphin high will be afterward. Now you understand the underlying biological mechanism that explains this.
How is this relevant to hyperthermic conditioning and sauna use?
Heat stress from heat exposure in a dry sauna has been demonstrated to cause a potent increase in beta-endorphin levels, even more than exercise alone.1,15
A study in rats explains this somewhat: dynorphin delivered directly into a part of the hypothalamus in the brains of rats triggers a drop in their body temperature, while blocking dynorphin with an antagonist was shown to prevent this same response. Similarly, mu receptor agonists have been shown to induce increases in body temperature in rats. 47 What this seems to imply is that perhaps, by deliberately manipulating your body temperature you are actually directly engaging the mu (endorphin) and kappa opioid (dynorphin) systems since they clearly play a role in temperature regulation in general.
To recap and drive the point home: acclimating your body to heat stress by intermittent whole-body hyperthermia via sauna use (“hyperthermic conditioning”) has been shown to:
Enhance endurance by:
- Increasing nutrient delivery to muscles thereby reducing the depletion of glycogen stores.
- Reducing heart rate and reducing core temperature during workload.
Increase muscle hypertrophy by preventing protein degradation through the following three means:
- Induction of heat shock proteins and a hormetic response (which has also been shown to increase longevity in lower organisms).
- Cause a massive release of growth hormone.
- Improving insulin sensitivity.
Hyperthermic conditioning also has robust positive effects on the brain:
- Increases the storage and release of norepinephrine, which improves attention and focus.
- Increases prolactin, which causes your brain to function faster by enhancing myelination and helps to repair damaged neurons.
- Increases BDNF, which causes the growth of new brain cells, improves the ability for you to retain new information, and ameliorates certain types of depression and anxiety.
- Causes a robust increase in dynorphin, which results in your body becoming more sensitive to the ensuing endorphins.
Life is stressful.
When you exercise, you are forcing your body to become more resilient to stress (somewhat paradoxically) through stress itself.
Hyperthermic conditioning is a novel and possibly effective tool that can improve your resistance to the sort of stress associated with fitness pursuits as well as some that are not traditionally associated with fitness such as the protective effects of HSPs on various types of stress. That being said, deliberately applied physical stress, whether heat stress or ordinary exercise, is something that requires caution.
You shouldn’t avoid it altogether, but you should use good common sense, not overwhelm yourself, and make sure to know your limits. (NOTE: you should not drink alcohol before or during sauna use as it increases the risk of death). 48 Personal variation probably comes into play when finding your own sweet spot for building thermal tolerance while avoiding over-extending yourself.
I believe that hyperthermic conditioning in general may be worth a closer look as a tool in the toolbox of athletes. Perhaps it can be used for much more than just relaxation?
But no matter how enthusiastic you might be, remember:
- Heat responsibly and with someone else, never alone.
- Never heat yourself while drunk, and friends don’t let friends sauna drunk.
- If you are pregnant or have any medical condition, saunas are not for you. Speak with your doctor before starting this or any regimen involving physical stressors.
Be careful, ladies and gents.
ABOUT THE AUTHOR: Dr. Rhonda Patrick
You can find more video and writing from Dr. Rhonda Patrick at her website, FoundMyFitness.com.
- Hannuksela, M. L. & Ellahham, S. Benefits and risks of sauna bathing. The American journal of medicine 110, 118-126 (2001). This is actually an important review article that covers some of the benefits of sauna use including the cardiovascular advantages and hormonal changes such as the boost in GH levels. I also like it because it covers some of the risks of alcohol use before or during the sauna.
- Ricardo J. S. Costa, M. J. C., Jonathan P. Moore & Neil P. Walsh. Heat acclimation responses of an ultra-endurance running group preparing for hot desert-based competition. European Journal of Sport Science, 1-11 (2011). The sample sizes in both studies referenced here and in #4 have small sample sizes but they are two independent studies that compliment each other. This study also reinforces the endurance enhancements in #5.
