Throughout a long evolutionary history, nature has endowed us with a fine-tuned thermal-regulatory mechanism. Despite its limitations, we still owe much of our survival to its automaticity (i.e., operates independently of volition) and plasticity.
It goes without saying that temperature can affect our health in so many different ways. When exposed to intense heat, our body, in an attempt to maintain its internal core temperature, would cool itself down by simultaneously triggering various heat-releasing mechanisms (e.g., sweating, panting, vasodilation). We might even reflexively grab a bottle of water, or head to a faucet and pour some water onto our skin. In any case, a body under such conditions are said to be under heat stress, with nausea, lethargy, dehydration and tiredness being some of its typical symptoms.
In fact, it’s fairly common to experience dizziness, while doing construction work or running in a soccer field under intense sunshine. In more extreme cases, heat stress can cause hyperthermia, leading to fainting or even death (this is especially true with immune-compromised elders). In less extreme cases, mild-but-chronic heat stress can induce accelerated tissue damages and promote systemic chronic inflammation.
On the other end of the spectrum, when the temperature regularly hits below 0°C, one easily find themselves shivering uncontrollably and their skin hardening. In such cases, blood circulation slows down and one is then forced to move faster — to counteract the rapid drop in internal core temperature and the potential of organ freezing. A body under such conditions is said to be under cold stress. and, as with heat stress, sometimes deserves special attention. Cold stress can render the body more prone to injuries (e.g., frostbite, leg twist), which in the more extreme cases can lead to suffocation and hypothermia (not to mention the potential of ears falling off and limbs frozen beyond repair). In the less extreme cases, mild-but-chronic cold stress can cause headaches and joint issues.
Hormetic Thermal Stress — Introduction
While even mild chronic thermal stress can negatively impact our health, it would be shortsighted to exclusively stay within our comfort zone. In fact, evolutionary clues suggest that a bit of both heat and cold stress, applied under carefully controlled conditions, can actually help streamline our thermal-regulatory mechanism, extend our thermal tolerance range and even potentially resolve certain temperature-related illnesses:
- The regular applications of hormetic cold stress enhance our ability to preserve and generate heat when it’s called for. For example, it prompts the contraction of the muscles underneath hair follicles, closing the pores while activating an efficient heat-generating mechanism via the burning of the then-acquired brown adipose tissues (a.k.a brown fat). In addition, cold stress triggers vasoconstriction (i.e., constriction of blood vessels), which minimizes heat loss during blood circulation, while simultaneously redirecting heat to the most vital organs — All of these to guarantee efficient heat conservation and distribution. In general, hypothermic conditioning has important implications in immune activation, weight loss and longevity.
- Hormetic heat stress, on the other hand, is also beneficial in terms of optimizing heat dissipation. For example, it promotes sweating (which also allows the body to release potential toxins), and vasodilation (i.e., opening of blood vessels). This would increase blood circulation and subsequently streamline the delivery of glucose, oxygen and other nutrients to our organs and muscles. Result? Improved endurance (by reducing the depletion of glycogen stores), lower risk of injury (by slowing down protein degradation, ultimately leading to an increase in net protein synthesis rate) and a more efficient thermo-regulatory mechanism.
Hormetic Thermal Stress — The Science
What does the scientific literature say about the effects of temperature on different creatures? Well, the experimental findings are not always rosy, but let’s find out more!
Due to their short lifespan, small size and low maintenance cost, the fruit fly model (Drosophilia Melanogaster) has been adopted extensively for a variety of research purposes. In particular, fruit flies were the subjects of several studies on the effects of thermal stress:
- Three previous studies collectively found that mild heat stress, especially when applied at a younger age, increases the lifespan of fruit flies (Le Bourg 2008).
- On the opposite end of the spectrum, a 2007 paper appearing in Biogerontology found that for young fruit flies (5 days of age), cold exposure (1 hour of 0°C water per day, for 10 days, with a 2-days break in between the 5th and the 6th day) led to increased longevity, increased resistance to heat stress and delayed behavioral aging (the latter being a phenomenon not observed in fruit flies undergoing heat stress).
- In a more recent study (Le Bourg 2010), it was found that for fruit flies soaked in 0°C water, one hour per week for 2 weeks, in addition to being more resistant to heat, survives longer after being infected by a fungus. In general, the earlier the soaking, the longer the after-the-infection lifespan, suggesting that the same cold stress that are beneficial for the younger flies are less beneficial for the older ones.
While fruit flies are physiologically very distinct from human, these studies can still provide a glimpse as to why cold stress and heat stress go hand in hand.
