In my first year of naturopathic medical school, stuck right in the middle of the standard basic science classes, was a class on hydrotherapy- evidence of how fundamental the use of hot and cold water as a healing modality was viewed in our training. They introduced it to us early, and it was a core piece straight through my time in clinic. We all rotated through our first year treating patients by administering various hot and cold applications as prescribed by the doctors on shift. Like other fundamental teachings in ND medical school (intestinal permeability as a driver of chronic disease comes to mind), hydrotherapy was once dismissed as quackery, but has since roared onto center stage with a bunch of good science behind it and rebranded as intermittent thermal stress. Make no mistake, while I acknowledge and appreciate the early understanding of “nature cure” doctors (and in fact, cultures and civilizations throughout history), my heart sings with the scientific validation being put behind this time honored and powerful tradition. And I say, bring it on!
In this blog, we’ll look at the latest research, uncovering transformative insights and practical applications of intermittent thermal stress, and explore how these hormetic stressors can change how we think, feel, and age – in support of closing the healthspan-lifespan gap.
“Hormesis is a biological process where a beneficial effect is triggered by a limited exposure to something harmful or challenging to the body. As the body responds to a mild stressor, it kicks on your bio-transformation system, which then releases a broad swath of antioxidant and anti-inflammatory molecules into circulation that provide benefits well beyond the stress exposure itself.” – excerpt from Younger You: Reduce Your Bio Age and Live Longer, Better by Dr. Kara Fitzgerald, ND.
Research on moderate heat stress
We have learned a lot about the physiological responses to heat stressfrom studying the time-honored practice of sauna culture in Finland. Finland consistently ranks as the happiest country on earth and this small nation of just over 5M people also boasts one of the longest healthy life expectancies (HALE) at 60 years, leading some to propose that sauna culture is the cornerstone of Finnish health and wellness. But, does the research back this up?
In the acute setting, both exercise and sauna raise the body’s core temperature, which results in a substantial redistribution of blood to the skin to facilitate sweating. Like exercise, sauna elevates heart rate, up to 150 bpm, which corresponds to a 60-70% increase in cardiac output. In fact, a recent study found that a 25-minute sauna session elevated heart rate and blood pressure to levels typical of light-to-moderate cardiovascular exercise (60-100 watts). What’s fascinating about these studies is that they open the possibility that individuals with limited mobility might experience exercise-like benefits from intermittent heat stress alone.
“A traditional Finnish sauna has dry air (humidity 10%-20%) with a relatively high temperature. The recommended temperature for a sauna is usually 80°C to 100°C at the level of the bather’s face. Humidity is temporarily increased by throwing water on the hot rocks of the sauna heater.” – Laukkanen et al.
But are there long-term effects of long-term sauna use? Several large-scale observational studies have examined cardiovascular and all-cause mortality in frequent sauna users. One famous study, published in JAMA in 2015, found that Finnish men were 63% less likely to die from sudden cardiac death and the risk for all-cause mortality was 40% lower. [this is extraordinary… with the exception of certain omega 3 and exercise studies, what comes close to showing such potent beneficial outcomes? Certainly no drug…] Importantly, both the frequency and duration of sauna were associated with the greatest benefits (more on that below). But, a limitation of the study was that it was only performed in male participants. A more recent study from the same group that included women also found a potent, inverse association between frequent sauna use and risk of cardiovascular disease mortality. While these observational studies cannot prove causation, the ability of sauna use to lower blood pressure, and the risk of hypertension also suggests that there are long-term benefits with respect to cardiovascular health.
We have gained a solid understanding of the cellular-level effects of intermittent heat stress, which further strengthens its significance in health and longevity. Heat shock proteins (HSPs), are cellular chaperones responsible for maintaining protein structure (loss of proteostasis is a hallmark of aging) Heat can irreversibly denature proteins, as demonstrated when boiling an egg, causing the liquid egg-white and yolk to solidify. To protect cells from heat stress, organisms have evolved HSPs that maintain proper protein structure. In the world of proteins, function follows form.
HSP activity plays a vital role in protecting against accumulation of misfolded protein aggregates seen in atherosclerosis, Alzheimer’s, and Parkinson’s disease. Sauna has been shown to increase HSP activity by nearly 50% in both men and women following a single 30 minute session. The beneficial effects of sauna on HSP activity might also synergize with other anti-inflammatory adaptations to heat stress – as sauna is associated with reduced C-reactive protein (pro-inflammatory) and elevated IL-10 (anti-inflammatory).
