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Too hot to handle: does heat exposure improve athletic performance?
Saunas have been around for centuries, and populations worldwide have benefited from their general health effects. However, as heat exposure gains popularity, athletes will undoubtedly seek guidance on its performance benefits. Chris Mallac discusses sauna use in athletic populations and the potential performance and recovery benefits for athletes.
Milwaukee Brewers relief pitcher Matt Bush sits in the dugout with a towel on his head after hitting a home run against the Milwaukee Brewers in the fifth inning at American Family Field. Mandatory Credit: Michael McLoone-USA TODAY Sports
The use of saunas has recently been made more popular among the mainstream public. This is because sauna use benefits general health, well-being, and longevity. Furthermore, sauna use is gaining popularity amongst athletes to boost performance. People have used heat and ‘heat exposure’ environments for centuries. This is evident in the sweat lodges of the American Indians, the Banya steam baths in Russia, and, more notably, the prevalence of saunas in many Finnish households.
Short-term heat exposure (usually 20-30 minutes) elicits a mild hyperthermic response, otherwise known as ‘hormetic stress.’ This activates numerous cardiovascular, neuroendocrine, and cytoprotective mechanisms to restore the disrupted homeostasis and prepare and adapt the body for future heat environments.
Types of saunas
Traditional Finnish saunas use rocks or heat elements to heat the air temperature to between 70-100°C with a relative air humidity of 10-20%(1). Conversely, far infrared saunas (FIRS) use 120-V infrared elements to radiate heat. The heat created by FIRS penetrates deeper into the adipose and neuromuscular tissues than a traditional Finnish sauna (depth of 3-4cm)(2). This leads to more profuse sweating at lower temperatures.
Figure 1: Traditional Finnish sauna
Duration and Temperature
Traditional sauna practice involves spending short periods in the sauna (5-20 minutes) and interspersing these short exposures with a cooling period. This can be as simple as a cold shower or more extreme as rolling in the snow or plunging into ice-cold water. Individuals typically repeat these cycles numerous times.
More recently, with the advent of FIRS, spending longer periods in the sauna, such as 20-30 minutes, is typical. Furthermore, there is no need to intersperse the heat exposure with cooling periods. Therefore, individuals can complete one session or two or more sessions per day for up to two hours.
Another type of sauna practice in Japan, known as Waon Therapy, exposes an individual to 60°C for 15-30 minutes, and then they lie on a bed wrapped in a thermal blanket for 30 minutes post sauna. This has been used successfully in reducing cardiac events in patients with chronic heart failure(3).
Benefits of sauna use
The observable benefits of heat environment exposure fall into three broad categories, and these benefits may be more important to some users than others.
- Health and Wellness
Sauna use has notable benefits to longevity and minimizing neurodegenerative and cardiovascular disease development. Researchers from the University of Jyväskylä in Finland have highlighted the benefits of long-term sauna use in preventing neurodegenerative and cardiovascular disease(4,5,6). The researchers found that men who sauna bathed frequently at temperatures of at least 78.9°C for at least 20 minutes suffered less from acquired neurodegenerative and heart diseases, irrespective of age, activity levels, and other lifestyle factors that may influence health. Furthermore, the researchers found that:
- Men who use the sauna 2-3 times per week were 27% less likely to die from cardiovascular-related diseases.
- Men who used the sauna 4-7 times per week were 50% less likely to die from cardiovascular-related diseases (indicating a dose-dependent interaction).
- Frequent sauna users were 40% less likely to die from all causes of premature death.
- Men who use the sauna 4-7 times per week have a 66% less chance of developing dementia and a 65% less chance of developing Alzheimer’s disease than men who only use it once per week.
- Those who used the sauna 4-7 times per week were 77% less likely to develop psychotic disorders.
- Longer durations (>19 minutes) had more robust effects on cardiovascular protection than moderate durations (11-18 minutes).
Heat Shock Proteins
The primary metabolic response of repeat sauna use is the release of Heat Shock Proteins (HSPs), found in all cells(7). The HSPs are usually dormant under normal homeotic conditions and facilitate several aspects of protein synthesis, such as assembly, folding, export, turnover, and regulation(8).
In the normal course of metabolic processes and immune function, reactive by-products (reactive oxygen species and reactive nitrogen species) can damage and disrupt the structure of proteins. Furthermore, these by-products can aggregate and clump together to form ‘plaques.’ An example of this is tau proteins found in the brain. As these unravel and form ‘tau tangles,’ they can clump together to form plaques. These amyloid plaques are the precursor to Alzheimer’s disease.
These intrinsically disordered proteins are features found in conditions such as;
- Cardiovascular disease
- Alzheimer’s disease
- Parkinson’s disease
- Huntington’s disease
Heat Shock Proteins are ‘molecular chaperones’ that prevent protein aggregation and facilitate the folding system of cellular proteins. In addition, they can chaperone improperly folded proteins along the protein degradation pathway(9).