Another favorite among the researchers, the rodent model sometimes provides great insight on the general behavioural changes a mammalian body undergoes when confronted with certain stress:
- In a 1986 experimental study by Holloszy and Smith, 6-months-old Long-Evans rats were gradually conditioned to stay in 23°C water. After 3 months of training, these rats were staying in water for 4 hours per day, 5 days per week (this treatment continued until the rats reached 32 months of age). The average age for this experimental group was found to be 968 (±141) days, while the same number for the control group was 923 (±159) days. However, the increase in longevity is not statistically significant, and while the cold-immersed rats did have less occurrence of sarcomas and carcinomas, they also suffered increased vascular pathologies (Le Bourg 2010).
In this particular study, the magnitude and duration of the cold immersion might have been too high for it exert any clinically-significant hormetic effects. As Le Bourg pointed out elsewhere:
“the problem of heterogeneity in populations has to be taken into account: a stress could be mild at young age but more severe in elderly people and, among them, even more deleterious for some persons.”
The scientific papers on the effects of thermal stress on human tends to fall into 2 groups: those focusing on reversing disease symptoms, and those focusing on life extension or delaying senescence:
- In a review article on sauna studies, Talebipour et al. suggest that when sauna is applied safely, the resulting vasodilation can improve blood circulation, leading to increased cardiac output and better peripheral endothelial functions in the arteries. This would then have important implications in the treatment of various cardiovascular diseases such as congestive heart failure and arterial hypertension. In fact, a recent February 2015 correlational study appearing on JAMA Internal Medicine only lend more support to the beneficial effects of (safely applied) sauna on improved cardiovascular functions:
“Compared with men having a sauna bathing session of less than 11 minutes, the adjusted hazard ratio for SCD[Sudden Cardiac Death] was 0.93 (95% CI, 0.67-1.28) for sauna bathing sessions of 11 to 19 minutes and 0.48 (95% CI, 0.31-0.75) for sessions lasting more than 19 minutes (P for trend = .002)”
- A 2003 review article appearing in Experimental Biology and Medicine repeated the same messages, while adding that 2 weeks of sauna therapy (15 minutes per day at 60°C) significantly reduces body weight, body fat and blood pressure in 25 obese patients, and that 30 minutes of hot tub for 3 weeks reduces blood glucose level in patients of type II diabetes.
- A 2006 paper appearing in the Journal of Molecular Medicine found that short-term heat stress can prevent the activation of certain pro-inflammatory genes that are in part responsible for the genesis of rheumatoid arthritis.
- A 2010 randomized clinical trial concluded that spa therapy, in conjunction with home-based exercises and pharmacological treatments, is more effective in treating knee osteoarthritis than home-based exercises and pharmacological treatments alone. This suggests that knee osteoarthritis can be improved via regular, local applications of hormetic heat stress.
And of course — There is more:
- A 2002 In vitro experiment found that mild heat stress exerts anti-aging effects on human skin fibroblast. Could this be liken to the exfoliation, whereby the microscopic peeling of outer-layer skin tissues trigger tissue regrowth?
- Recently, there has been an insurgence of hydrotherapy in the literature, which leads some researchers to hypothesize a biochemical mechanism through which brief repeated exposures to cold stress activate the immune system to fight against non-lymphoid cancerous tumours (the researchers did quote a study saying that water at 14°C or below could cause cutaneous pain, and as such is best avoided).
- A December 2012 paper published in Scandinavian Journal of Clinical and Laboratory Investigation documents the effects of thermal stress on winter swimmers, and reports that while 3 minutes of swimming in 0°C water might not lead to signs of lysosomal damages, 30 minutes of 60°C sauna sessions do. Although it’s unclear whether the said lysosomal damages are hormetic in nature, the researchers did conclude the following:
“regular winter swimming combined with sauna in accordance with the theory of hormesis may lead to some adaptation and thermotolerance, which protects cellular membranes against the deleterious effects of thermal and other stress factors.”
Here is Dr. Rhomda Patrick talking the benefits of hyperthermic conditioning, putting an emphasis on the biochemical changes following the exposure to hormetic heat stress:
From our end, we also have some additional and corroborating findings. In particular:
- An appropriate magnitude and duration of cold stress cancels out chronic/unpleasant heat stress, and vice versa. It therefore makes sense to talk about targeted stress and its reverse stress.
- By carefully tweaking the magnitude of reverse stress, one can undo the symptoms of certain temperature-related chronic illnesses (e.g., residual burning, frostbite, arthritis), and encourage favourable functional adaptations such as thermal resistance and accurate relative-temperature perception.
After a bit of self-experimentation, these observations give rise to a biohack which we refer to as the reverse-temperature method. In the application post, we outline how we can apply this method in various contexts to reverse certain temperature-related illnesses and further provide additional health benefits.
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