What Does Not Kill

Hormesis: Stimulus at the Boundary

When I tell a patient undergoing cancer treatment, "You should exercise," many are taken aback. "I'm already this worn out, and you want me to strain my body even more?" It is a fair enough reaction. And yet there is a direction that more than twenty years of research has pointed to, without wavering: appropriate exercise reduces the side effects of cancer treatment and lowers the risk of recurrence and death in a range of cancers. In 2019 the American College of Sports Medicine convened its experts and came to declare, officially, that "exercise is safe for cancer patients and should be recommended." How does that very exercise, which stresses the body, bring the body to life? That secret is the key to today's story.

Exercise physiology has a concept called hormesis. It is the principle that a small dose of stress makes an organism stronger instead of weaker. Drawn from the same root as "toxin," the word carries the paradox that a little poison awakens immunity and triggers adaptation. Exercise is precisely this. Exercise tears the muscle in fine tears, burdens the heart, exposes the cells to oxidative stress. And yet the body responds to that small damage and recovers stronger than before. A stimulus that does not kill is what brings us to life.

"What does not kill me makes me stronger." This line Nietzsche left behind is, in fact, also a line of cell biology. There is only one condition. It must be only as much as does not kill. Too great, and it is not training but destruction; too absent, and it is not safety but decay.

A boundary hardens by being struck. A boundary untested is no boundary at all.

I too play often at that boundary of "only as much as does not kill." The day I climbed the mountains of Banff in Canada, I left the cable car behind and walked up the six-kilometer trail on foot.

Two-thirds of the way up, perhaps? My heart rate is 165. Given my age, that is nearly the maximum. … I feel dizzy. … Reaching the summit, the scene spreading out before me was rapturous.— 2023, Facebook

I drove my heart until I was dizzy, but because it was only as much as would not kill me, I came down sturdier. Imagine a body raised in a sterile room. An immune system that has met no germ at all is not strong but weak. Because it has never met an enemy, it collapses before a real one. This is one reason allergy and autoimmune disease are rising today. We are raised in too clean a world, our boundaries left untested. It is the same logic by which a child raised only in a greenhouse grows not strong but weak. The Age of Coddling I spoke of in Chapter One is precisely this sterile room, this greenhouse. A tenderness that erases every wound keeps us from growing.

This principle—that difficulty only as much as does not kill makes us grow—I learned from the body. But there was one who saw the same thing in civilization: the historian Arnold Toynbee. He held that civilizations, too, grow through "challenge and response." Civilization does not arise in comfortable places, he said, but in the answering of difficulty. Moreover, he said there is a fitting size to a challenge. Too small, and it is no stimulus at all; too great, and it does not stimulate but crushes the man beneath it. The principle of hormesis I saw in the cell, he confirmed anew in civilization.

Here the story of Chapter One returns. The one who only swells avoids real stimulus. He refuses the small poison of failure. And so he never grows strong. A muscle swollen without stimulus is in fact not muscle but edema (浮腫). It looks large on the surface, yet has no strength. So to refuse the wound is not safety. It is the refusal of growth. The living thing is hurt, little by little, at the boundary, and through that wound it grows. This truth will come to find us again, later, under the name "the wounded healer." For now, let us remember only this: the right poison is medicine. And the wound is proof that the boundary is alive.

References

Friedrich Nietzsche, Twilight of the Idols · Arnold Toynbee, A Study of History (challenge and response) · ACSM Exercise–Oncology Roundtable (2019)