- King, D. S., Costill, D. L., Fink, W. J., Hargreaves, M. & Fielding, R. A. Muscle metabolism during exercise in the heat in unacclimatized and acclimatized humans. J Appl Physiol 59, 1350-1354 (1985). This study shows that glycogen utilization is decreased in runners after heat acclimation. The sample size is small but ref #7 (another small sample) is an independent study that shows the same effect.
- Scoon, G. S., Hopkins, W. G., Mayhew, S. & Cotter, J. D. Effect of post-exercise sauna bathing on the endurance performance of competitive male runners. Journal of science and medicine in sport / Sports Medicine Australia 10, 259-262, doi:10.1016/j.jsams.2006.06.009 (2007). This study shows the effect of preconditioning the body to heat stress by using a sauna for at least 30 min directly after after training session. While the study sample is small, other studies referenced in #2, #5 reinforce and compliment this. I also have some anecdotal data. I did some serious experimentation with the sauna a couple of years ago when I had access to a sauna. I would sit in the sauna for up to 60 min. until I pushed myself to extreme physical discomfort about 4-5 times a week. I substantially (and I know this is just anecdote) increased my running PRs.
- Michael N. Sawka, C. B. W., Kent B. Pandolf. Thermoregulatory Responses to Acute Exercise-Heat Stress and Heat Acclimation. Handbook of Physiology, Environmental Physiology (2011). This is a good review article that covers many of the mechanisms that underly the endurance enhancements as a consequence of heat acclimation.
- Garrett, A. T., Creasy, R., Rehrer, N. J., Patterson, M. J. & Cotter, J. D. Effectiveness of short-term heat acclimation for highly trained athletes. European journal of applied physiology 112, 1827-1837, doi:10.1007/s00421-011-2153-3 (2012).
- Kirwan, J. P. et al. Substrate utilization in leg muscle of men after heat acclimation. J Appl Physiol (1985) 63, 31-35 (1987). The findings in this study reinforce the data in ref #3. Both small sample sizes but multiple studies showing the same effect makes the argument stronger.
- Selsby, J. T. et al. Intermittent hyperthermia enhances skeletal muscle regrowth and attenuates oxidative damage following reloading. J Appl Physiol (1985) 102, 1702-1707, doi:10.1152/japplphysiol.00722.2006 (2007). This is an important paper because it shows that intermittent hyperthermia can enhance the regrowth of skeletal muscle in rats after disuse via induction of heat shock proteins. Having a quantitative way to non-invasively measure muscle mass in humans is difficult. Even though the experiment was done in rats (N=40) this is a good study because it also shows mechanism.
- Naito, H. et al. Heat stress attenuates skeletal muscle atrophy in hindlimb-unweighted rats. J Appl Physiol 88, 359-363 (2000). This study demonstrates that HSP induction by intermittent hyperthermia in rats can prevent muscle atrophy during muscle disuse. Again, this study was in rats but it shows mechanism has has a good sample size (N=40).
- Kokura, S. et al. Whole body hyperthermia improves obesity-induced insulin resistance in diabetic mice. International journal of hyperthermia : the official journal of European Society for Hyperthermic Oncology, North American Hyperthermia Group 23, 259-265, doi:10.1080/02656730601176824 (2007). This study was done in mice (N=20) but it demonstrates a very important mechanistic finding that hyperthermia increases the expression of glucose transporters in skeletal muscle, thus improving insulin sensitivity. Exercise (which elevates core body temp.) is known to improve insulin sensitivity. This is a cool mechanism by which this can occur.
- Yamada, P. M., Amorim, F. T., Moseley, P., Robergs, R. & Schneider, S. M. Effect of heat acclimation on heat shock protein 72 and interleukin-10 in humans. J Appl Physiol (1985) 103, 1196-1204, doi:10.1152/japplphysiol.00242.2007 (2007). This study includes a relatively small human sample size (N=12) but it is a very important because it demonstrates that heat acclimation causes a higher induction of heat shock proteins upon later exercise. This is the fundamental concept behind hyperthermic conditioning.
- Moseley, P. L. Heat shock proteins and heat adaptation of the whole organism. J Appl Physiol (1985) 83, 1413-1417 (1997). This is a review article that explains some of the functions of HSPs and reinforces the data from reference #11 demonstrating that heat acclimation can increase the expression of HSPs.
