Balneotherapy, Immune System, and Stress Response: A Hormetic Strategy?

Open-access review published in the International Journal of Molecular Sciences (2018), proposing a hormetic framework for understanding why balneotherapy works. Hormesis — where mild doses of a stressor produce beneficial adaptive responses while large doses are harmful — is a well-established concept in toxicology and exercise physiology. Gálvez and Torres-Piles apply it to the combined thermal and chemical stimulus of balneotherapy.

The Hormesis Model

The central argument: balneotherapy subjects the body to a controlled, low-level physiological stress — heat stress, osmotic challenge, and chemical stimulation from humic substances. This mild stress activates adaptive responses without causing damage: increased antioxidant enzyme production, immune cell mobilisation, anti-inflammatory cytokine shifts, and heat shock protein expression. The body emerges from the treatment in a better-regulated state than before it entered.

This explains something that simple pharmacology does not: why a 20-minute peat bath produces anti-inflammatory and immune effects that persist for hours or days afterward. The compounds absorbed during treatment do not stay at therapeutic concentrations that long. But the adaptive response triggered by the combined stimulus continues to operate after the stimulus has ended.

Why Moderate Intensity Matters

Hormesis predicts a dose-response curve with an optimum — too little stimulus produces no meaningful adaptation, too much causes harm rather than benefit. In the peat therapy literature, this maps onto temperature guidelines (38–42°C maximum), treatment duration (15–30 minutes, not hours), and the structured rest period after treatment. The therapeutic protocols developed empirically across 200 years of Central European spa medicine are consistent with a hormetic framework, even though the practitioners did not describe them that way.

Limitations

Hormesis is a theoretical model, not a measured mechanism for balneotherapy specifically. The review synthesises plausible evidence but does not provide direct experimental data showing that balneotherapy’s benefits are driven by adaptive hormetic responses rather than direct pharmacological effects. Both mechanisms may contribute simultaneously.