Heat: the ultimate way to progress? Acclimation, real levers, false good ideas

Amateur or elite, there's one opponent we all face on equal terms: heat. And with climate change, it's showing up on more and more of our races.

The real question is no longer how to avoid it — but how to make it an ally.

Heat isn't in your head — it's in the numbers

• Climate Central (2025): of 221 marathons analyzed worldwide, 86% will see conditions worsen by 2045 — including the 7 majors.
• PLOS ONE (2012): marathon performance peaks between 3.8 and 9.9°C; beyond, it drops off.
• Case study — Chicago 2007, 25°C: 30.7% DNFs, 66 runners hospitalized, one death.

On the body side, Cyril Schmit describes two mechanisms: a cardio/peripheral effect (blood rushes to the skin, the heart overcompensates) and a nervous/central effect (the brain eases off to protect you). And no, sweating a lot isn't a flaw: it's often the sign of a well-acclimated body.

The good news: heat can also help you progress

It all depends on the distance:

• On short efforts (800 to 1500 m): about +1°C ≈ +5% power.
• On long efforts: the opposite, roughly −0.5% per degree, tapering off.

True or bullshit? Sorting the advice you read everywhere

• Cold bath / pre-cooling before a hot start → true, it works.
• Training in thick clothing to adapt → useful but to be framed with caution.
• Ice in the mouth during the race → real but modest effect, especially on ultras.
• Depriving yourself of water or taking diuretics to habituate the body to dehydration → dangerous, to be banned.
• Eating hyper-protein to better tolerate heat → social media myth.

What if the problem also came from the races themselves?

Heat raises a real organizational question: earlier starts, reinforced aid stations… and maybe, tomorrow, night marathons. In the meantime, summer remains the season when most amateurs wreck their prep — I laid out the right reflexes in training on vacation.