Acute and chronic effects of hot water immersion on inflammation and metabolism in sedentary, overweight adults.
Level 3 - non-randomized controlled study
Controlled clinical trial with a non-randomized matched control group
PubMed 30335579 · doi:10.1152/japplphysiol.00407.2018
What was done
Ten sedentary, overweight men (BMI 31.0 ± 4.2 kg/m²) underwent an acute session comparing 1 hour of hot water immersion at 39°C (HWI) versus 1 hour resting at ambient temperature (AMB). Venous blood was collected pre-session, immediately post-session, and 2 hours post-session to measure monocyte intracellular heat shock protein-72 (iHsp72), plasma extracellular Hsp72 (eHsp72), interleukin-6 (IL-6), fasting glucose, insulin, and nitrite. Participants then completed a 2-week intervention of 10 HWI sessions (INT) and were compared with 8 BMI-matched sedentary control subjects (BMI 30.0 ± 2.5 kg/m²; CON).
What was found
Acute HWI significantly increased plasma IL-6 from 1.37 ± 0.94 to 2.51 ± 1.49 pg/ml (P < 0.001) and nitrite from 271 ± 52 to 391 ± 72 nM (P = 0.04) compared with AMB, while monocyte iHsp72 expression was unchanged (P = 0.57). Following the 2-week intervention, fasting glucose decreased from 4.44 ± 0.93 to 3.98 ± 0.98 mmol/l (P = 0.04), fasting insulin decreased from 68.1 ± 44.6 to 55.0 ± 29.9 pmol/l (P = 0.04), and eHsp72 decreased by 17 ± 41% (P = 0.03) compared with CON. Resting iHsp72 expression was unchanged (P = 0.59).
Why it matters
Passive heating via hot water immersion can acutely increase nitric oxide bioavailability and improve fasting glycemic markers over two weeks, offering a potential alternative for individuals unable to exercise.
Limits
The study was small (n=10 intervention, n=8 control) and exclusively enrolled sedentary men. The chronic phase used a non-randomized matched control group, the intervention lasted only 2 weeks, and longer-term clinical outcomes were not assessed.
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