- Kuennen, M. et al. Thermotolerance and heat acclimation may share a common mechanism in humans. American journal of physiology. Regulatory, integrative and comparative physiology 301, R524-533, doi:10.1152/ajpregu.00039.2011 (2011). This study is another small human sample size (N=8) but it reinforces the data from ref #11 because it demonstrates that some of the positive effects of heat acclimation are due to increased expression of HSPs. The study even shows specificity here by administering an HSP inhibitor, which ameliorates the positive effects of heat acclimation.
- Leppaluoto, J. et al. Endocrine effects of repeated sauna bathing. Acta physiologica Scandinavica 128, 467-470, doi:10.1111/j.1748-1716.1986.tb08000.x (1986). This is a very important study because it shows the profound hormonal responses to repeated sauna use in humans (N=17). By day 3, growth hormone increased 16-fold, highlighting the importance of hyperthermic conditioning.
- Kukkonen-Harjula, K. et al. Haemodynamic and hormonal responses to heat exposure in a Finnish sauna bath. European journal of applied physiology and occupational physiology 58, 543-550 (1989). Even though the human sample size in this study is small (N=8), it shows that varying temperatures and durations differentially affect hormones. Small sample or not, the fundamental chemical changes in this study are reinforced from the data referenced in #1 and #4.
- Velloso, C. P. Regulation of muscle mass by growth hormone and IGF-I. British journal of pharmacology 154, 557-568, doi:10.1038/bjp.2008.153 (2008).
- Coleman, M. E. et al. Myogenic vector expression of insulin-like growth factor I stimulates muscle cell differentiation and myofiber hypertrophy in transgenic mice. The Journal of biological chemistry 270, 12109-12116 (1995). In this study mice were engineered to constitutively express high levels of human IGF-1 in their muscle stem cells. This caused the proliferation and differentiation of myoblasts and caused muscle hypertrophy.
- Barton, E. R., Morris, L., Musaro, A., Rosenthal, N. & Sweeney, H. L. Muscle-specific expression of insulin-like growth factor I counters muscle decline in mdx mice. The Journal of cell biology 157, 137-148, doi:10.1083/jcb.200108071 (2002).
- Healy, M. L. et al. High dose growth hormone exerts an anabolic effect at rest and during exercise in endurance-trained athletes. The Journal of clinical endocrinology and metabolism 88, 5221-5226 (2003).
- Ftaiti, F. et al. Effect of hyperthermia and physical activity on circulating growth hormone. Applied physiology, nutrition, and metabolism = Physiologie appliquee, nutrition et metabolisme 33, 880-887, doi:10.1139/H08-073 (2008). This study shows that hyperthermia SYNERGIZES with exercise to increase growth hormone levels in humans. So you can feel the burn from your routine and then jump immediately in the sauna for amplified effects. Again, small sample (N=8) but its conclusion is logical and intuitively follows the other studies. Anything that substantially increases core temperature should increase growth hormone and the effects should potentiate each other.
- Louard, R. J., Fryburg, D. A., Gelfand, R. A. & Barrett, E. J. Insulin sensitivity of protein and glucose metabolism in human forearm skeletal muscle. The Journal of clinical investigation 90, 2348-2354, doi:10.1172/JCI116124 (1992). This study demonstrated that insulin stimulated BCAA uptake in the forearm (post-absorptive and insulin infusion) The sample size in this human study was good (N=39).
- Lecker, S. H., Goldberg, A. L. & Mitch, W. E. Protein degradation by the ubiquitin-proteasome pathway in normal and disease states. Journal of the American Society of Nephrology : JASN 17, 1807-1819, doi:10.1681/ASN.2006010083 (2006). This is a review article that covers the mechanism by which insulin decreases protein degradation: proteasome inhibition.
