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

Hormesis is a biphasic effect: a low dose of a stressor (such as heat, mild radiation, a small amount of a toxin, or a brief period of fasting) activates protective processes, while the same stressor at a high dose is simply harmful. The stimulatory response is modest: measurable effects typically reach no more than 30 to 60 percent above baseline values.

Under the hood, mild stress activates a network of signalling molecules that help cells repair themselves and stay protected. Think of proteins that clear out harmful substances, antioxidant enzymes, and molecules that monitor the cell's energy balance (such as the energy sensor AMPK and the stress protector Nrf2). After this activation, cells are better equipped to withstand a subsequent assault, a phenomenon also known as preconditioning. In addition, these adaptations sometimes leave a kind of 'memory' in the cell, so that the protection persists for longer.

Fasting and a low-carbohydrate diet are a concrete example: ketone bodies, the fuel your body produces when sugar is scarce, cause a mild oxidative stimulus in the mitochondria (the cell's power plants). That stimulus sets repair processes in motion, including improved mitochondrial function and reduced inflammation. In animal research into heart disease, this was found to increase resistance to tissue damage following a blood-clotting event.

In experimental models, primarily in animals and in the laboratory, hormesis is associated with a longer lifespan and a slowing of ageing. The mechanism is plausible: through repeated mild stress, the repair capacity of cells is continually reinforced. Whether this translates directly into a longer, healthier lifespan in humans has not yet been thoroughly investigated.

Hormesis also has a dark side. The same epigenetic 'memories' that make cells more resilient may, over the long term, also contribute to chronic disease processes. More worryingly, cancer cells appear to hijack hormesis mechanisms. Under stress, they form specialised survival structures that help them withstand chemotherapy. Hormesis is therefore not a simple case of 'what doesn't kill you makes you stronger'; it depends strongly on which cells receive the stimulus and in what context.