What is hormesis (what does not kill you makes you stronger)?

Hormesis describes a biological pattern in which a low, mild dose of a stressor -- think heat, brief fasting, or exercise -- prompts the body to make protective adaptations, while that same stressor at a high dose is harmful. It is therefore not a case of 'more is better', but of a narrow zone of beneficial dosing. Research supports this principle across multiple types of stressors and in multiple types of organisms.

Exercise and fasting are the best-studied examples. Physical activity causes slight damage to muscle tissue and produces mild mitochondrial stress -- the cell's power plants are, in a sense, stimulated. This leads to recovery and strengthening: cells become more resilient. The same applies to intermittent fasting: the period without food activates stress-resistance mechanisms, in part through proteins involved in cell maintenance. Crucially, recovery is part of the process. Without sufficient recovery after the exertion or the fast, no benefit occurs -- only damage.

Regular sauna use (temperatures of 45 to 100 degrees Celsius) is another example of heat stress as a hormesis trigger. Large observational cohort studies found a dose-dependent association between frequency of sauna use and a reduced risk of disease and mortality. This is an association, not a proven causal relationship, but the biological responses -- such as neuroendocrine and cardiovascular adaptation -- align well with the hormesis model.

Plant-derived compounds such as sulforaphane (from broccoli), curcumin (from turmeric), and resveratrol activate protective enzyme systems in cells at low doses. Notably, plants produce these compounds precisely as a defence against herbivory; at high doses they are toxic to humans. The evidence that they do anything beneficial in humans at common, low doses is preliminary and promising, but optimal dosing has not yet been established.

A specific form is called mitohormesis: a small amount of stress to the mitochondria -- for example via certain plant compounds or exercise -- leads to protective responses that extended lifespan in animal models ranging from worms to mammals. Whether this works equally strongly in humans is still unknown; human evidence is limited. The same applies to mild oxygen deprivation (hypoxic conditioning), which is being investigated in heart and brain conditions, but where the correct dosing is far from settled and incorrect application can be dangerous.

The most important safety point is the inverted U-shape of hormesis. Overtraining, excessively prolonged fasting, or high doses of phytochemicals exceed the biological threshold within which the body can adapt, and at that point become harmful. Multiple sources state this explicitly. More, harder, or higher doses are therefore not synonymous with greater benefit; the principle only works within a limited margin that is not the same for everyone.