Can heat, such as a sauna, damage your cells or actually make them stronger?

Heat does something clever inside your cells: it activates an ancient protective system. As soon as the temperature rises, cells begin producing special proteins that act like molecular first responders. They prevent other proteins from clumping together and help damaged proteins return to their normal shape. This system operates in virtually all living organisms and is one of the best-understood stress responses in biology.

After the heat stimulus, the cell also actively clears up the damage: damaged proteins are tagged so they can be broken down or repaired. That clean-up process is indispensable. Cells in which it did not work properly failed to restore normal functions such as protein synthesis and transport. The body therefore treats mild heat as a training stimulus, not as pure damage.

Gradual acclimatisation to heat, of the kind you build up through regular sauna visits, also improves resilience at the whole-body level. Organs become better protected against serious heat damage. There is even a so-called cross-protection effect: people accustomed to heat also appear to be more resistant to other forms of stress. This effect has been demonstrated in both animals and humans, although the mechanism has not yet been fully explained.

There is also a downside. New research in mice and humans suggests that repeated heat stress may affect metabolism over the longer term. Heat may raise a signalling substance through the skin that permanently alters brain cells, promoting fat storage. In mice that were subsequently given a high-fat diet, abdominal fat accumulated more quickly. Vitamin A appeared to attenuate this effect in both mice and humans. This is preliminary and requires more research, but it shows that 'more heat' is not automatically better.

Extreme heat is outright dangerous. Heatstroke, in which the gut wall leaks and toxins enter the bloodstream, can lead to inflammation, clotting problems, and simultaneous failure of multiple organs. The risk is greater if you also have a fever or recently had an infection. In addition, prolonged heat stress impairs fertility in several animal species and in humans as well, in both men and women.