Stronger Through Stress: The Hormesis Principle for Greater Resilience

Contrary to the popular belief that stress is inherently harmful, the principle of **hormesis** teaches us something surprising: small, controlled doses of a potentially harmful stressor can trigger a positive, adaptive response in the body. Known as a biphasic dose-response relationship, this concept shows that moderate challenges strengthen our resilience. Science journalist Richard Friebe puts it well: "What doesn't kill you makes you tougher." In modern daily life, we often lack the mild stress signals our ancestors were exposed to, such as hunger, cold, or physical exertion. That makes it more important than ever to deliberately build these back into our routines, to "train" our cells' own protective and repair mechanisms.

Published on 2026-08-25 · Wissenschaft & Forschung

How hormesis works at the cellular level

The effects of hormesis can be described at the molecular and cellular level. Three mechanisms are named most often – but they are not equally well supported, and we say at each point how well.

• Autophagy: Literally “self-eating”, a cellular clean-up and repair process. In cell and animal studies it is triggered by temporary nutrient withdrawal, which inhibits a signalling pathway called mTOR (mechanistic target of rapamycin). The cell begins recycling damaged components. Important caveat: autophagy is very hard to measure in a living human, and we found no human study that measured it directly under intermittent fasting. The mechanism is well understood – that fasting switches it on to a meaningful degree in people is a plausible assumption, not a measured finding.
• Mitochondrial biogenesis: Mitochondria are the power plants of our cells, and their function declines with age. High-intensity interval training (HIIT) stimulates the formation of new mitochondria. The trigger is not, as often claimed, a lack of oxygen in the muscle, but the energy state of the cell: the signalling chain runs through AMPK, p53 and PGC-1α. In a study using muscle biopsies, p53 activation after intense interval training was markedly stronger when carbohydrate stores were low – an energy signal, not an oxygen signal.
• Antioxidant defence: This is the best-supported part. Mild stressors such as physical exertion activate the body’s own antioxidant defences – among them superoxide dismutase and glutathione peroxidase.

The finding that shows the principle most clearly

39 men trained for four weeks; half of them additionally took 1,000 mg of vitamin C and 400 IU of vitamin E per day. Insulin sensitivity improved only in those without the supplements. Likewise, only without supplements did the body’s own protective enzymes increase.

The antioxidants cancelled the training effect. That is the heart of the matter: the mild oxidative stress of exercise is the mechanism, not the damage. Buffer it away and you buffer away the adaptation too.

This does not apply to the amounts found in food – the point is about high-dose supplements.

Putting it into practice

Physical activity

Regular movement benefits sleep and the cardiovascular system. HIIT is a time-efficient way to trigger a hormetic response.

On the widespread advice not to train hard in the evening: it is more sweeping than the data support. A review of 15 studies with 194 participants found that high-intensity exercise two to four hours before bedtime does not disrupt the sleep of healthy adults; the only measurable change was 2.3 per cent less REM sleep. Regular evening training did not disrupt sleep either. Two hours’ distance is enough, according to the evidence.

Cold exposure – please read this part carefully

A cold shower and immersion in cold water are not the same thing, and the difference is not academic.

On immersion in cold water, the so-called cold shock response sets in within seconds. It can be fatal before core body temperature even begins to fall – through involuntary inhalation of water, through hyperventilation-induced reduced blood flow to the brain, and through cardiac arrhythmia. “Autonomic conflict” and underlying ion channel disorders are discussed as triggers of sudden arrhythmic death in susceptible people – that is, pre-existing conditions they typically do not know they have.

If you want to try cold water: start slowly, never alone, and if you have heart disease, high blood pressure or are pregnant, talk to a doctor first.

A cold shower is a different matter and normally harmless.

What cold actually does: In a study in which five healthy men slept for four months at 24 °C, 19 °C, 24 °C and 27 °C for one month each, brown adipose tissue measurably increased under cold – and decreased again under warmth. The effect is reversible. What improved was diet-induced thermogenesis and post-meal insulin sensitivity. Cold-induced heat production, by contrast, did not change.

This matters, because that is exactly what the popular claim rests on – that cold makes you burn calories. The study gives nothing for it. And: this was four weeks at 19 °C overnight, not thirty seconds in a cold shower.

Heat

Regular sauna sessions are also described as a hormetic stimulus, partly through the formation of heat shock proteins. We did not check this against the primary literature for this article – it appears here as a common explanation, not as a verified finding.

Time-restricted eating (intermittent fasting)

Intermittent fasting is well studied – mainly with regard to weight, and there the result is more sober than is often claimed.

