Acute 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.