- Chow, L. S. et al. Mechanism of insulin’s anabolic effect on muscle: measurements of muscle protein synthesis and breakdown using aminoacyl-tRNA and other surrogate measures. American journal of physiology. Endocrinology and metabolism 291, E729-736, doi:10.1152/ajpendo.00003.2006 (2006). This study used multiple different methods to measure protein synthesis and degradation in 18 humans after insulin infusion. The insulin levels were raised to physiologically relevant postprandial levels.
- Guillet, C., Masgrau, A., Walrand, S. & Boirie, Y. Impaired protein metabolism: interlinks between obesity, insulin resistance and inflammation. Obesity reviews : an official journal of the International Association for the Study of Obesity 13 Suppl 2, 51-57, doi:10.1111/j.1467-789X.2012.01037.x (2012).
- Selsby, J. T. & Dodd, S. L. Heat treatment reduces oxidative stress and protects muscle mass during immobilization. American journal of physiology. Regulatory, integrative and comparative physiology 289, R134-139, doi:10.1152/ajpregu.00497.2004 (2005). This study just reinforces and compliments the protective effect that HSPs have on muscle mass during disuse. It reinforces data referenced in #9.
- Nath, K. A. et al. Induction of heme oxygenase is a rapid, protective response in rhabdomyolysis in the rat. The Journal of clinical investigation 90, 267-270, doi:10.1172/JCI115847 (1992). This reference is relevant to the mechanism by which hyperthermic conditioning may protect against rhabdomyolysis: induction of HSP32.
- Wei, Q., Hill, W. D., Su, Y., Huang, S. & Dong, Z. Heme oxygenase-1 induction contributes to renoprotection by G-CSF during rhabdomyolysis-associated acute kidney injury. American journal of physiology. Renal physiology 301, F162-170, doi:10.1152/ajprenal.00438.2010 (2011).
- Khazaeli, A. A., Tatar, M., Pletcher, S. D. & Curtsinger, J. W. Heat-induced longevity extension in Drosophila. I. Heat treatment, mortality, and thermotolerance. The journals of gerontology. Series A, Biological sciences and medical sciences 52, B48-52 (1997). This reference, as well as the two immediate ones following, back up the notion that heat shock extends lifespan in lower organisms via HSP induction.
- Lithgow, G. J., White, T. M., Melov, S. & Johnson, T. E. Thermotolerance and extended life-span conferred by single-gene mutations and induced by thermal stress. Proceedings of the National Academy of Sciences of the United States of America 92, 7540-7544 (1995).
- Tatar, M., Khazaeli, A. A. & Curtsinger, J. W. Chaperoning extended life. Nature 390, 30, doi:10.1038/36237 (1997).
- Singh, R. et al. Anti-inflammatory heat shock protein 70 genes are positively associated with human survival. Current pharmaceutical design 16, 796-801 (2010). This study was a longitudinal cohort of a Denmark population (N=168) that found a slight increase in longevity (1 year) in females that had a polymorphism in the HSP70 gene that was associated with increased HSP expression upon heat stress.
- Yenari, M. A., Giffard, R. G., Sapolsky, R. M. & Steinberg, G. K. The neuroprotective potential of heat shock protein 70 (HSP70). Molecular medicine today 5, 525-531 (1999).
- Duveau, V., Arthaud, S., Serre, H., Rougier, A. & Le Gal La Salle, G. Transient hyperthermia protects against subsequent seizures and epilepsy-induced cell damage in the rat. Neurobiology of disease 19, 142-149, doi:10.1016/j.nbd.2004.11.011 (2005).
- Lundgren, J., Smith, M. L., Blennow, G. & Siesjo, B. K. Hyperthermia aggravates and hypothermia ameliorates epileptic brain damage. Experimental brain research. Experimentelle Hirnforschung. Experimentation cerebrale 99, 43-55 (1994).
- Laatikainen, T., Salminen, K., Kohvakka, A. & Pettersson, J. Response of plasma endorphins, prolactin and catecholamines in women to intense heat in a sauna. European journal of applied physiology and occupational physiology 57, 98-102 (1988). This study reinforces ref #15 in terms of the norepinephrine response but this demonstrates it in women. Also, the smaple size is small (N=11), so it good to have multiple studies showing similar effects.