Sauna culture might partially explain why Finland is a world leader in healthspan, but is it also behind their standing as the world’s happiest nation? In a double-blind study published in JAMA Psychiatry in 2016, patients suffering from depression were subjected to whole body hyperthermia (core temperature raised to 38.5℃). Compared to placebo, hyperthermia produced significant and persistent antidepressant effects. Similarly, frequent sauna use is associated with a reduced risk of psychotic disorders. Taken together, the effects of intermittent heat stress – through sauna in particular – appear to have profound effects on both physical and mental health.
How to implement heat therapy
Now I know you’re probably saying, “I don’t live in Finland and don’t have access to a sauna!” If you happen to be one of my devoted followers from Finland, “Hyvää päivää! (Good day!)” But if not, I’ve got you covered. There are sauna locations in the United States and other countries. Additionally, other forms of intermittent heat stress, like infrared saunas or hot baths are also associated with improved cardiovascular function and biomarker profiles – albeit not to the same degree as saunas. And in terms of terms of the duration, temperature, and frequency – the consensus from the literature is that sauna sessions lasting more than 19 minutes at 174℉ (79℃), at least four-to-seven times a week provide the most benefit, with modest benefits seen at two-to-three sessions per week.
Hydrate well when engaged in sauna treatment. I recommend using an electrolyte replacement formula to put back in what you’ve lost during sweating, like you would for exercise. Here’s our recipe.
Research on moderate cold stress
When it comes to harnessing extreme temperatures for our well-being, we can’t overlook the invigorating effects of moderate cold stress, too. From the Nature Cure doctors I mentioned above, to ancient Greeks and Romans and modern high-performance athletes, the use of cold therapy has left an enduring mark throughout history.
While the concept of cold exposure’s impact on inflammation has been passed down through the ages, there is remarkable research on the effects of cold exposure on longevity in many organisms – whether it is fruit flies, nematodes, or mammals – moderate cold exposure tends to increase life expectancy across the board.
Research on cold exposure has mainly focused on its effects on regulating metabolism, where mitochondria are the literal metabolic powerhouses of the cell. Mitochondrial dysfunction is a hallmark of aging and is caused, in part, by impaired mitochondrial biogenesis. Cold water immersion has been shown to stimulate mitochondrial biogenesis in brown adipose tissue (which is brown because it’s dense with mitochondria and beneficial for energy production, glucose and insulin homeostasis) and skeletal muscle. Additionally, intermittent cold stress results in a 45% increase in brown adipose tissue volume and a 2.2-fold increase in oxidative capacity. The collective findings from these studies indicate that cold stress can enhance mitochondrial health and resilience, likelyleading to improved metabolism.
Interestingly, HSPs are also induced following cold shock, and there are also specialized cold-shock proteins that appear to be specific for cold stress. A study published in April 2023 found that cold shock prevents disease-related accumulation of misfolded proteins – further exploration of these connections holds intriguing possibilities for understanding the potential impact of cold stress on longevity.
How to implement cold therapy
When it comes to applying intermittent cold therapy, start by gradually introducing cold exposure to your routine, choose a safe and controlled method, such as cold showers or baths. Start with shorter durations and gradually increase over time as you monitor your body’s response – mild discomfort is normal, but if you experience excessive shivering or discomfort, it’s important to stop and warm up. In terms of time and temperature, 15 minutes at 50℉ (10℃) appears to support mitochondrial biogenesis.
While hot and cold therapies can offer a range of potential benefits, and are generally well-tolerated and safe, it’s important to exercise caution and be aware of potential contraindications. Individuals with certain medical conditions, such as cardiovascular diseases, circulatory disorders, Raynaud’s disease, immunodeficiency, or uncontrolled hypertension, should consult with their healthcare provider before engaging in hot or cold therapies. Pregnant people, young children, the elderly, and individuals with impaired temperature sensitivity are advised not to use hot and cold therapies. Finally, avoiding alcohol or other drugs while engaging in hot or cold therapies is imperative.
As a functional medicine practitioner, I am equally inspired by innovative and centuries-old approaches to optimize health and promote longevity. Intermittent thermal stress is a smart and reasonably affordable addition to a core longevity program such Younger You. And as I wrap up this exploration of intermittent hot and cold stress and their impact on our health, I am reminded of the power of hormesis – the concept that small doses of stress can activate our body’s adaptive mechanisms and promote resilience.
From the ancient practices of sauna culture to the latest cutting-edge research on cellular adaptations, we have seen how intermittent thermal stress can stimulate beneficial physiological responses. By embracing these stressors in a mindful and controlled manner, we have the potential to enhance our well-being, optimize our metabolism, and even extend our healthspan and lifespan. It’s fascinating to witness the interplay between our bodies and the environment, and how harnessing the power of acute stressors can truly transform our lives.