Finnish sauna heat exposure induces stronger immune cell than cytokine responses

ABSTRACT

Regular exposure to Finnish sauna bathing (FSB) has been associated with reduced morbidity and mortality. This study aimed to examine the acute effects of FSB on immune cell mobilization, circulating cytokines, and their associations with changes in body temperature. A total of 51 adults − 27 women (mean age 50 ± 9 years, body mass index (BMI) 27 ± 5 kg/m2) and 24 men (mean age 50 ± 10 years, BMI 27 ± 3 kg/m2) – were exposed to a 30-minute session of acute FSB at a temperature of + 73°C. Venous blood samples were collected at baseline, immediately after and 30 minutes after the FSB and analyzed for immune cells and 37 cytokines. Subjects were allowed to drink water throughout. FSB increased body temperature from 36.4 ± 0.5°C to 38.4 ± 0.7°C, without altering plasma volume. Total white blood cell (WBC) count rose significantly and remained slightly elevated 30 minutes post-sauna in women. Neutrophil and lymphocyte counts increased immediately after the FSB but returned to baseline after 30 minutes, whereas MXD cells (monocytes, eosinophils, basophils) remained elevated. The levels of only two cytokines changed significantly. Although only a few correlations were observed between changes in immune cells and cytokines, 18 significant associations were identified between changes in body temperature and circulating cytokines – particularly immediately post-sauna – but not with WBC changes. Thus, a 30-minute session of acute FSB induces immune cell mobilization. The observed associations between changes in body temperature and circulating cytokines suggest that sauna-induced heat stress, along with immune activation, may partly mediate the health benefits of FSB.

Introduction

Increasing temperatures and especially extreme heat waves due to climate change threatens the nature and well-being of humans and other animals. While chronic heat stress is generally harmful to health, short-term exposure to acute heat – even at high intensities – can produce beneficial effects. Finnish sauna bathing represents one such acute heat stress exposure, where individuals are subjected to relatively dry air at high temperatures (typically 70–100 °C) for 10–30 minutes per session. This traditional practice has been associated with a range of benefits, including reductions in the risk of hypertension, sudden cardiac death, venous thromboembolism, stroke, dementia and Alzheimer’s disease and psychosis. Additionally, sauna use has been linked to lower risks of respiratory conditions such as pneumonia, chronic obstructive pulmonary and other respiratory diseases. It may also mitigate the adverse effects of low socioeconomic status, improve physical fitness, and ultimately contribute to decreased overall morbidity and mortality. Although the physiological mechanisms mediating these health effects have been partially explored – particularly those related to cardiovascular function – the exact pathways remain incompletely understood.

The protective effects of heat therapies may, in part, be mediated through immunological responses. In our previous population-based studies, regular Finnish sauna bathing was associated with lower levels of systemic inflammation as indicated by reduced C-reactive protein (CRP) levels. Additionally, acute sauna exposure has been linked to increased white blood cell (WBC) counts. However, the specific effects of sauna-induced heat stress on leukocyte subpopulations and circulating cytokines – which play key roles in mediating immune responses – have not been fully characterized. Therefore, the aim of the present study was to investigate the acute effects of Finnish sauna bathing on immune cell mobilization and a comprehensive panel of circulating cytokines in middle-aged men and women, including their interrelationships and associations with changes in body temperature.