- Salbaum, J. M. et al. Chlorotoxin-mediated disinhibition of noradrenergic locus coeruleus neurons using a conditional transgenic approach. Brain research 1016, 20-32, doi:10.1016/j.brainres.2004.03.078 (2004).
- Gregg, C. et al. White matter plasticity and enhanced remyelination in the maternal CNS. The Journal of neuroscience : the official journal of the Society for Neuroscience 27, 1812-1823, doi:10.1523/JNEUROSCI.4441-06.2007 (2007).
- Christman, J. V. & Gisolfi, C. V. Heat acclimation: role of norepinephrine in the anterior hypothalamus. J Appl Physiol (1985) 58, 1923-1928 (1985).
- Wigal, S. B. et al. Catecholamine response to exercise in children with attention deficit hyperactivity disorder. Pediatric research 53, 756-761, doi:10.1203/01.PDR.0000061750.71168.23 (2003).
- Goekint, M., Roelands, B., Heyman, E., Njemini, R. & Meeusen, R. Influence of citalopram and environmental temperature on exercise-induced changes in BDNF. Neuroscience letters 494, 150-154, doi:10.1016/j.neulet.2011.03.001 (2011). This study had an N=8 (okay, tiny) but… it demonstrated that hyperthermia and exercise synergize to elevate BDNF. This is awesome. Who doesn’t want more BDNF?
- van Praag, H., Christie, B. R., Sejnowski, T. J. & Gage, F. H. Running enhances neurogenesis, learning, and long-term potentiation in mice. Proceedings of the National Academy of Sciences of the United States of America 96, 13427-13431 (1999).
- Maniam, J. & Morris, M. J. Voluntary exercise and palatable high-fat diet both improve behavioural profile and stress responses in male rats exposed to early life stress: role of hippocampus. Psychoneuroendocrinology 35, 1553-1564, doi:10.1016/j.psyneuen.2010.05.012 (2010).
- Pedersen, B. K. Muscle as a Secretory Organ. Comprhensive Physiology (2013).
- Koltyn, K. F., Robins, H. I., Schmitt, C. L., Cohen, J. D. & Morgan, W. P. Changes in mood state following whole-body hyperthermia. International journal of hyperthermia : the official journal of European Society for Hyperthermic Oncology, North American Hyperthermia Group 8, 305-307 (1992).
- Liu, X. L. et al. [Therapeutic effect of whole body hyperthermia combined with chemotherapy in patients with advanced cancer]. Zhong nan da xue xue bao. Yi xue ban = Journal of Central South University. Medical sciences 31, 350-352 (2006).
- Narita, M. et al. Heterologous mu-opioid receptor adaptation by repeated stimulation of kappa-opioid receptor: up-regulation of G-protein activation and antinociception. Journal of neurochemistry 85, 1171-1179 (2003). This study was done in mice but shows that repeated activation of kappa opioid receptor causes mu opioid receptor to become more sensitive to beta-endorphin. This study provides a mechanism by which the dysphoric feeling from exercise or heat stress can ultimately result in a better “endorphin high.”
- Xin, L., Geller, E. B. & Adler, M. W. Body temperature and analgesic effects of selective mu and kappa opioid receptor agonists microdialyzed into rat brain. The Journal of pharmacology and experimental therapeutics 281, 499-507 (1997).
- Heckmann, J. G., Rauch, C., Seidler, S., Dutsch, M. & Kasper, B. Sauna stroke syndrome. Journal of stroke and cerebrovascular diseases : the official journal of National Stroke Association 14, 138-139, doi:10.1016/j.jstrokecerebrovasdis.2005.01.006 (2005). This reference is only an N=1 where a a man had consumed several glasses of wine before he got in the sauna and was, subsequently, found dead. Alcohol consumption while in the sauna can cause severe dehydration, hypotension, arrhthymia, and embolic stroke. This is also reviewed in reference #1.
APL (American Performance Labs) is a research group dedicated to the collection, analysis, and dissemination of published research and articles on the science of hyperthermia and the various applications, technologies and protocols for the use of hyperthermic conditioning.