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

Heat immediately puts cells to work protecting themselves. As soon as the temperature rises, cells produce extra heat shock proteins. These proteins act as chaperones: they capture damaged proteins, prevent them from clumping together, and help the cell restore its normal function. This protective system is evolutionarily ancient and operates in virtually all animals. Afterwards, cells attach a chemical tag to the most severely damaged proteins to mark them for breakdown. Without that repair mechanism, the cell cannot properly recover after the heat stimulus.

Repeated heat exposures can also make cells more resilient to other types of stress, not just heat itself. This effect has been demonstrated in people who train in warm conditions, and in animal research. Whether ordinary sauna visits provide the same protection cannot be stated precisely on the basis of the available studies.

Extreme heat is a different matter. During heatstroke, the intestine loses adequate blood flow, bacterial toxins leak into the blood, and dying cells send out alarm signals that trigger severe inflammation throughout the entire body. This can lead to multi-organ failure. This danger is separate from normal sauna use in healthy individuals, but it shows that heat in high doses genuinely causes harm.

There are also specific risk groups for whom heat deserves extra attention. Heat above normal body temperature can impair fertility in both men and women and disrupt early embryonic development. There is furthermore evidence, based on associative research, that heat stress during pregnancy may influence the brain development of the child and increase the risk of psychiatric disorders. Causality has not yet been established. Finally, a new study in mice and humans shows that heat stress can, via a signalling pathway from the skin, leave a lasting imprint on the brain cells that regulate metabolism, potentially making a person more susceptible to abdominal fat and metabolic problems later in life. This is a striking finding from a single study and requires replication before any conclusions may be drawn.

For ageing cells, a separate caveat applies. As cells grow older, they do activate the protective response upon heat exposure, but as a result they may be less able to produce the normal emergency structures that cells typically deploy under stress. Whether this represents a relevant risk during sauna use at an older age cannot be inferred from this laboratory research.