Nuclear factor-erythroid factor 2-related factor (Nrf2) is a transcription factor that regulates a host of genes in the cell cytoplasm in normal and stressed situations. These factors have unique functions, such as detoxification, antioxidative defense, autophagic degradation, and protection against chronic inflammation(10).
Heat exposure activates Nrf2. This then upregulates an HSP known as heme-oxygenase-1 (HO-1). The upregulation of HO-1 inhibits the expression of several pro-inflammatory molecules involved in the pathophysiology of cardiovascular disease(11).
The pro-inflammatory cytokine Interleukin-6 (IL-6) is released in a heated environment and plays a role in the early stages of inflammation by dampening the inflammatory cascade by activating Interleukin-10 (IL-10). Interleukin-10 inhibits tumor necrosis factor alpha (TNF–α) and C-reactive proteins (CRP), reducing the inflammatory cascade(12-14).
Health and Wellness
The stress adaptation response to repeated sauna bathing influences hemodynamic and endocrinological parameters(15). Researchers at RMIT University in Australia summarized the influences as follows:
- It increases cardiac output and reduces peripheral vascular resistance.
- Decreased systolic and diastolic blood pressure
- Increased heart rate variability (HRV)
- Improved flow-mediated arterial and vasodilation of blood vessels
- Increased nitric oxide metabolites in the bloodstream
- Decreased total and low-density lipoproteins cholesterol levels
- Increased serum levels of growth hormones
- Decreased fasting blood glucose levels
Therefore, regular sauna use benefits those suffering from high blood pressure, peripheral vascular disease, high cholesterol, and type 2 diabetes.
For the athlete, the noted benefits of repeat sauna use are in the realm of athletic performance.
Acute exposure to heat causes a transient change in plasma volume. This acts like a ‘reservoir’ of fluid for future sweating episodes to protect the body from increases in core temperature. In this sense, saunas improve heat tolerance. In addition, when undertaken repeatedly, the body will acclimatize to the heat. This compensatory mechanism is known as ‘hormesis.’ Therefore, an athlete preparing for an upcoming event in a hotter environment will benefit from short-term sauna use in the weeks preceding this event. Researchers in Australia found that 30 minutes of daily sauna use for ten days results in a 17.8% expansion of plasma volume by day four. This was associated with a trend back towards pre-sauna levels by days 7-10(16).
In another study, athletes who sauna bathed for 30 minutes immediately after a running training session improved their run time to exhaustion by 32%. These subjects underwent four sessions per week over three weeks. The 32% increase in run time correlated to a 2% increase in the 5000m time trial time(17). Although this study is small, it provides insight into the potential effects of sauna use to improve performance.
Frequent sauna use mimics exposure to moderate exercise intensities. Therefore, it is a viable alternative to exercise for those with injury or chronic disease who cannot participate in exercise(18).
Preventing Muscle Atrophy
Far infrared saunas are a useful adjunct in the prevention of muscle atrophy. This is important to the athlete who has recently been injured and cannot train (e.g., postsurgical immobilization).
A few studies have shown the benefits of heat therapy in the prevention of muscle atrophy;
- In a rodent study, researchers showed that alternate day heating in a 41-degree environment for 30 minutes and reloading a recently immobilized limb (seven days) caused an elevation in heat shock proteins. This resulted in a 30% greater increase in muscle mass following seven days of immobilization compared with reloading alone(19).
- In a human study, researchers subjected participants to direct local muscle heating via short wave diathermy for two hours per day for six days and found marked increases in the HSP-70 and -90 and phosphorylation of AMP-activated protein kinase and ERK ½ phosphorylation(20).
The results suggest that heat use during injury/recovery may be beneficial in preventing the loss of muscle tissue and improving cellular functions(20).
Growth Hormone Release
Heat exposure results in a transient increase in growth hormone levels. For example, high-frequency use in an 80-degree sauna causes a transient increase in circulating growth hormone levels(21). Two 30-minute sauna sessions in the morning (30-minute break between sessions) and two 30-minute sessions at night for seven days increased GH levels by 16-fold after the first exposure. The GH levels remain elevated 4-5x on day three and 3x on day seven. This highlights that high-frequency sauna use over a week (two hours per day) may cause a transient increase in GH levels(21). This may be important for an athlete who needs to maintain or restore muscle mass quickly (such as after an injury).
Sauna use improves health, well-being, longevity, and athletic performance. Heat exposure through sauna use will improve cardiovascular fitness by developing more efficient cooling mechanisms and improvements in cardiac function. For the injured athlete, the increase in HSPs and growth hormone will alleviate the inevitable muscle atrophy associated with the lack of training. Heat exposure continues to gather popularity within sports medicine, and high-quality evidence is required to support its use in athletic populations. However, the positive health benefits of sauna use may provide athletes with a tool to enhance their performance or recovery from injury.
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