Hormesis from Low-Dose Radiation

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Hormesis was first shown to extend life more than a century ago when low doses of radiation were shown to increase the lifespan of a type of beetle. Scientists were hoping irradiation with x-rays could kill off the eggs of the grain pest, but to their surprise, at low doses, it actually made the beetles live longer. This was subsequently replicated with gamma rays, and in about a dozen other insects from house flies and crickets to wasps and mosquitoes: a strongly consistent 20 to 60 percent increase in lifespan. What is going on? Hormesis is thought to be an adaptive response to anticipated potential threats. The body is able to leverage the initial insult to prompt a compensatory counter response that more than covers the cost of the original damage and yields a net benefit in the end.

In the case of radiation, at high enough doses, it kills by damaging DNA. Animals have a whole DNA repair apparatus, but it’s presumably too metabolically costly to have it run at full tilt all the time even when we don’t really need it. But ramping up DNA insults with low dose radiation signals to the animal that they are in a DNA destructive environment; so, their body starts ramping up DNA repair to compensate. And a happy side effect of all that extra DNA protection is a longer life. That which didn’t kill them made them stronger.

Hormesis is not just for insects. It may explain the findings of the study “Low-dose radiation from A-bombs elongated lifespan and reduced cancer mortality relative to un-irradiated individuals.” The atomic bombs dropped on Hiroshima and Nagasaki killed approximately 200,000 people instantaneously, and survivors exposed to high levels of radiation suffered from high rates of cancer and shortened lives. However, those exposed to lower levels of radiation living farther from the blast site appeared to end up with lower cancer mortality and longer average lifespans. Exceptional medical services provided to the surviving victims under the Law Concerning Relief to Atomic Bomb Survivors may explain some of the longevity benefit, but it also could have been a hormetic effect. “Nothing in life is to be feared,” Madame Curie, who won the Nobel Prize for discovering radioactivity, is quoted as saying, “it is only to be understood.” Of course, this is coming from a woman who died of bone marrow failure from radiation exposure, such that her remains were interred in a lead-lined coffin.

Why would animals evolve to be able to withstand radiation? Because we’re bombarded day and night by cosmic rays from the universe. Our bodies are exposed to about 20,000 hits of radiation every second. But what if we weren’t? Scientists have found that single-celled organisms like paramecia don’t grow as well in lead boxes, and the thicker the lead walls, the more their growth is stunted. Human cells grown under similarly shielded conditions show an increase in levels of DNA damage and mutations. This suggests the natural background radiation of the Earth has the hormetic benefit of keeping our body on its toes.

Even a foot (0.3048 m) of lead only blocks about a third of cosmic rays. To really see what life would be like without constant bombardment, scientists descended more than a mile (1.6 km) down into the Earth into SNOLAB, the Sudbury Neutrino Observatory, set at the bottom of a nickel mine in Canada. Contrary to expectations, fish raised deep underground grew bigger and heavier than those raised on the surface. The researchers suggested the silencing of cosmic radiation may have been offset by increased radiation from radon gas trapped in the mine. Bottom line: we don’t know enough about low-level radiation to exploit any hormetic effects without potentially being exposed to unacceptable risks. Thankfully, there are salutary ways to harness hormesis for health and longevity.