Comparison of thermoregulatory, cardiovascular, and immune responses to different passive heat therapy modalities.
Level 3 - non-randomized controlled study
Within-subject crossover comparative physiological study with unstated randomization
PubMed 40332494 · doi:10.1152/ajpregu.00012.2025
What was done
Twenty healthy young adults (10 females, 10 males; age 24 ± 4 years) completed three acute passive heating protocols separated by at least one week: hot water immersion (45 min at 40.5°C), traditional dry sauna (3 × 10 min at 80°C), and far-infrared sauna (45 min at 45–65°C). Core temperature was measured at baseline and every 5 minutes during heating. Cardiac output was measured before, midway, and at the end of heating via open-circuit acetylene wash-in. Venous blood was drawn at baseline, ~20 min, 24 h, and 48 h postheating to evaluate inflammatory cytokines and immune cell populations.
What was found
Change in core temperature from baseline to end of heating was greatest in hot water immersion [+1.1°C (0.9, 1.3)] compared with traditional sauna [+0.4°C (0.2, 0.6), P < 0.01] and far-infrared sauna [+0.0°C (-0.1, 0.2), P < 0.01], with traditional greater than far-infrared (P = 0.02). Cardiac output increased most in hot water immersion [+3.7 L/min (2.9, 4.4)] versus traditional sauna [+2.3 L/min (1.8, 2.8), P < 0.01] and far-infrared sauna [+1.6 L/min (1.2, 2.1), P < 0.01], with no difference between traditional and far-infrared (P = 0.06). Hot water immersion was the only modality that increased postheating interleukin-6 [+0.386 pg/mL (0.093, 0.680), P = 0.04], natural killer cells at 24 h (P = 0.03), and CD8+ T cells at 24 h (P = 0.02) and 48 h postheating (P = 0.03).
Why it matters
This study shows that hot water immersion provides a substantially stronger thermal, cardiovascular, and immune stimulus than traditional sauna or far-infrared sauna in protocols reflecting common practice.
Limits
The sample size was small (n = 20) and limited to young, healthy participants, preventing generalization to older adults or clinical populations. Only acute single-session effects were measured, and the abstract does not state whether session order was randomized.
Cited by
- supports Hot tub immersion provides the same deliberate heat exposure benefits as a sauna.