Two of the hottest (and coldest) wellness trends that are taking over Hollywood are therapies claimed to use extreme temperatures to stimulate the body’s natural healing processes. But does the science really stack up?

If you have spent any amount of time on the internet lately, you may have heard of infrared saunas and whole-body cryotherapy, wellness trends that boast a whole range of health benefits, from melting away fat and boosting heart health to aiding in exercise recovery and flushing out toxins.

Sounds pretty hopeful - but before you jump on the bandwagon, let’s reveal the facts and myths behind these popular treatments, including how they work, whether they really are beneficial, and what risks they may pose.

Infrared sauna

Infrared saunas have become increasingly popular in recent years as more people seek natural and holistic ways to improve their well-being. In a traditional sauna, known as a Finnish sauna, the body gets hot mainly from the warm air and steam around it. By contrast, an infrared sauna uses electrically heated panels to emit infrared light, penetrating the skin and heating the body directly without warming the air. Finnish saunas typically reach 176°F to 212°F, whereas the temperature of infrared saunas is usually lower, at 113°F–140°F.

The benefits of saunas are attributed to a concept known as hormesis. Hormesis is based on the idea that when the body is exposed to a mild increase in temperature, molecules called heat shock factors (HSFs) become activated. These molecules switch on the production of other molecules called heat shock proteins (HSPs). HSPs are proteins that help other proteins stay healthy and work well. They do this by helping them fold into the right shape, fix them if they get damaged, put them together or take them apart, and get rid of them if they are not needed. Cells need healthy and functional proteins, because defective proteins can cause diseases like cancer and brain diseases. HSPs can fix or remove these proteins to prevent or treat these diseases.

Infrared saunas are thought to boost the heart’s function in particular. To confirm this theory, one 2018 systematic review analyzed seven studies to assess whether time spent in an infrared sauna may help to improve outcomes in people with heart failure. Specifically, the researchers looked at studies that included 491 adult subjects with heart failure, including 275 subjects who used an infrared sauna for treatment and 216 subjects who did not. Researchers evaluated the subjects’ outcomes such as the severity of symptoms, blood pressure, levels of brain natriuretic peptide (BNP) - a hormone produced by the heart that helps regulate blood pressure - and imaging indicators that examined the structure and activity of the heart.

While infrared sauna usage did not affect the subjects’ blood pressure, using a sauna resulted in other short-term heart-related improvements. Sauna users who spent 15 minutes at 140°F followed by a 30-minute rest in a warm environment, five times a week for 2 to 4 weeks saw the most impact. Specifically, a reduction in their BNP was seen, signifying an improvement in cardiac function. BNP levels are elevated in patients with heart failure, as the heart has to work harder to pump blood around the body. A reduction in BNP levels means that the heart is less stressed and more efficient. In addition, the sauna users’ ejection fraction improved by 1.45% compared to the non-sauna users. Ejection fraction is the percentage measurement of how much blood the heart pumps out with each contraction, where an ejection fraction of 40% or less indicates heart failure. The sauna users also saw a 1.3% higher improvement in how well their blood vessels widened in response to greater blood flow, and they also experienced a 38% reduction in cardiac events compared to non-sauna users.

Despite these encouraging results, the researchers did caution that because of bias and imprecision in the studies they evaluated, the quality of the evidence was not very high. Also, the benefits were not long lasting. In other words, according to this review, the evidence that sauna bathing helps heart failure is weak and inconsistent, and further research is needed to reach clear findings.

Whole-body cryotherapy

Whole-body cryotherapy is a trendy treatment particularly for athletes that briefly exposes the body to subzero temperatures. Cryotherapy sessions can reach as low as -166°F to -220°F and typically last two to three minutes. It can involve either standing in a can-shaped device that is open on top where only the body is frozen while the head stays at normal temperature, or seated in a closed chamber where the whole body, including the head, is frozen by liquid nitrogen.

Many people swear by cryotherapy’s benefits for ailments like rheumatoid arthritis, asthma, anxiety, chronic pain, and even weight loss, and it has become especially popular among athletes seeking to avoid injury and reduce harmful inflammation after physical activity to promote recovery. It is believed to be beneficial because it lowers the temperature of bodily tissues, which in turn cools down the body’s core temperature. This sensory shock is claimed to cause the activation of the autonomic nervous system, a network of nerves that controls the body’s involuntary actions such as the heart rate and the breathing rhythm. The theory is that the cold helps stimulate the body’s natural healing abilities to reduce inflammation and pain or swelling in the same way that ice is used.

Though these outcomes appear optimistic, the FDA and other experts claim these benefits are not backed up by any reliable evidence. For example, one study recruited 40 healthy men and divided them into five groups, where each group experienced a different temperature in a whole-body cryotherapy device (−166 °F, −76 °F, 14 °F, and a control temperature of 75 °F). Each group went into the device for five days in a row, for three minutes each time. The researchers measured how the cryotherapy affected their heart rate variability, the fluctuation in the time intervals between consecutive heartbeats, as well as the amount of norepinephrine in the blood, a hormone and neurotransmitter involved in the body’s stress response that increases alertness, heart rate, and blood pressure. These metrics can signify how fast post-exercise recovery may occur.

After five sessions of −166 °F exposure, heart rate variability increased by 20 ms, indicating that the body was able to restore its balance and adapt to the physical stress imposed by the exercise more efficiently. However, norepinephrine also increased, suggesting that the body was still in a state of stress and had not returned to its normal resting state. Higher norepinephrine also suggested that the nervous system was overworked or fatigued by the exercise.

In sum, spending time in the coldest temperature (−166 °F) just once can enhance autonomic nervous system activity, which is beneficial for recovery, as indicated by the higher heart rate variability. However, this condition also increases norepinephrine level, which is a marker of stress and may impair recovery. Temperatures of −76 °F and 14 °F do not have a significant effect on autonomic nervous system activity or norepinephrine level. The researchers also noted that the autonomic response to cold appeared to diminish over time, as the norepinephrine increase was no longer observed on day 5.

The risks you should know

Although more evidence is needed to determine the true extent of how using saunas can benefit health, saunas are generally considered safe for most people. But if you have uncontrolled high blood pressure, heart disease, low blood pressure, or other serious heart issues, you should check with your doctor before using a sauna. Also, avoid alcohol and medications that may impair sweating and produce overheating, stay in no more than 15 to 20 minutes, cool down gradually, drink plenty of water, and stop if you feel unwell.

And while cryotherapy may not have proven long-term benefits for healing, it has many obvious risks. For example, using liquid nitrogen for cooling can cause asphyxiation, a condition where the oxygen level in a closed room drops due to the presence of nitrogen vapors. This can make the user faint from lack of oxygen. Also, the user may suffer from frostbite, burns, and eye damage because of the very low temperatures.

The bottom line

Infrared saunas and whole-body cryotherapy are the latest wellness crazes that promise to transform health, with quicker exercise recovery and improving cardiovascular wellness. However, these claims are not well supported by the actual evidence, particularly regarding the quality of research and long-term results. So, do not rush to the sauna or cryotherapy chamber without first consulting your doctor. This is especially crucial if you have any underlying medical conditions, as extremely hot or cold temperatures can lead to some severe health risks.