What is hormesis (what doesn't kill you makes you stronger)?

Hormesis is the phenomenon whereby a low dose of something that is harmful in larger amounts actually produces a beneficial effect. The dose-response curve takes an inverted-U shape: a little stress helps, too much is harmful. Reviews of thousands of studies observed this pattern in approximately 350 investigated cases, across a wide range of organisms and substances. The maximum stimulating effect at a low dose was, on average, about 50% higher than at zero exposure.

The mechanism: cells respond to mild stress by activating signalling pathways that produce protective proteins. Think of enzymes that neutralise harmful substances, and proteins that repair damaged other proteins. This 'tune-up' makes the cell more resilient to a subsequent insult. Timing plays a key role here: periodic stress gives that tune-up room to work, while continuous stress overloads the system.

The best-known practical example is physical exercise. Training causes temporary muscle damage and oxidative stress, but the body adapts and becomes stronger. This improves cardiac function, muscle mass and the immune system. Overtraining reverses this effect: training too much and too frequently actually leads to greater oxidative damage and an increased risk of infection. The hormesis zone sits somewhere between complete inactivity and overtraining.

In animal research, exposure to mild stressors such as heat fluctuations, low doses of reactive compounds or caloric restriction sometimes extends lifespan. This is linked to greater resilience of the organism. Effects are, however, modest, and dependent on the right dose and the right timing. Caloric restriction and intermittent fasting work through comparable pathways, but whether this leads to the same extension of lifespan in humans has not been proven.

Plant compounds such as sulforaphane (found in broccoli), curcumin and resveratrol activate comparable protective pathways in cells at low doses. At high doses, those same substances can be harmful. Whether you actually benefit from this in practice through food or supplements has not been sufficiently demonstrated in humans, so do not overclaim.