Compared with continuous calorie restriction, intermittent fasting has a slight edge on body weight; on BMI there is no difference. A second review of 18 studies reaches the same picture: comparable benefit, no special path.

In short: intermittent fasting works about as well as steady calorie reduction. Some people find it easier to stick to – that is a good reason to try it. That it slows ageing cannot be shown in humans, and so we do not claim it here.

The table below summarises the strategies and what can actually be supported for each:

Conclusion

Properly dosed, stress can be a tool for greater resilience – the principle is real, and the antioxidant finding demonstrates it about as clearly as anything could. At the same time, a good deal of what circulates under the heading of hormesis has been extrapolated from cell studies to everyday life. In this article we have noted at each point what is measured and what is assumed.

And one thing matters more than any effect: with cold water, safety comes before benefit.

Note: This article is not a substitute for medical advice. If you have a pre-existing condition – particularly of the heart or circulation – are pregnant, or take medication, speak to your doctor before cold exposure, sauna sessions, intense training or longer fasting periods.

Frequently Asked Questions

What is hormesis and how can it improve health?

Hormesis describes how a mild stimulus triggers an adaptive response that makes the body more resilient. It is best supported for physical exertion: the mild oxidative stress it produces activates the body’s own antioxidant defences – making it the mechanism rather than a side effect.

Is a cold shower the same as an ice bath?

No, and the difference is substantial. On immersion in cold water the cold shock response sets in within seconds and can be fatal through water inhalation, reduced blood flow to the brain and cardiac arrhythmia – before body temperature even falls. A cold shower is normally harmless. If you want to immerse: start slowly, never alone, and clear it with a doctor first if you have heart or circulatory conditions or are pregnant.

Does cold burn calories?

Not in the way the claim suggests. In the study usually cited for it, brown adipose tissue did increase under cold – but cold-induced heat production did not change. What improved was diet-induced thermogenesis and post-meal insulin sensitivity. And it involved four weeks at 19 °C overnight, not brief cold showers.

Does intermittent fasting slow ageing?

There is no solid human evidence for that. What exists is weight research: intermittent fasting works about as well as continuous calorie restriction, with no difference in BMI. If you find it easier to stick to, that is a good reason – but it is not a special path.

Why is high-intensity interval training effective for cell health?

Because it strongly alters the energy state of the muscle cell and thereby triggers the AMPK–p53–PGC-1α chain, at the end of which new mitochondria are built. The frequently cited explanation via a lack of oxygen in the muscle is not the established pathway.

Should I avoid intense training in the evening?

Not as a rule. According to a review of 15 studies, high-intensity exercise two to four hours before bedtime does not disrupt the sleep of healthy adults; nor did regular evening training. Two hours’ distance is usually enough.

Are antioxidant supplements useful for exercise?

Probably not. In a controlled trial, four weeks of training improved insulin sensitivity only in participants who took no vitamin C or E – 1,000 mg of vitamin C and 400 IU of vitamin E per day cancelled the training effect. This does not apply to vitamin C and E from food.

References

• Leuci L, Melau J, Messina A, Tipton M, Carenzo L: Beyond hypothermia: mechanisms of death, rescue, and prevention in cold water immersion. J Appl Physiol 2026. doi:10.1152/japplphysiol.00578.2026
• Ristow M, Zarse K, Oberbach A et al.: Antioxidants prevent health-promoting effects of physical exercise in humans. PNAS 2009. doi:10.1073/pnas.0903485106
• Lee P, Smith S, Linderman J et al.: Temperature-acclimated brown adipose tissue modulates insulin sensitivity in humans. Diabetes 2014. doi:10.2337/db14-0513
• Frimpong E, Mograss M, Zvionow T, Dang-Vu TT: The effects of evening high-intensity exercise on sleep in healthy adults: a systematic review and meta-analysis. Sleep Med Rev 2021. doi:10.1016/j.smrv.2021.101535
• Bartlett JD, Louhelainen J, Iqbal Z et al.: Reduced carbohydrate availability enhances exercise-induced p53 signaling in human skeletal muscle. Am J Physiol Regul Integr Comp Physiol 2013. doi:10.1152/ajpregu.00498.2012
• Zhang Q, Zhang C, Wang H et al.: Intermittent fasting versus continuous calorie restriction: which is better for weight loss? Nutrients 2022. doi:10.3390/nu14091781
• Enríquez Guerrero A, San Mauro Martín I, Garicano Vilar E, Camina Martín MA: Effectiveness of an intermittent fasting diet versus continuous energy restriction. Eur J Clin Nutr 2020. doi:10.1038/s41430-020-00821-1