When the human body is pushed to the absolute limit by athletes who run 226 km non-stop, it makes a radical decision — it shuts down reproduction and tissue repair to keep only the systems that ensure immediate survival functioning.

Written by Douglas Avila

Published on 18/04/2026 at 21:36

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A study published in Evolutionary Human Sciences analyzed ultramarathon and Ironman triathlon athletes and revealed that, under extreme physical stress, the body enters selective shutdown — prioritizing immune defense above all else

Imagine running 226 kilometers non-stop.

That’s what an Ironman athlete does: 3.8 km of swimming, 180 km of cycling, and 42 km of running.

All in up to 17 hours.

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What the body shuts down — and what it keeps

The discovery is surprising for the clarity of the choice.

Under extreme physical stress, the body prioritizes immune defense over two fundamental functions:

Reproduction — reproductive hormones are suppressed
Tissue repair — healing and cellular regeneration are slowed down

All available energy is redirected to keep the immune system functioning.

From an evolutionary perspective, surviving an infection is more urgent than reproducing or healing a wound.

The logic of ancestors

The researchers argue that this mechanism is an evolutionary inheritance.

Our ancestors faced extreme stress during hunts or escapes from predators.

With limited energy, it was impossible to keep all systems at maximum.

Evolution “programmed” the organism to prioritize what keeps the individual alive now.

During an escape, any open wound becomes a gateway for infections.

Keeping immunity active was a matter of life or death.

Ironman triathlete swimming in open water — The Ironman requires 3.8 km of swimming, 180 km of cycling, and 42 km of running — 226 km in up to 17 hours that push the body to its absolute limit

Athletes as living laboratories

Ultramarathon and Ironman athletes are the living humans who come closest to the stress faced by ancestors.

They spend hours in maximum continuous effort.

The body enters “selective shutdown”.

It’s not total collapse — it’s a strategic redistribution of resources.

The consequences for modern athletes

This mechanism explains known phenomena in elite sports:

Post-event performance drop — the body did not repair tissues during the event
Increased susceptibility to diseases after marathons — the immune “boost” depletes reserves
Hormonal changes in endurance athletes — especially reduction of reproductive hormones

The human body is paying a price for immediate survival.

Soldiers, firefighters, astronauts

The discovery goes beyond sports.

Soldiers in combat, firefighters in fires, and astronauts on long missions face comparable stress.

Understanding how the body redistributes resources can lead to treatments that optimize this response.

Moreover, it can help understand why researchers isolated in Antarctica for 14 months suffer profound physiological changes.

The mechanism may be the same.

Scientist analyzing blood samples from athletes in the lab — Researchers analyzed biomarkers from extreme athletes and discovered active redistribution of biological resources during maximum effort

Caveats

The study is based on extreme athletes — sedentary populations may respond differently.

The immune prioritization is adaptive in the short term, but if chronic, it can lead to risks such as infertility.

The data is preliminary and subject to peer review.

Still, the discovery reveals something profound: when pushed to the limit, the body knows exactly what to do — and the first thing it protects is not the heart or the brain, but the system that defends us against invisible invaders.

When the human body is pushed to the absolute limit by athletes who run 226 km non-stop, it makes a radical decision — it shuts down reproduction and tissue repair to keep only the systems that ensure immediate survival functioning.

A study published in Evolutionary Human Sciences analyzed ultramarathon and Ironman triathlon athletes and revealed that, under extreme physical stress, the body enters selective shutdown — prioritizing immune defense above all else

What the body shuts down — and what it keeps

The logic of ancestors

Athletes as living laboratories

The consequences for modern athletes

Soldiers, firefighters, astronauts

Caveats

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When the human body is pushed to the absolute limit by athletes who run 226 km non-stop, it makes a radical decision — it shuts down reproduction and tissue repair to keep only the systems that ensure immediate survival functioning.

A study published in Evolutionary Human Sciences analyzed ultramarathon and Ironman triathlon athletes and revealed that, under extreme physical stress, the body enters selective shutdown — prioritizing immune defense above all else

What the body shuts down — and what it keeps

The logic of ancestors

Athletes as living laboratories

The consequences for modern athletes

Soldiers, firefighters, astronauts

Caveats

Forget petroleum-based asphalt: a Scottish company transforms 684,000 plastic bottles into one kilometer of road that is 60% more resistant and up to 10 times more durable, and the technology has already started replacing traditional asphalt in more than 30 countries.

Austrian engineers have developed an inflatable concrete technology that transforms flat slabs into domes using only air pressure, eliminating scaffolding and temporary forms, and potentially changing the way bridges, tunnels, and curved roofs are constructed worldwide.

With billions in loans, China is rebuilding Nigeria’s railway network from scratch, abandoning the British colonial tracks that have turned to scrap and creating a corridor of over 1,300 km to take the load off the roads of Africa’s largest economy.

Australia prepares to dredge 96.5 million m³ in Moreton Bay to deepen, widen, and realign channels of the Port of Brisbane, in a project of up to 25 years designed to accommodate larger ships and enhance maritime safety and regional port efficiency.

A robot named Walter does the work of five bricklayers per hour and can save the construction industry, where the average age of professionals is 46 years and almost no one wants to learn the trade anymore, in the United Kingdom.

Cristiano Ronaldo builds a billion-dollar mansion in Portugal with 54,000 square feet, a 3-acre plot facing the Atlantic, private spa, indoor and outdoor pools, and an estimated value of an impressive R$ 385 million.

For the first time in history, a submarine cable will descend to four thousand meters deep under the ice of the North Pole to ensure that the internet between Europe and Asia no longer depends on conflict zones in the Middle East.

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While Brazil has been discussing the Maracanã renovation for 25 years and struggles to keep the Arena MorumBis open, Abu Dhabi signs a $1.7 billion deal to build the world’s second Sphere on an artificial island, with 20,000 seats, by 2029.

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For the first time in history, a submarine cable will descend to four thousand meters deep under the ice of the North Pole to ensure that the internet between Europe and Asia no longer depends on conflict zones in the Middle East.